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Abstract

The origin of ancient massive dolostones, i.e. continuous dolostone sequence with a thickness >100 m and a platform-wide distribution, is the key issue of the ‘Dolomite Problem’ that cannot be clearly demonstrated by any existing dolomitization model individually or sequentially. It has been proposed that the massive dolostone could be generated by the stacking of multistage dolomitization events linked to the sea-level fluctuation, which results in repeatedly occurring of limestone precipitation-dolomitization cycles. However, the sequence of dolomitization events cannot be differentiated by any sedimentological or traditional geochemical techniques. Here we report Mg isotopic compositions of the massive dolostone (δ26Mgdol) from the middle Cambrian Qinjiamiao Formation (QJM) in the Yangtze Platform, South China, which consists of cyclic depositions of shoaling upward sequences. The stratigraphic variation of δ26Mgdol is coincident with the depositional cycles, suggesting the dolomitization might be periodic and be coupled with the sea-level oscillation. As dolomitization fluids experience changes in δ26Mg values during dolomitization processes, the intra-cycle stratigraphic δ26Mgdol profile reflects the processes of dolomitization. Our study indicates that the massive dolostone could be generated by the temporal and spatial stacking of multiple dolomitization events that are associated with sea-level fluctuation. If this model can be verified by other massive dolostone successions, the origin of massive dolostone may be resolved.

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... To ensure the thorough purification of Mg, each sample needs to pass through the No.1 ion exchange chromatography column 3 times and the No.2 ion exchange chromatography column 2 times. After purification, Ca/Mg, Al/Mg, Na/Mg, K/Mg, and Fe/Mg were less than 0.05, and the magnesium recovery rate was greater than 99% (Ning et al., 2020). ...
... According to the variation of δ 26 Mg value with depth (Fig. 7), the δ 26 Mg of the shallow dolomite interval (3608-3624 m) is basically unchanged vertically, which is consistent with the far-source seepage reflux dolomitization model of the early diagenetic stage. Its essence is that, in areas far from the source area of dolomitization fluids, such as the subtidal zone, Mg 2+ -rich fluids may undergo lateral migration under the action of hydrostatic pressure gradient; thus, as the dolomitization fluid migrates, the δ 26 Mg may remain unchanged in a vertical profile with equal distance from the source region (Ning et al., 2020;Xia et al., 2021). ...
... The δ 26 Mg upward heavier interval (3713∼3709 m) is similar to the penecontemporaneous sabkha-type dolomitization model in the confined or semi-confined environment with periodic Mg 2+ replenishment. Fundamentally, as 24 Mg preferentially enters the dolomite lattice formed in the early stage, with the continuous formation of dolomite, the Mg isotope value of the sedimentary seawater becomes heavier, the Mg isotope value of the dolomite formed in the later stage correspondingly becomes heavier, and the δ 26 Mg in the vertical profile shows an upward increasing trend (Ning et al., 2020;Xia et al., 2021). This model can explain the formation process of grain dolomite and aphanocrystalline dolomite of well ZG58 during the near surface to penecontemporaneous stage. ...
... However, it is essential to recognize that the massive dolostone successions in geological history perhaps formed via the stacking of a multistage of dolomitization events (Lumsden and Caudle, 2001;Meister et al., 2013). The study by Ning et al. (2020) has shown that the stacking of multistage dolomitization events in the tidal plat environment, which are linked to sea-level fluctuation, can lead to various stratigraphic cycles of δ 26 Mg values in a completely dolomitized carbonate profile. A recent study suggested that the fluid flow event on a basin scale during deep burial stage can also cause the created gradients in Mg isotope compositions of dolomite bodies (Kimmig et al., 2021). ...
... Therefore, exploring the dolomitization processes and mechanism is the key to elucidate the Mg isotopic variability in dolostones. There are two possible scenarios accounting for the formation of massive dolostone sequences in the Quse Formation: 1). the stacking of multistage dolomitization event caused by the periodic downward migration of dolomitization fluid at early diagenesis stage (Ning et al., 2020); 2). syn-depositional origin (Machel and Mountjoy, 1986;Hu et al., 2022b). ...
... During the downward migration of dolomitization fluid, due to the preferential uptake of 24 Mg by dolomite, dolostones that formed at the early stage in the upper part of the succession are always isotopically lighter than that formed in deeper sediments (Huang et al., 2015;Ning et al., 2020). This feature seems to resemble the stratigraphic variability as shown in Fig. 3. ...
Article
Magnesium isotopes of marine dolomites has the potential to reconstruct the Mg isotopic signals of coeval seawater. However, a massive dolostone sequence can form via the coupling effects of multiple dolomitization events, which can potentially reset the Mg isotopic compositions of dolomites during each migration of the dolomitization fluid. How to distinguish changes of seawater chemistry and early diagenetic effects which have been recorded in dolomites is crucial for the reconstruction of seawater Mg isotopic signals. Here we studied the Mg-Sr-C-O isotope systematics and trace elements of a dolostone sequence from the Qiangtang Basin in the hinterland of Tibet. Multiple lines of evidence consistently indicate the lack of deep-burial diagenesis and high-temperature alteration in the carbonates. The stratigraphic variations of isotopic and elemental compositions of the dolostone profile are generally characterized by the co-variation between Mg-C isotopes and trace element contents. Our modelling results indicated that a single downward dolomitization can cause the large Mg isotopic variability of dolomites, ca. 1‰, within a dolomitization front. However, when the limestones in the profile were completely dolomitized, the coupling effects of multiple downward dolomitizations will homogenize the δ26Mg values of dolostones. Therefore, we attribute the observed co-variation in isotopic and elemental signals in the dolostone sequence to the changes in the chemistry of evolved seawater within the platform. The reconstructed seawater δ26Mg values from the Carnian to Early Jurassic are lower than previous model-based estimations. This result implies an overestimation of the production of shallow-water carbonate in literature during Late Triassic to Early Jurassic. Our study highlights the importance of utilizing multiple geochemical proxies to identify the controlling factors driving Mg isotope variability in dolomites.
... The formation of ancient massive dolomite may be related also to sea level fluctuations. Ning et al. (2020) proposed a model for the formation of massive dolostone in the Yangtze Platform, South China, by the stacking of multi-stage dolomitization associated with sea-level fluctuations. During the period of sea level rise, limestone began to be deposited in the subtidal zone and intertidal zone (Fig. 12A). ...
... Each kind of dolomitization has certain environmental limitations. The formation of massive dolostones in marine environments cannot be explained by a single dolomitization event (Chang et al., 2020;Ning et al., 2020) rather, it involves multiple dolomitization processes. ...
... The massive dolostone formation is directly linked to the fluctuation of sea-level, resulting in the vertical stacking (C) and lateral shifting (D) of dolomitization. Reprinted from Ning et al., 2020, Copyright (2020, with permission from Elsevier. and Lonnee, 2002; Davies and Smith Jr., 2006). ...
Article
Dolomite can form as a major mineral component of marlstones and limestones and is an important sink for magnesium in the marine environment. Dolomite is widely distributed in time and space and is considered an intrinsic ingredient in the evolution of the Earth’s crust. The crystal structure and genesis of dolomite record fundamental geochemical and environmental processes of significance in the Earth’s element cycle process, carbon storage, and paleoenvironments, with relevance to the exploration for minerals and fossil fuels, management and maintenance of the environment, and the use of carbonate minerals by industry. The review brings together new advances and insights from recent studies on dolomite structure, geological genesis, laboratory synthesis, and applications. In the mantle, dolomite may adapt to increasing pressure by structural rearrangement and undergoes crystal phase transitions. At present, four high-pressure polymorphs have been identified. The phase transitions allow dolomite to survive subduction into the mantle, possibly into the transition zone, but stability is not fully predictable and is influenced by factors that include initial degree of cation ordering in dolomite and Fe and Mn substitution for Mg in the dolomite crystal lattice. The presence of Fe and Mn is influenced by the environment of dolomite formation. The key factors controlling formation of dolomite, including transition or recrystallization from precursor high-Mg calcite or proto-dolomite, at low temperatures remain ambiguous. Sulfate-reducing bacteria, methanogens, and aerobic bacteria, the exudates or relevant extracellular polymeric substances, fluctuating environmental conditions, and the negatively charged surfaces of clay minerals all can mediate high-Mg calcite/proto-dolomite formation at low temperature. As for secondary dolomite, formed by Mg2+ replacement of Ca2+ in carbonate minerals, several models have been proposed and widely adopted, including: near-surface dolomitization, burial dolomitization, and hydrothermal dolomitization. The formation of massive deposits of dolomite in marine sediments probably involves multiple dolomitization processes. Yet the “dolomite problem” remains enigmatic. Mg isotope analysis, an emerging technology, offers a new approach to further investigate the genesis of dolomite. In the laboratory, synthesis of dolomite at low temperature has yet to be achieved. Fundamental scientific research on dolomite is expected to inform the sustainable use of dolomite resources. Traditional uses of dolomite typically in construction materials, refractory, and flux continue. Now, the use of dolomite and its calcined products is being expanded into environmental protection, soil improvement, thermochemical energy storage and biomedical materials.
... Mg isotopes can effectively trace the source of Mg-rich fluids and reconstruct the evolution process of dolomitization (Huang et al., 2015;Ning et al., 2020;Peng et al., 2016). Clumped isotope temperature measurement technology has significant implications for analyzing the diagenetic temperature of dolomite, restoring oxygen isotope values (combined with oxygen isotope values) for the study of diagenetic environments and fluids (Shenton et al., 2015). ...
... The thickbedded Huanglong dolomite is transformed from stacked dolomitization of many thin-bedded limestones layer by layer. Li et al. (2023a) conducted Mg isotope tests on dolomite, dolomitic limestone, and limestone of the Ordovician Yingshan Formation in the Tarim Basin and also concluded that the vertical variation of the δ 26 Mg values is closely related to sedimentary Ning et al., 2020;Murray et al., 2021;Shalev et al., 2021;Zhu et al., 2022;Li et al., 2022Li et al., , 2023aLi et al., , 2023b. ...
Article
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The 10 000‐m ultradeep dolomite reservoir holds significant potential as a successor field for future oil and gas exploration in China's marine craton basin. However, major challenges such as the genesis of dolomite, the formation time of high‐quality reservoirs, and the preservation mechanism of reservoirs have always limited exploration decision‐making. This research systematically elaborates on the genesis and reservoir‐forming mechanisms of Sinian–Cambrian dolomite, discussing the ancient marine environment where microorganisms and dolomite develop, which controls the formation of large‐scale Precambrian–Cambrian dolomite. The periodic changes in Mg isotopes and sedimentary cycles show that the thick‐layered dolomite is the result of different dolomitization processes superimposed on a spatiotemporal scale. Lattice defects and dolomite embryos can promote dolomitization. By simulating the dissolution of typical calcite and dolomite crystal faces in different solution systems and calculating their molecular weights, the essence of heterogeneous dissolution and pore formation on typical calcite and dolomite crystal faces was revealed, and the mechanism of dolomitization was also demonstrated. The properties of calcite and dolomite (104)/(110) grain boundaries and their dissolution mechanism in carbonate solution were revealed, showing the limiting factors of the dolomitization process and the preservation mechanism of deep buried dolomite reservoirs. The in situ laser U‐Pb isotope dating technique has demonstrated the timing of dolomitization and pore formation in ancient carbonate rocks. This research also proposed that dolomitization occurred during the quasi‐contemporaneous or shallow‐burial periods within 50 Ma after deposition and pores formed during the quasi‐contemporaneous to the early diagenetic periods. And it was clear that the quasi‐contemporaneous dolomitization was the key period for reservoir formation. The systematic characterization of the spatial distribution of the deepest dolomite reservoirs in multiple sets of the Sinian and the Cambrian in the Chinese craton basins provides an important basis for the distribution prediction of large‐scale dolomite reservoirs. It clarifies the targets for oil and gas exploration at depths over 10 000 m. The research on dolomite in this study will greatly promote China's ultradeep oil and gas exploration and lead the Chinese petroleum industry into a new era of 10 000‐m deep oil exploration.
... First, precipitates of microbial experiments exhibit different morphologies of spheroids and dumbbell-shaped crystals (Vasconcelos et al., 1995;Warthmann et al., 2000;Kenward et al., 2013) in contrast to rhombic crystals of dolomite found in nature (Fairchild, 1983;Rosen and Coshell, 1992;Tucker, 1992). Second, the mode of dolomite formation in microbial experiments is direct primary precipitation from saturated solutions, which forms crystals in µm size; however, natural dolomite is a product of diagenetic replacement of precrusor calcium carbonate (Kaczmarek et al., 2017), which forms massive dolomite with a thickness that reaches >100 m (Ning et al., 2020). Therefore, it is likely that the crystal shape, mode of formation, and size are interrelated. ...
... Many reasons can stand behind this failure, individually and collectively. First, the replacement reactions require a pumping system of fluids with a high Mg content in order to be able to dolomitize calcium carbonates (Ning et al., 2020). In our experiments, we had a limited volume of fluids for replacement reactions (≥ 75 ml). ...
Article
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Recent laboratory experiments have exhibited microbes as promising agents in solving the perplexing origin of ancient dolomite by demonstrating microbial capability to mediate dolomite nucleation and growth. However, dolomite crystals from laboratory experiments have shown irrelevant characteristics to ancient dolomite from mineralogical and petrological perspectives. A major irrelevant characteristic is that ancient dolomite was assumed to be formed after the replacement of Ca by Mg in precursor CaCO 3 in a process known as diagenesis, which contrasts with the primary precipitation process observed in laboratory culturing experiments. Considering dolomite microbial experiments, one can imply the involvement of microbes in the formation of ancient dolomite, as microbes have shown the ability to overcome the dolomite kinetic barrier. Despite that fact, the ability of microbes in mediating dolomite diagenesis has not been investigated. In this study, microbes were applied to mediate replacement of Ca by Mg in different CaCO 3 precursors. The microbial replacement experiments were based on the enrichment of aerobic halophilic heterotrophic microbial consortia sampled from sediments collected from Al-Subiya sabkha in Kuwait. Two experiments were performed in saturated media at 35 • C for 14 and 30 days simulating the conditions of microbial dolomite experiments. The change in mineralogy was examined via powder X-ray diffraction (XRD), and the change in texture and compositional microstructures was examined using scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). The effect of microbes on the alteration of CaCO 3 precursors was studied by comparing biotic experimentations with abiotic controls. The biotic samples were shown to result in the favorable conditions for dolomite formation including an increase in pH and alkalinity, but no changes were observed in mineralogy or compositional microstructure of CaCO 3 precursors. Our results suggest the inability of aerobic halophilic heterotrophic microbial consortia to introduce Mg replacement on CaCO 3 precursors in a timely manner that is comparable to primary precipitation in microbial dolomite experiments. The inability of the enriched microbial consortia to mediate replacement can be ascribed to different factors controlling the diagenetic process compared to primary precipitation in microbial dolomite experiments.
... Zhang, Xu, Konishi, Kemp, et al., 2012) or very high-Mg calcite (VHMC; Gregg et al., 2015); Warthmann et al. (2000) might not synthesize dolomite because they did not list the characteristic (104) peak of dolomite; Wright and Wacey (2005) reported aragonite rather than dolomite (see their Figure 14a); and the X-ray diffraction (XRD) data in Deng et al. (2010) paper did not fit with the dolomite spectra. Neither the thickness nor the area of these microbial "dolomites" reached the grade of dolostone (Compton, 1988;Ning et al., 2020), and how much microbes contributed to the ancient dolomitic formation has been questioned (Petrash et al., 2017). In contrast, several studies have found some abiotic factors that stimulate dolomite nucleation and growth, including water-level fluctuations (Lumsden & Caudle, 2001;Ning et al., 2020;R. ...
... Neither the thickness nor the area of these microbial "dolomites" reached the grade of dolostone (Compton, 1988;Ning et al., 2020), and how much microbes contributed to the ancient dolomitic formation has been questioned (Petrash et al., 2017). In contrast, several studies have found some abiotic factors that stimulate dolomite nucleation and growth, including water-level fluctuations (Lumsden & Caudle, 2001;Ning et al., 2020;R. Wang et al., 2018), systems with catalysts (H. ...
Article
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Plain Language Summary Dolostone is common in ancient rocks but rare in modern marine sediments, and dolomite is notoriously difficult to synthesize without microbes at room temperature; this is known as the “dolomite problem”. In many studies, microbes have often been associated with dolomite, and synthesis by microbial mediation has succeeded. However, the mineralogy of microbial dolomites has been questioned and not all massive dolostones have been found with microbial structures. Hydrothermal–burial alteration is another promising solution, but recent studies have demonstrated that dolostones could also form at surface temperatures (<60°C). In this study, we investigated a Holocene lacustrine dolomite layer (12‐cm‐thick) precipitated without microbes or burial alteration. Our results demonstrated that these dolomites have obvious cation ordering and euhedral crystals, and their oxygen isotope values are similar to ostracods in the same horizon, suggesting a primary origin. These euhedral and rhombic dolomite grains are consistent with dolomite precipitated slowly in an abiotic way rather than by microbial mediation. We infer the dolomite layer might be an aftermath of the day and night temperature cycling of shallow lake water resulting from local warming and aridification. This study provides evidence that partially ordered dolomite, as an intermediate phase of ideal dolomite, can precipitate abundantly in ambient environments.
... Zhang, Xu, Konishi, Kemp, et al., 2012) or very high-Mg calcite (VHMC; Gregg et al., 2015); Warthmann et al. (2000) might not synthesize dolomite because they did not list the characteristic (104) peak of dolomite; Wright and Wacey (2005) reported aragonite rather than dolomite (see their Figure 14a); and the X-ray diffraction (XRD) data in Deng et al. (2010) paper did not fit with the dolomite spectra. Neither the thickness nor the area of these microbial "dolomites" reached the grade of dolostone (Compton, 1988;Ning et al., 2020), and how much microbes contributed to the ancient dolomitic formation has been questioned (Petrash et al., 2017). In contrast, several studies have found some abiotic factors that stimulate dolomite nucleation and growth, including water-level fluctuations (Lumsden & Caudle, 2001;Ning et al., 2020;R. ...
... Neither the thickness nor the area of these microbial "dolomites" reached the grade of dolostone (Compton, 1988;Ning et al., 2020), and how much microbes contributed to the ancient dolomitic formation has been questioned (Petrash et al., 2017). In contrast, several studies have found some abiotic factors that stimulate dolomite nucleation and growth, including water-level fluctuations (Lumsden & Caudle, 2001;Ning et al., 2020;R. Wang et al., 2018), systems with catalysts (H. ...
... Recent studies on late Cenozoic and modern dolomitization indicate that δ 26 Mg dolomite values are controlled by the hydrologic conditions of early diagenesis (Blättler et al., 2015;Ahm et al., 2018;Higgins et al., 2018;Ning et al., 2020;Shalev et al., 2021), while the diffusion of porewater in carbonate sediments can cause stratigraphic variability in δ 26 Mg dolomite values (Huang et al., 2015). As massive dolostone sequences are usually deposited over a long time, they often experience considerable sea-level changes (Jones and Luth, 2003;Zhao and Jones, 2012;van der Meer et al., 2019). ...
... Therefore, we infer that the highfrequency eustatic change did not impact the long-term Mg isotope compositions of the dolomites and the type of dolomitization, possibly because this high-frequency eustatic change did not change the marine connectivity between the platform and open ocean. Ning et al. (2020) showed that in nearshore deposits, the δ 26 Mg dolomite values of shallowing-upward sequences display considerable variation, with a total range of~0.5‰. We note that in each depositional cycle of that study, the sedimentary environment evolved gradually from lower intertidal to evaporated supratidal conditions, a change that could be caused by basin restriction or a strong dolomitization gradient, such as that in evaporitic tidal flats (Shalev et al., 2021). ...
Article
Marine dolomitization processes are characterized by complex variations in hydrological conditions and pore-fluid chemistry. Deposition of massive dolomitized carbonates on the Comanche Platform of south Texas, USA, during the Albian coincided with multiple fluctuations in sea level, thereby providing an ideal setting to study the response of magnesium isotopes to dolomitization during eustatic sea-level change. In this study, we conducted Mg, C, and O isotope analyses and complimentary mineralogical and petrographic investigations of dolomitized massive carbonates of the Albian Edwards Group in the Comanche Platform. Petrographic observations indicate that the carbonate rocks in the studied units were not altered by deep burial diagenesis or hydrothermal fluids. Based on our petrographic observations and the trace element and CO isotope data, we infer the dolomites were formed via syn-depositional dolomitization during a period of low sea level. The δ²⁶Mg values of the dolomites increase rapidly from −2.5‰ to −1.8‰ in the basal part of the unit, reflecting a change in fluid chemistry caused by dolomitization in a restricted marine environment. Subsequently, δ²⁶Mgdolomite values gradually decrease back to their initial value of approximately −2.5‰ due to seawater replenishment during transgression. The observed variability in dolomite δ²⁶Mg values reflects changes in the connectivity of the platform with the open ocean during marine transgressions. However, the δ²⁶Mgdolomite values do not vary with the high-frequency eustatic sea-level change recorded in the lithological variations, indicating uniform hydrologic conditions of the massive dolomitization system despite the hydrodynamic variations in the sedimentary environment. Therefore, we propose that massive dolomitization systems are mostly fluid-buffered and, as a result, Mg isotopes of dolomites can be used to trace changes in the paleo-marine environment, such as basin connectivity.
... 15 Recent studies on the genesis of dolomite reservoirs have shown that dolomite can always be formed from matrix dolomite produced by dolomitization, but not necessarily dolomite reservoirs. 16,17 Primary sedimentary facies and early dolomitization are the keys to the development of high-quality reservoirs, and the later tectonic fluid acidification has a vital effect on the formation and maintenance of reservoirs. 18 The formation of good dolomite reservoirs is not only bound to limestone and presedimentary environments 19 but also closely related to porosity evolution during late diagenetic processes. ...
Article
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The hydrocarbon reserves in carbonate rocks account for about half of the global hydrocarbon reserves and are an important reservoir type. The Ordovician Majiagou Formation in the central Ordos Basin is a representative weathering crust reservoir. The Caledonian movement uplifted the stratum as a whole, and subsequently, 120 million years of exposed weathering, denudation, and leaching created this unique karst paleomorphology. Dolomite reservoirs have developed dissolved pores and microfractures, which are the best reserved spaces for natural gas and good hydrocarbon migration channels. This paper takes the Ma5Member (hereinafter referred to as Ma51+2) carbonate reservoir in Gaoqiao Gas Field as the research target, based on the core, thin section, cathodoluminescence, logging data, etc., and systematically study the effect of karstification on the reservoir and the genesis of the dolomite reservoir. The results show that the depositional period of the Ordovician Majiagou strata is a regression cycle and the depositional environment is a limited evaporative tidal flat. The reservoir lithology of Ma51+2 is mainly gypsiferous dolomite and micrite dolomite. The reservoir space types consist of intergranular pores, gypsum mold pore, intragranular dissolution pores, intercrystalline pores, and microfractures. The porosity has values from 0.3 to 11.2% (mostly less than 5.0%) with an average of 3.3%, and the permeability ranges from 0.003 to 13.2 mD (mostly less than 1 mD) with an average of 0.36 mD. Karstification is divided into three periods, including syngenetic karst, eogenetic karst, and burial karst. The sedimentary microfacies determine the material basis of the reservoir, and multistage karstification finally modifies the physical properties. By deeply exploring the formation mechanism and influencing factors of the carbonate reservoir in the Ordovician Majiagou Formation, it provides an important theoretical basis and practical guidance for oil and gas exploration and development. At the same time, it also has important reference value for understanding and predicting the development law and distribution characteristics of carbonate reservoirs under similar geological background.
... The "Dolomite Problem" refers to the ongoing difficulty in understanding the conditions and processes behind the precipitation of dolomite as opposed, for example, to its more prevalent counterpart, calcite (CaCO 3 ). While calcite precipitation can be readily simulated in the laboratory and explained by established geochemical principles, dolomite formation has proven to be very challenging (Meister et al., 2023;Ning et al., 2020;Petrash et al., 2017). In the laboratory, dolomite precipitation from supersaturated solutions can be easily accomplished exclusively at high temperatures (70°C or more) (Montes-Hernandez et al., 2014). ...
Article
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The mineral Dolomite CaMg(CO3)2 is a common constituent of sedimentary rocks. Despite centuries of research, the mechanism of its formation remains elusive and debated. Recent studies have shown the presence of silica in solution promote the incorporation of Mg into the carbonate mineral, forming crystal phases that may be precursors to dolomite. The goal of this study was to evaluate with laboratory experiments whether dissolved silica may play a role for dolomite formation in sabkha (i.e., salt flats) environments. Several models for dolomite formation are based on the studies of sabkhas, which are often cited as modern analogue for ancient dolomite-rich sedimentary sequences. We performed long-incubation time (i.e., up to 600 days) laboratory precipitation experiments at 30 °C with solution mimicking the sabkha pore waters (characterized by a salinity of 23 % and Mg: Ca ratio of 15) to which we added different concentrations of Si. Our results revealed a positive of 24 h (p-value <0.001) between Si concentration in solution and the mol% Mg of the carbonate minerals forming in the experiment. With 2 mM of Si, the bulk precipitate was comprised of 90 % stoichiometric dolomite with possible signs or ordering. Moreover, the rhombohedral morphology of the crystals is analogue to that of natural dolomite previously described from sabkha sediments. Our results suggest that dissolved Si may play an important role for dolomite formation in evaporitic environments.
... Currently, DAR and Raley fractionation models have been developed within marine dolomite to quantitatively constrain dolomite origins [15,16]. These models, in conjunction with sequence stratigraphy, have effectively elucidated the genesis of thick dolomite layers [17]. ...
Article
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The investigation of magnesium (Mg) isotopes in dolomite has mainly focused on marine dolomite environments, leaving a significant gap in the understanding of their dynamics within lacustrine settings, especially in saline lake basins. In this study, a total of 16 sediment core samples from Well BX-7 in the Qianjiang Depression were sequentially selected for scanning electron microscope observation, whole-rock analysis for major and minor elements, and isotopic measurements including δ18Ocarb, δ13Ccarb, δ26Mgdol, and δ26MgSi. In addition, two intact cores were subjected to detailed analysis on the centimeter scale. Sedimentation models were established to elucidate dolomite formation under contrasting climatic conditions, specifically humid climates with a significant riverine Mg input versus relatively dry conditions with a lower Mg input. Furthermore, a quantitative model was developed to assess the magnesium flux and isotopic mass balance within lacustrine systems, simulating the magnesium isotope variations in lake water under different climatic scenarios. The dolomite sample data at a smaller scale (sampling interval ≈ 3~5 mm) demonstrate a consistent trend with the established model, providing additional confirmation of its reliability. Dolomite precipitated under humid climatic conditions exhibits a lower and relatively stable δ26Mgdol, lower δ18O, and higher CIA, indicating higher river inputs and relatively stable Mg isotope values of lake water controlled by river input. Nevertheless, dolomite formed under relatively dry climatic conditions shows a relatively high δ26Mgdol, higher δ18O, and lower CIA, suggesting reduced river inputs and weathering intensity, as well as relatively high magnesium isotope values of the lake water controlled by dolomite precipitation. This study contributes to the understanding of magnesium isotopes in lacustrine dolomite systems.
... The Mg isotope variability of dolostones within a sedimentary unit mainly stemmed from the dolomitization processes (23,24). Isotopic evolution of Mg in dolomitization fluids can occur along a diffusion/advection profile on a scale of meters to hundreds of meters (17,25,26) or across a carbonate platform on a scale of hundreds of kilometers (Supplementary Materials and figs. S4 and S5) or even within an enclosed basin on the continental scale (19,27). ...
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The surficial cycling of Mg is coupled with the global carbon cycle, a predominant control of Earth’s climate. However, how Earth’s surficial Mg cycle evolved with time has been elusive. Magnesium isotope signatures of seawater (δ ²⁶ Mg sw ) track the surficial Mg cycle, which could provide crucial information on the carbon cycle in Earth’s history. Here, we present a reconstruction of δ ²⁶ Mg sw evolution over the past 2 billion years using marine halite fluid inclusions and sedimentary dolostones. The data show that δ ²⁶ Mg sw decreased, with fluctuations, by about 1.4‰ from the Paleoproterozoic to the present time. Mass balance calculations based on this δ ²⁶ Mg sw record reveal a long-term decline in net dolostone burial (NDB) over the past 2 billion years, due to the decrease in dolomitization in the oceans and the increase in dolostone weathering on the continents. This underlines a previously underappreciated connection between the weathering-burial cycle of dolostone and the Earth’s climate on geologic timescales.
... Island dolostones are the result of long-term and multistage dolomitization [19], and these multiple stages of dolomites coexist on a crystal scale, making it difficult to obtain their in situ chemical compositions [20]. Most chemical studies of Cenozoic dolostones work on whole rocks, such as carbon/oxygen isotopes, elemental, and other metal isotope data. ...
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Cenozoic dolomitization of reefal carbonates has been widely found on many tropical islands worldwide. However, most ages and geochemical data obtained from bulk samples prevent a clear understanding of the previous complex diagenetic processes of these island dolostones due to a lack of in situ age and fluid composition. In this study, one deep borehole penetrated Cenozoic carbonates on Meiji Island in the southern South China Sea and massive dolostones with thicknesses over 400 m were uncovered. The in situ U–Pb geochronology and elemental analysis were conducted on the lower Nanwan Formation (upper Miocene) comprising undolomitized calcite (bioclast), replacive dolomite, and dolomite cement. Strontium isotope ages and U–Pb dates show that the penecontemporaneous replacive dolomitization occurred at 11.0–8.5 Ma, close to the deposition of precursor limestone. The dolomite cement precipitated at 8.5–6.0 Ma. In situ elemental analyses indicate that the formation of replacive dolomite and dolomite cement in the Nanwan Formation was probably controlled by seawater. The higher Mg/Ca ratio and lower Mn and Sr contents in dolomite cements show that their fluid underwent more evaporation. The dolomite content is positively related to the porosity of reefal limestones in the Nanwan Formation, suggesting that primary voids play an important role in fluid transportation during following dolomitization. Coralline algae and lime mud with algal fragments are beneficial for the rapid nucleation of dolomite. This study demonstrates that in situ elemental analysis using laser ablation has great potential for identifying the source of multistage dolomitizing fluids and can help refine the existing dolomitization model of isolated atolls.
... The lack of Mg isotope fractionation during carbonate dissolution (Jacobson et al., 2010;Perez-Fernandez et al., 2017) made the Mg flux from carbonate weathering has the similar Mg isotope composition with parent carbonate rocks. According to the literature (Gao et al., 2016a;Hu et al., 2019;Hu et al., 2017;Hu et al., 2021;Li et al., 2016b;Ning et al., 2019;Ning et al., 2020;Peng et al., 2016), the average δ 26 Mg value of high-Mg carbonates (including dolomite and high-Mg calcite) exposed in Eastern Asian Continent is ca. -2.0 ± 0.8‰ (2SD, n = 224). ...
Article
In the last decade, much attention has been devoted to Mg isotopic behavior during various weathering processes. Nonetheless, whether Mg isotopes in terrigenous siliciclastic sediments in modern oceans can reflect continental weathering regimes remains unresolved. Understanding the major controls of Mg isotope fraction-ation in siliciclastic sediments is thus a prerequisite for decoding past Mg isotope records for tracing continental weathering history. In this study, we present mineralogical, elemental, and Mg isotopic compositions of terrigenous siliciclastic sediments in the Changjiang (Yangtze River) estuary and the adjacent continental shelf of the East China Sea. δ 26 Mg values of the clay-sized sediments vary from − 0.15‰ to 0.00‰ and lack of the correlation with the proportion of different clay minerals. Our results suggest that the incipient to intermediate weathering processes are characterized by the progressive leaching of isotopically light Mg from silicates, leading to the enrichment of 26 Mg in secondary clay minerals. Therefore, Mg isotopic variability of clay-sized sediments is primarily determined by catchment weathering, rather than by provenance lithology or mixing of clay minerals. An isotopic mass balance calculation indicates that the δ 26 Mg value of silicate weathering flux on a continental scale is ca.-0.60‰ to − 0.40‰. The new constraint on the δ 26 Mg value of silicate weathering flux provides a new opportunity to better understand the Mg cycle. The Mg flux from silicate weathering in the Changjiang catchment is estimated as ~15-17 × 10 10 mol/yr, accounting for ca. 50% to 60% of the total Mg influx of Changjiang River into the ocean. In addition, numerical modeling with the up-to-date flux and isotope fractionation data have indicated that a slight enhancement of silicate weathering can potentially drive the rapid rise of Mg/Ca in seawater since late Cenozoic, but maintain the constancy of seawater δ 26 Mg as reported by literature. Our study demonstrates that Mg isotopes of clay-sized sediments can be used to constrain the relevant δ 26 Mg value of silicate weathering flux on a continental scale, which is a key parameter to trace the Mg cycle on Earth's surface.
... Due to the complexity of Mg isotopic fractionation in carbonate formation processes, a considerable variation of Mg isotope compositions (>5‰) has been observed in modern natural carbonates (Li et al., 2012;Kimmig and Holmden, 2017;Saenger and Wang, 2014;Teng, 2017). Significant Mg isotope fractionation in carbonates has made Mg isotopes a promising tool for recording geological processes, including dolomitization (Higgins et al., 2018;Huang et al., 2015;Kimmig et al., 2021;Mavromatis et al., 2014;Ning et al., 2020), hydrothermal activity (Hu et al., 2019;Lavoie et al., 2014;Walter et al., 2015), and marine Mg cycling (Hu et al., 2021;Kimmig and Holmden, 2017;Liu et al., 2014;Pokrovsky et al., 2011;Zhang et al., 2022). Thereby, the constraints on Mg isotope behavior during stromatolite growth can potentially decode the microbe-mineral interactions during microbial mat calcification. ...
... Additionally, many continental dolomitized platforms have never suffered significant burial . Furthermore, recent geochemical analyses based on high-frequency sampling of massively dolomitized platforms, points to dolomite replacement occurring on a cycle-by-cycle basis within the environment of deposition (Manche and Kaczmarek 2019;Ning et al. 2020). Finally, rapid denudation of deposits above the Stokes surface in highly arid settings, where sediment stabilization by land plants and meteoric cementation is largely absent, necessitates lithification by dolomite replacement in the marine realm before exposure to achieve rockrecord retention (Rivers et al. 2021a). ...
Article
The “dolomite problem” is the product of two distinct observations. First, there are massive amounts of ancient marine limestone (CaCO3) deposits that have been replaced by the mineral dolomite (MgCa(CO3)2). However, recent (Holocene and Pleistocene) marine deposits contain relatively minuscule amounts of dolomite, although the occurrence of small quantities of dolomite is observed in many modern settings, from deep marine to supratidal. Second, low-temperature synthesis of dolomite in laboratory settings has been elusive, particularly in comparison to the ease with which common marine calcium carbonate minerals (aragonite and calcite) can be synthesized. Since low-temperature solid-state diffusion can be discounted as a method for Mg incorporation into calcium carbonate (as it operates on time scales too long to matter), the replacement of CaCO3 by dolomite is one of dissolution followed by precipitation. Therefore, an often overlooked but required factor in the replacement of limestone by dolomite is that of undersaturation regarding the original calcium carbonate mineral during replacement. Such conditions could conceivably be caused by rapid dolomite growth relative to aragonite and calcite dissolution–precipitation reactions, but laboratory studies, modern systems analyses, and observations of ancient deposits all point to this possibility being uncommon because dolomite growth is kinetically inhibited at low temperature. Pressure solution by force of dolomite crystallization is a second possible driver for CaCO3 undersaturation, but requires a confining stress most likely attained through burial. However, based on petrographic observations, significant amounts of ancient dolomite replaced limestone before burial (synsedimentary dolomite), and many such platforms have not suffered any significant burial. Because these possibilities of undersaturation caused by dolomite precipitation and crystal growth can be largely discounted, the undersaturation required for “dolomitization” to proceed is most likely to be externally forced. In modern natural systems, undersaturation and selective CaCO3 dissolution in marine porewaters is very common, even in warm-water environments, being forced by the breakdown of organic matter. Such dissolution is frequently attended, to varying degrees, by precipitation of a kinetically-less-favored but thermodynamically more stable phase of CaCO3. Laboratory studies as well as observations of modern systems show that when undersaturation is reached with respect to all common marine CaCO3 phases, dolomite assumes the role of this kinetically-less-favored precipitate. This degree of undersaturation is uncommon in modern shallow marine pore systems in warm-water settings, but it was more common during times of elevated atmospheric CO2, and ocean acidification. Furthermore, because oxidation of organic matter drives dolomite formation, near-surface organic-rich deposits such as the remains of microbial mat communities, were more predisposed to dolomite replacement in the acidified oceans of the ancient past relative to contemporaneous deposits that contained less organic matter. These observations lend to a more harmonious explanation for the abundance and occurrence of dolomite through time.
... It should be noticed that the presented interpretations mentioned previously are preliminarily made on the current database available. Additional investigations are needed to better understand the origin of hydrothermal D3 dolomites in the examined carbonate interval, such as U-Pb dating (Salih et al., 2020;Pan et al., 2021) and Mg-isotope constraining (Ning et al., 2020;Mansurbeg et al., 2021). ...
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The Middle-Upper Cambrian Xixiangchi carbonates in the Sichuan Basin have been pervasively dolomitized. In the presented work, petrographic investigation revealed three generations of the Xixiangchi dolomites, consisting of dolomicrite (D1, 5–20 μm) with a planar-s to non-planar texture, fabric destructive dolomite (D2, 50–150 μM) with a planar-s to planar-e texture, and saddle dolomite (D3, 300 μm to 4 mm) with a planar-s to planar-e texture. D1 and D2 dolomites are presented as matrix dolomites, whereas D3 dolomites are observed as fracture-filling dolomites. Compared with the matrix D1 and D2 dolomites, which are interpreted as products of dolomitization under near-surface or at shallow burial conditions, the depleted δ¹³C and δ¹⁸O values of D3 than D1 and D2 dolomites are probably caused by the temperature-controlled isotopic fractionation within an increasing fluid–rock interaction at burial. The enriched Mn, Sr, and Ba concentrations of D3 than D1 and D2 dolomites suggest a newly introduced type of diagenetic fluids, which is probably related to the upwelling of magmatic activities (Emeishan large igneous province). By contrast, the abnormally depleted Fe concentration in D3 dolomites is attributed to its preferential incorporation into other solid phases rather than its true concentration. The similar rare earth element (REE) partition patterns of D1 and D2 dolomites demonstrate similar dolomitization fluids related to seawater or marine-origin fluids. The hydrothermal-derived D3 dolomites exhibit a different REE partition pattern by contrast. The negative Eu anomalies of D3 dolomites may represent hydrothermal fluid cooling or an association with intermediate-felsic igneous rocks. The findings of the presented work would enhance our understanding on the hydrothermal dolomitization of the Middle-Upper Cambrian Xixiangchi Formation.
... The timescale of carbonate recrystallization is typically on the order of millions of years for typical deep-sea sediments (Fantle et al., 2010), and approaching 1 Myr for more rapidly accumulating platform sediments Staudigel et al., 2021). Evidence from the geologic record suggests much shorter timeframes (sub-100 Kyr) timeframes for early diagenetic dolomitization and lithification (Manche and Kaczmarek, 2019;Ning et al., 2020), and stratal geometries of dolomite in the Capitan Reef Complex indicate that dolomitization occurred on the timescale of high frequency sequences (sub-Myr) (Mutti and Simo, 1993;Mutti and Simo, 1994;Garcia-Fresca et al., 2012;Wu et al., 2020). An ideal ratio exists between the rates of MSR and recrystallization that will result in maximally elevated δ 34 S CAS values, with a maximal Δ 34 S CASseawater of between 1 and 3‰ for three modern sites in slope settings (Rennie and Turchyn, 2014). ...
Article
The Capitan Reef Complex in West Texas is famous for its high prevalence of early marine cements, unusual for a Phanerozoic platform, leading some to suggest that Precambrian styles of carbonate sedimentation enjoyed a Permian encore. Here, we use patterns of stable Ca, Mg, C and S isotopes to better understand the environmental driver(s) of the enigmatic cementation. We find that calcite that is the most enriched in ⁴⁴Ca has δ³⁴S values that approach the inferred composition of Permian seawater sulfate. Microbial sulfate reduction in pore fluids must have been spatially and temporally coincident with recrystallization of primary carbonate phases, such that substantial ³⁴S-enriched sulfate was incorporated into diagenetic calcite under relatively closed-system conditions. Moreover, the magnitude of ³⁴S-enrichment of carbonates relative to seawater was strongly influenced by local diagenetic conditions, with fluid-buffered early marine cements, shelf, reef, and upper slope preserving more seawater-like S isotope ratios than the more sediment-buffered lower slope. Some samples are far more ³⁴S-enriched relative to seawater than those from modern sites in similar depositional environments, possibly responding to specific combinations of sedimentary parameters (e.g., grain size, porosity, organic matter rain rate). Additionally, the sulfate concentration in the Delaware Basin might have been slightly lower than modern levels, leading to more extensive isotopic evolution of sulfate in pore waters during carbonate recrystallization. Based on the data and a numerical model of carbonate recrystallization, we suggest that one driver of the extensive seafloor cement precipitation in the Capitan Reef Complex was a Permian water column [Ca²⁺]:[SO4²⁻] ratio somewhere between 1 and modern seawater.
... In this case, a short period of time has become a crucial factor limiting the degree of kinetic reaction (i.e., dissolution/precipitation processes of dolomite). A statistical truism that the dolomite abundance (relative to limestone) progressively increases with stratigraphic age has also highlighted the importance of time accumulation for dolomite precipitation occurring slowly at geological timescales (Given and Wilkinson, 1987;Warren, 2000;Ning et al., 2020). For equilibrium minerals (e.g., calcite), however, the reaction will reach thermodynamic equilibrium in a relatively short period depending on the mineral saturation state. ...
Article
Eogenetic karst has been regarded as a dominant origin of hydrocarbon reservoirs, whereas quantifying the fluid-rock interactions and their impacts on porosity evolution during the palaeokarst process remain persistent challenges that limit the accuracy of reservoir quality prediction. This study investigates the dissolution of carbonate rocks with potential controlling factors by reactive transport modeling that couples fluid flow, mineral reactions and porosity changes in a one-dimensional vadose meteoric water-rock system. Simulation results of base case scenario and sensitivity analyses show that the duration of subaerial exposure and recharge capacity of rainwater significantly determine the karst-affected depth and porosity increment. The amount of calcite dissolution is also affected by a downward decrease in calcite solubility (temperature-dependent) and the enrichment of Ca²⁺ and HCO3– in the lower part. The atmospheric carbon dioxide concentration has a minor impact on the vertical extent of karst, while it facilitates the dissolution rate under high pCO2 conditions. The influence of atmospheric pCO2 variation over geological time on the porosity increment was reconstructed under hydrogeological conditions of the base case scenario (exposure time = 130 ka; rainfall = 628 mm/a). The differences in the dissolution rates and extent of karst between limestone and dolostone can be interpreted as the results of different rate-determining reaction mechanisms, i.e., thermodynamically controlled calcite and kinetically controlled dolomite.
... In order to have a faster change in δ 26 Mg seawater , the mass of the seawater in a model must be reduced to be <20% of global oceans (Fig. 7). A Mg isotope study on dolostones from hinterland-attached carbonate platforms shows that δ 26 Mg of dolomite can increase rapidly in response to basin restriction and this has been confirmed by an independent study of Ning et al. (2020). The Permian-Triassic transition was associated with a global regression event as evidenced from sedimentary disconformities in numerous sedimentary records from the oceans, particularly around the Paleotethys (Yin et al., 2014) (Appendix 4 Fig. ...
Article
The Permian-Triassic transition witnessed the largest mass extinction event in Earth's history, multiple mechanisms have been proposed to explain the cause of this catastrophe, however, relatively less attention has been paid to the paleogeography and major element chemistry of seawater and its possible link to mass extinction during this interval. Syndepositional massive marine dolostones could record the Mg isotope signature of contemporaneous seawater that hold clues of ancient Mg cycling. In this study, we investigated Mg isotopes of dolomite from three widely spaced carbonate sections in the Paleotethys to trace oceanic Mg cycling during this critical period. The latest Permian dolostones from all studied sections have similar δ 26 Mg values around −2.1 ± 0.1. However, a remarkable and consistent increase in δ 26 Mg dolomite across the extinction interval occurred in both eastern and western margins of the Paleotethys. The results suggest that δ 26 Mg of seawater in Paleotethys fluctuated by 0.4 within ∼750 kyr across the Permian-Triassic transition. Modeling reveals that the high rate of change in δ 26 Mg seawater required an extremely short residence time for seawater Mg. This is attributed to dramatically intensified dolomitization in a restricted oceanic environment, with Mg isotope evidence revealing that the Paleotethys Ocean experienced transient restriction events, in association with radical changes in the major cation composition of seawater, around the end-Permian mass extinction event (EPME).
Article
Zoned dolomite crystals, characterized by their dirty core and clear outer rim, are common in most island dolostones. The conditions under which these dolostones formed, however, remains controversial. To explain the origin of island dolostones, here, in situ determinations of Mg isotopic compositions, major (Ca, Mg) and trace (Fe, Mn, Sr, Na) elemental concentrations are carried out for the cores and rims of zoned dolomite crystals for dolostone samples from the Sanya Formation (Lower Miocene) and Meishan Formation (Middle Miocene) of well XK‐1 drilled on Shidao Island, the Xisha Islands. For all of the dolomite crystals, both cores and rims are formed of high‐Ca calcian dolomite, but the cores have higher %Ca, Sr and Na concentrations than the rims. Moreover, the cores (−3.85 to −2.95‰) have ca 0.2 to 0.5‰ lower δ ²⁶ Mg values than the rims (−3.34 to −2.60‰). The difference in δ ²⁶ Mg values between the dolomite crystal core and the rim cannot be explained by the presence of calcite inclusions or dolomite recrystallization, but rather reflect the nature of Mg isotopic fractionation due to the growth of the dolomite crystals during different stages of replacement. For zoned dolomite crystals, the progressive decrease in Ca and trace element concentrations but increase in δ ²⁶ Mg values from dirty core to clear rim demonstrate that: (i) the dolomite crystal cores grow via a diffusion‐limited process; and (ii) the rims form through an incremental process (interface‐controlled) whereby the zone of dissolution/dolomite precipitation was very thin and simply repeated many times until it had fully developed. This growth model of zoned dolomite crystals may be applied to dolostones that share similar zoned patterns in petrography and geochemistry throughout the world.
Article
The DOUNCE (DOUshantuo Negative Carbon isotope Excursion) was marked by a significant shift in δ13Ccarb from ∼ + 5 ‰ down to ∼ − 12 ‰ in the upper part of the Ediacaran Doushantuo Formation of South China. As an equivalent event of the Shuram/Wonoka anomaly, the DOUNCE event is the largest negative δ13Ccarb excursion in geological history and denotes a global ocean oxygenation event. Consequently, it has been widely used as a chemostratigraphic tool for correlating the Ediacaran strata globally. Nonetheless, the DOUNCE exhibits variable stratigraphic expressions across sections and depositional environments, raising questions about its representation as a primary indicator of the Ediacaran seawater δ13C value. Such variability casts doubt on the reliability of the DOUNCE for global correlation, and its implications for the carbon cycle, oceanic oxygenation, and biological evolution during the Ediacaran period. To elucidate the DOUNCE event as a synchronous global occurrence and a chemostratigraphic tool, we have compiled the “DOUNCEraq”, a global-scale database comprising 9375 valid δ13Ccarb analyses from 156 sections/boreholes documenting the DOUNCE/Shuram/Wonoka event. Our meta-analysis of DOUNCEraq highlights the global scope of the DOUNCE event and reveals the presence of an instant rise stage post the abrupt δ13Ccarb decline as an inherent feature of the DOUNCE pattern. Moreover, it also emphasizes the impacts of palaeolatitude, palaeocontinent, water depth, and lithology on the DOUNCE's pattern and variability: (1) lower pre-DOUNCE δ13Ccarb values and smaller shift magnitudes within 30–0°N compared to the southern hemisphere; (2) compared to the shallower sections, deep-water sections exhibit lower pre-DOUNCE and DOUNCE nadir δ13Ccarb values with smaller shift magnitudes relative to shallower sections; (3) dolostones demonstrate lower pre-DOUNCE values, higher values at the DOUNCE nadirs, and smaller shift magnitudes compared to limestones. Such local impacts on the DOUNCE pattern provide important constraints on the causes of the DOUNCE event and could be explained within the DOC-oxidation hypothesis via regulating oxidants supply. Overall, the present meta-analysis enhances our understanding of the DOUNCE's global stratigraphic expressions and provides important constraints on the DOUNCE causes.
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Magnesium isotope ratios in carbonate rocks and minerals play an important role in tracing geological and biological processes. We report δ ²⁶ Mg DSM3 values for the first time in ten carbonate reference materials (GBW07865, GBW07114, GBW(E)070159, GBW07136, GBW07108, GBW03109a, GBW07120, GBW07214a, IAEA‐B‐7 and IAEA‐CO‐8) with calcium to magnesium mass ratios around 1300 g g ⁻¹ and ultra‐low Mg mass fractions of 295 μg g ⁻¹ . A combination of AG MP‐50 (100–200 mesh) and AG 50W‐X12 (200–400 mesh) resins for the matrix extraction and multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS) were used for the isotopic measurements. This measurement procedure is applicable to materials with high and low Mg mass fractions (e.g., carbonates, lunar highlands anorthosites, ice core). It was validated using various types of reference material. In‐house synthetic solutions with 1000 and 1300 g g ⁻¹ [Ca]/[Mg] mass ratios yielded δ ²⁶ Mg DSM3 values at 2 s = ±0.05‰ ( n = 62), ‐4.89‰ and ‐4.89‰, respectively, indistinguishable from those of the pure Mg solutions, at ‐4.91‰. δ ²⁶ Mg DSM3 values in well characterised carbonatite and carbonate reference materials (such as JDo‐1, COQ‐1, GBW07133, GBW07217a and GBW07129), with varying MgO and CaO mass fractions, were in agreement with literature values.
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Investigating the carbonate preservation efficiency (CPE) of continental crust is crucial to understand the global carbon cycle, which requires constraints on initial carbonate abundances (ICAs) of crustal rocks. To link Mg isotopes to ICAs, we present elemental and Mg isotopic data for Himalayan carbonate‐bearing and carbonate‐free metasedimentary rocks. Given no evident melt extraction or external‐fluid infiltration, ICAs of these samples can be independently estimated by elemental data. Despite different carbonate species in the protoliths, all the samples show congruent relationship between their δ²⁶Mg and ICAs, owing to the elevated carbonate δ²⁶Mg and Mg/Ca in protoliths of calcite‐rich samples resulting from diagenetic processes. When collated with literature data, we suggest the observed correlation here can be applied to most carbonate‐bearing (meta‐)sedimentary rocks. Based on a steady state box‐model, we constrained the modern net carbonate accretion flux (9.50−5.56+9.50 9.505.56+9.50{9.50}_{-5.56}^{+9.50} Tmol/year) and the average time‐integrated CPE (∼80−43+20 8043+20{80}_{-43}^{+20}%) for continental crust.
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This paper investigates the origin of ultra-deep dolostone and the factors influencing large-scale dolostone reservoirs, focusing on the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin. The study involves petrology, microscale X-ray diffraction, trace element analysis, and C-O-Sr-Mg isotope experiments to provide a detailed analysis. The research findings indicate that the Dengying and Longwangmiao formations comprise six types of matrix dolostone and four types of cement. The Dengying Formation, which developed under a sedimentary background of a restricted platform, contains special microbial and microcrystalline dolostones. The dolomite grains are small (<30 µm) and have a low order degree (Min=0.55), with large unit cell parameters and an extremely high Na content (Max=788 ppm). The 87Sr/86Sr value of the dolostone is consistent with contemporaneous seawater, while the δ13C and δ18O values are lower than those of the contemporaneous seawater. The δ26Mg value is small (Min=−2.31‰). Powder crystal, fine-crystalline, and calcite dolostones with coarser and more ordered crystals exhibit similar δ13C and 87Sr/86Sr values to microbial and microcrystalline dolostone. During the sedimentary period of the Dengying Formation, ancient marine conditions were favorable for microbial survival. Microorganisms induced the direct precipitation of primary dolomite in seawater, forming microbial and microcrystalline dolostones during the seawater diagenesis period. During the subsequent diagenesis period, dolostones underwent the effects of dissolution-recrystallization, structures, and hydrothermal fluids. This resulted in the formation of dolostone with coarser crystals, a higher degree of order, and various types of cement. The Longwangmiao Formation was developed in an inter-platform beach characterized by special particle dolostone. The particle dolostone has a large grain size (>30 µm), high order degree (Min=0.7), small unit cell parameters, high Na content (Max=432 ppm), and low Fe and Mn content. The δ26Mg and δ13C values are consistent with the contemporaneous seawater, while the δ18O and 87Sr/86Sr values are higher than those of the contemporaneous seawater. There is mutual coupling between multiple-period varying δ26Mg values and sedimentary cycles. The dolostone in the Longwangmiao Formation resulted from the metasomatism of limestone by evaporated seawater. The thickness and scale of the dolostone in the Longwangmiao Formation are controlled by the periodic changes in sea level. The period of dolostone development from the Sinian to the Cambrian coincides with the transition from Rodinia’s breakup to Gondwana’s convergence. These events have resulted in vastly different marine properties, microbial activities, and sedimentary climate backgrounds between the Sinian and the Cambrian. These differences may be the fundamental factors leading to the distinct origins of dolostone formed in the two periods. The distribution of sedimentary facies and deep tectonic activities in the Sichuan Basin from the Sinian to the Cambrian is influenced by the breakup and convergence of the supercontinent. This process plays a key role in determining the distribution, pore formation, preservation, and adjustment mechanisms of ultra-deep dolostone reservoirs. To effectively analyze the genesis and reservoir mechanisms of ultra-deep dolostone in other regions or layers, especially during the specific period of supercontinent breakup and convergence, it is crucial to consider the comprehensive characteristics of seawater properties, microbial activities, sedimentary environment, and fault systems driven by tectonic activities. This can help predict the distribution of high-quality and large-scale ultra-deep dolostone reservoirs.
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In the Western High of the Sea of Marmara, where pervasive gas hydrate and hydrocarbon gas seepage occurs, late Pleistocene-Holocene sediments are composed of a lower lacustrine and an upper marine unit. The sedimentary evolution, gypsum formation and carbonate anomaly, under the complex sedimentary environment at cold seep sites on the Western High, have not been systematically well explained. The transition from the lacustrine to marine unit occurs at around 7.5 meters below the sea floor (mbsf), with a sulfidization front present in the lacustrine sediments just below it, and the marine unit includes a sapropel layer between 5.20 and 7.20 mbsf. The sedimentary sequence is characterized by authigenic minerals including pyrite, carbonates, and gypsum in the upper 0.50 mbsf, corresponding to the present-day sulfate-methane transition zone (SMTZ) near the seafloor. Mineralogical, chemical and isotopic compositions (13C, 18O, 34S, 26Mg) of the sediments in the core studied show their close relationship with intensive sulfate-driven anaerobic oxidation of methane (AOM) and other hydrocarbons. The dissolution of carbonate, related to the gas and oil migration, and organic matter enrichment might be the main cause of carbonate deficiency in the sapropel unit. Heavy carbon isotopic composition (δ13CCarb = 6.98‰) of carbonate at 4.30 mbsf implies that the carbon might be the residual by-product of subsurface biodegradation of seeping petroleum. The formation of secondary gypsum and partial dissolution of carbonate in the Sea of Marmara is commonly considered of being associated with the aerobic oxidation of pyrite. However, the euhedral authigenic gypsum studied might be with different origins, possibly including anaerobic oxidation of Fe-sulfides, carbonate deficiency in the sapropel unit and AOM. Gypsums below SMTZ are with the size of hundreds of microns. Linear pits and grooves are developed on these gypsum surfaces, probably due to corrosion by the methane-rich fluid of latest hydrocarbon seepage. Platy hexagonal gypsum in SMTZ, without any corrosion on crystal surface, might be associated with the changing flux of the acidic composition (e.g. CO2) in the seepage. Depleted-26Mg by about 1‰ in the SMTZ might be caused by the upward seepage of hydrocarbons. Magnesium isotope of authigenic carbonate could be a valid proxy for the recognition of SMTZ in a hydrate geo-system.
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Early lithification of carbonate mud during the subaerial exposure stage, under semiarid conditions, has been proposed to facilitate dolomite formation. However, how the biogeochemical processes during subaerial diagenesis promote dolomite formation remains unclear. Here, we employ a multiproxy approach to investigate the process of dolomite formation by analyzing modern dolomite crusts forming in lagoon Brejo do Espinho (LBE). Petrological analysis reveals that the crusts comprise coexisting high-Mg calcite (HMC) and dolomite. Low Fe and Mn concentrations indicate the formation of dolomite under oxic conditions, while a higher Sr concentration in well-lithified crust suggests primary bacterial-induced dolomite precipitation. The Mg isotopic composition of the crusts exhibits a lighter value compared to that of modern sabkha dolomite, suggesting different dolomitization processes and Mg sources. More negative δ ¹³C values of the crusts than unlithified carbonate mud in LBE, indicating the incorporation of ¹³ C depleted organic carbon. The biogeochemical processes related to decaying organic matter during subaerial diagenesis generate partially oxidized organic matter that promotes Mg ²⁺ dehydration and enhances the dissolution of primary HMC, ultimately triggering the transition of HMC to dolomite or/and direct dolomite precipitation. The ancient "dolomite factory" operated through cyclic deposition of carbonate sediments and penecontemporaneous subaerial diagenesis. Thematic collection: This article is part of the Towards unravelling the ‘Dolomite Problem’: new approaches and novel perspectives collection available at: https://www.lyellcollection.org/topic/collections/towards-unravelling-the-dolomite-problem
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灯影组白云岩是四川盆地超深层油气勘探的重点领域,但目前人们对该套白云岩成因争议仍较大,且缺乏系统研究.通过对四川盆地灯影组白云岩C-O-Sr 同位素和稀土元素数据的系统分析来研究白云石化流体的化学性质和成因,进而约束白云岩的差异性成因机制. 研究表明:(1)灯影组白云岩碳同位素值较均一,δ13C 值基本分布在0‰~+5.0‰ 之间,而氧同位素值变化较大. 近地表环境基质白云岩和早期白云石胶结物δ18O 均大于-8.0‰,埋藏环境白云石胶结物δ18O 均小于-8.0‰,而热液白云石化胶结物δ18O 均小于-10.0‰.(2)基质白云岩和早期白云石胶结物具有与同期海水相似的87Sr/86Sr 值(0.708~0.709),指示其继承于海水流体;而埋藏环境白云石胶结物87Sr/86Sr 比值明显大于同期海水,指示其为地层流体和深部热液流体来源.(3)灯影组白云岩稀土元素均亏损轻稀土元素、富集重稀土元素. 基质白云岩和早期白云石胶结物可见Ce 负异常、未见Eu 明显异常,说明白云石化流体来源于海水;埋藏环境白云石胶结物可见明显Eu 正异常. 不同沉积环境下白云石化流体化学性质和来源的不同,是四川盆地灯影组白云岩成因的控制因素. 近地表环境下海水来源的白云石化流体,主要受新元古代震旦系灯影组“文石‒白云石海”环境下高频海平面波动的控制;而在埋藏环境中地层流体和深部热液来源的白云石化流体,则主要受后期构造作用控制. 本研究可为认识白云岩成因、晚埃迪卡拉系海水化学条件及超深层油气勘探开发提供了有益参考。
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The Middle−Upper Xixiangchi Formation of the Sichuan Basin consists of mixed carbonates and siliciclastics. The carbonates are dominated by dolomitized lime-mudstones and dolo–grainstones of shallow warm-water environments. Petrographic examination of carbonates reveals two types of dolomite: (i) fabric retentive dolomicrite (D1, micritic to near-micritic, 4–50 μm) and (ii) fabric-destructive dolomite (eu-to subhedral crystals with cloudy cores and clear rims, 50–300 μm). The mean ∑REE values of D1 (15.3 ± 6.3 ppm) and D2 (14.6 ± 7.1 ppm) and the nearly identical mean PAAS-normalized patterns that coincide at some points, suggest an origin from similar parent fluids that were not dramatically changed by water-rock interaction through basinal sedimentary rocks. The estimated Sr/Ca molar ratios of D1 (0.0043–0.0055) parent fluids and their estimated δ¹⁸O values imply an origin from a mixture of meteoric and seawaters, which is also consistent with the petrographic evidence. The partially coinciding normalized REE pattern of D2 with that of the earlier D1 suggests that the latter was formed at a mid-burial setting rather than deep burial, which is consistent with higher homogenization temperatures of primary two-phased fluid inclusions retained in D2 (Th = 118.1 ± 5.3 °C). Therefore, D1 is suggested to have formed (1) during a penecontemporaneous period that resulted from a slight evaporative pumping effect within a near-surface diagenetic environment, and/or (2) during post-penecontemporaneous or penecontemporaneous seepage-reflux dolomitization, induced by sea-level fluctuations, during early stages of diagenesis at near-surface conditions. D2 formed likely through the recrystallization of D1 or a continuation of reflux dolomitization of the same precursor at mid-burial rather than deep burial settings.
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Dolomite genesis is a century-old mystery in sedimentology. To reveal the mechanism of dolomite genesis, two core problems need to be addressed. The first is the origin and migration mechanism of Mg2+-rich fluids during the dolomitization process. The second is the kinetic barrier caused by Mg2+ hydration during dolomite precipitation at low temperatures. To address these problems, our study, based on detailed petrological, sedimentological, geochemical (major and trace elements), and isotopic (C-O-Mg) analysis, clarified the source and migration of Mg2+-rich fluids and the kinetic barrier mechanism of low-temperature dolomite precipitation in the Upper Sinian Qigebulake Formation and the Lower Cambrian Xiaoerbulake Formation in the Tarim Basin. First, we found that the Mg2+-rich fluids required for the dolomitization of dolomite in the Xiaoerbulake Formation were primarily derived from the Early Cambrian marine fluid. At the interface of the sedimentary cycle, δ26Mg values fluctuated considerably, indicating that the sequence interface was the starting point and channel for the migration of dolomitized fluids. Sea level variation plays a major role in controlling the dolomitization process of the Xiaoerbulake Formation. Second, the Qigebulake Formation contains low-temperature dolomite with Mg2+-rich fluids supplied by seawater, microorganisms, and sedimentary organic matter. Comprehensive analysis shows that the dolomite of the Qigebulake Formation was formed by microbial induction by anaerobic methane bacteria. Finally, the properties and sources of dolomitization fluids and the formation process of dolomite were the reasons for the difference in the Mg isotope composition of dolomite during the Sinian-Cambrian transition. This study reveals the genetic mechanism of the Sinian-Cambrian dolomite in the Tarim Basin and establishes a new method to explain the genesis of microbial dolomite by C-O-Mg isotopes, providing a reference for the reconstruction of the formation and evolution of dolomites.
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The Upper Cambrian Xixiangchi Formation in the Southern Sichuan Basin, China, has favorable hydrocarbon accumulation conditions. The accumulation factors and enrichment conditions of this formation were key considerations in this study. By analyzing core, thin section, seismic, and geochemical data, the research shows that there are many sets of granular and crystalline dolomite reservoirs in the Xixiangchi Formation vertically, with thin thickness of single-reservoir. During the transformation of karst and tectonism, dissolution pores and fractures developed to form an ideal reservoir space. The reservoir of the Xixiangchi Formation is connected to the Lower Cambrian source rock through a fault system. The high-energy shoal facies of the Xixiangchi Formation are located on the oil and gas migration path, providing an appropriate reservoir space for forming the source reservoir configuration relationship between the lower generation and upper reservoir. The key factors affecting hydrocarbon accumulation in the Xixiangchi Formation are sufficient oil and gas supply, development of inherited paleo-uplift, effective transportation system, and favorable reservoir-forming combination. The inherited paleo-uplift controls the distribution of gas reservoirs. Owing to the short migration distance of oil and gas, hydrocarbon is found near source hydrocarbon accumulation, and the paleo-uplift slope area should be targeted for exploration in future studies.
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Weathering has always been a concerned around the world, as the first and most important step in the global cycle of elements, which leads to the fractionation of isotopes on the scale of geological age. The Middle Ordovician Majiagou Formation in Daniudi area of the Ordos Basin had experienced weathering for >130 Myr. Through thin section observation, major and trace element analysis, carbon, oxygen, and magnesium isotopes composition analysis, the dolomitization modes and weathering of ancient dolomite in Daniudi area were analyzed in detail. The results showed that the Sabkha and brine-reflux dolomitization modes had developed, and the Mg isotopes in different layers of the karst crust were fractionated by various factors. The vertical vadose zone was affected by weathering, the Mg isotope of dolomite (δ²⁶Mgdol) showed a downward decreasing trend; the horizontal underflow zone was controlled by diagenesis and formation fluid, δ²⁶Mgdol showed a vertical invariance and negative; the main reason for Mg isotope fractionation in the deep slow-flow zone was the brine-reflux dolomitization mode during early burial period, which showed a vertical downward increase. Finally, the Mg isotope characteristic data of the ancient weathering crust were provided and the process of Mg isotope fractionation in the karst crust was explained.
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Recovering the Mg isotopic composition of seawater using penecontemporaneous dolomite is possible; however, the Mg isotopic composition of dolomite may be changed by later diagenesis and hydrothermal activities. Solving this problem requires further research on the behavior of Mg isotopes in dolomitization systems. Diagenesis and hydrothermal fluids have significantly altered the considerable amount of dolomites in the Lower Cambrian Qiulitage Formation in the Gucheng region of the Tarim Basin. Thus, this formation is excellent for examining the behavior of dolomite Mg isotopes after diagnostic and hydrothermal alterations. The results of major and trace elements and C–O–Mg isotope analysis show that the dolomite of Qiulitage Formation is penecontemporaneous dolomite, and the Mg in dolomite is mainly from the Early Cambrian seawater. The dolomites of the Qiulitage Formation in the Gucheng area are formed by carbonate platform facies. The stratigraphic variability of Mg isotopes is small, averaging −2.08‰± 0.23‰, which indicates that the Mg isotope of the dolomitized fluid in the large-scale dolomitization process of the carbonate platform is uniform and balanced with the seawater in the same period. In addition, the Mg isotopic composition of dolomite is stable, and the δ²⁶Mg value is not affected by burial diagenesis and hydrothermal fluid activities. we found that the Mg isotopic composition and stratigraphic age of the dolomites of the Lower Cambrian Qiulitage Formation are consistent with the simulated results, the δ²⁶Mg value of the ancient seawater in the Early Cambrian is estimated to be about −0.08‰±0.23‰. This study shows that the δ²⁶Mg of penecontemporaneous massive dolomite may have been used to invert the Mg isotopic composition of seawater during the same period, within a confidence range of ±0.2‰, using massive dolomite, and provided a new idea for quantitatively reconstructing the marine Mg cycle in the geological history.
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The pervasive dolostone from the Middle Ordovician Zhuozishan Formation in the Ordos Basin (Northern China) are considered as potential hydrocarbon reservoirs. Three types of matrix dolomites (Md) are recognized: 1) very fine to fine crystalline, nonplanar-a to planar-e dolomite (Md1); 2) fine to medium crystalline, nonplanar-a to planar-e dolomite (Md2); and 3) medium crystalline, planar-s to nonplanar-a dolomite (Md3). The initial dolomitization of Md1 and Md2 dolomites is inferred to have taken place in the presence of seepage reflux of slightly evaporated seawater in the near-surface to shallow burial settings, and insignificant recrystallization occurred in the process of being buried again, according to petrography, trace elements, and oxygen-carbon-strontium isotopic geochemistry. Coarser crystals with cloudy cores and clear rims consist of the Md3 dolomites, and the texture shows irregular contacts between planar-s and nonplanar-a crystal. Geochemical analysis reveals that the Md3 dolomites have a negative shift in δ¹⁸O values with increasing Mn contents, and a negative δ¹⁸O trend with depth in the same well, while the δ¹³C does not tend to correlate with the oxygen isotopes. Additionally, the ⁸⁷Sr/⁸⁶Sr ratios of Md3 dolomites are relatively higher due to contribution of radiogenic ⁸⁷Sr. Petrological attribute and geochemical evidence suggest that the massive Md3 dolomites can be due to significant recrystallization of earlier formed dolomites during burial period. The unevenly distributed pores in Md3 dolomites suggest the heterogeneous structure of the precursor dolomites, which may be the Md2 dolomites. The residual particles of Md2 dolomites indicate the precursor limestones were deposited in a relatively high-energy part of the shoal. Hence, the wide distributed Md3 dolomites show salient facies-controlled characteristics, which suggests strategies for hydrocarbon exploration of potential heterogeneous dolomite reservoir.
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The Guanwushan Formation (GWSF) of Devonian dolomite are extensively developed in the northwest of Sichuan Basin in the Upper Yangtze region, but the properties of dolomitization fluid and the geneses are still unclear. Three types of dolomites can be divided by petrological characteristics: the fine-microcrystalline dolomites (FMD), the fine crystalline dolomites (FCD) and the medium crystalline dolomites (MCD). The order degree of these three types of dolomites increased in turn, and they all showed dark cathodoluminescence (CL) luminescence. The total amount of Rare Earth Elements (∑REE) of the dolomite was low, while the dolomite enriched with light REE and lacking heavy REE presented a distribution pattern consistent with that of limestone. The weak negative anomalies of the Ce and Eu indicated that the dolomites were formed in a weak redox environment with relatively low temperature. The dolomitization fluids were inherited from the original seawater. The respective δ13CPDB values of the three types of dolomites varied a little, indicating that they were not affected by the biological effects. Specifically, the δ18OPDB values of the FMD and FCD dolomites were higher than that of the limestone, indicating that the dolomitization fluid was influenced by evaporation at the penecontemporaneous stage. The interpretations were also supported by the 87Sr/86Sr ratios, as the 87Sr/86Sr ratios of FMD comparable to the Middle Devonian seawater. The δ18OPDB value of the MCD dolomite was lower than that of the limestone. It also showed poor automorphic extent, which clarified that the dolomite experienced more intense dolomitization in greater burial depth and at higher temperatures.
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The dolomite explosion in the terminal Ediacaran period has great geological significance for understanding the paleoenvironmental evolution of the Precambrian-Cambrian transition. However, dolomite origins remain controversial. Abundant dolomites in the Ediacaran Dengying Formation in the northern Yangtze Block provide a window for solving the problem. In this paper, the genesis of dolomites and the paleoenvironment of the terminal Ediacaran were studied by facies, trace elements (including rare earth elements, REEs), and carbon, oxygen, and strontium isotopes of the Dengying Formation in the Xichuan area of South Qinling, Northern Yangtze Block. The results indicate that the Dengying Formation was deposited on a carbonate platform, and four types of dolomite (i.e., micro-crystalline dolomite, fine-medium crystalline dolomite, brecciated dolomite, and saddle dolomite) are identified. According to the differences in the ∑REEs, δ13C, Z value and 87Sr/86Sr values in different types of dolomite, it is concluded that micro-crystalline dolomite mainly formed in the early diagenetic stage, fine-medium crystalline dolomite in the middle diagenetic stage, and saddle dolomite in the late diagenetic stage, while brecciated dolomite formed in the epigenetic stage. Moreover, it is determined that the dolomitization models for the Dengying Formation include the seepage-reflux, mixed water, burial, and hydrothermal dolomitization models. In addition, micro-crystalline dolomite has a Sr/Ba value far greater than 1, the Z value is usually greater than 125, δEu is positive, V/(V + Ni) is less than 0.74, and δ13C is positive, indicating that the Dengying Formation dolomite was deposited in a shallow water environment with high salinity, weak oxidation, and dry and warm climate. By comparing the characteristics of dolomite in other parts of the world in the same period, it indicates that the global climate warmed up, the ocean appeared oxygenation process, and extensive retrogression events occurred in the Precambrian-Cambrian boundary.
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Dolomite (CaMg(CO3)2) plays a key role in the global carbon cycle. Yet, the chemical mechanisms that catalyze its formation remain an enigma. Here, using batch reactor experiments, we demonstrate an unexpected acceleration of dolomite formation by zinc in saline fluids, reflecting a not uncommon spatial association of dolomite with Mississippi Valley-type ores. The acceleration correlates with dissolved zinc concentration, irrespective of the zinc source tested (ZnCl2 and ZnO). Moreover, the addition of dissolved zinc counteracts the inhibiting effect of dissolved sulfate on dolomite formation. Integration with previous studies enables us to develop an understanding of the dolomitization pathway. Our findings suggest that the fluids’ high ionic strength and zinc complexation facilitate magnesium ion dehydration, resulting in a dramatic decrease in induction time. This study establishes a previously unrecognized role of zinc in dolomite formation, and may help explain the changes in dolomite abundance through geological time.
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Cooling of Earth's climate over the Cenozoic has been accompanied by large changes in the magnesium and calcium content of seawater whose origins remain enigmatic. The processes that control these changes affect the magnesium isotopic composition of seawater, rendering it a useful tool for elucidating the processes that control seawater chemistry on geologic timescales. Here we present a Cenozoic magnesium isotope record of carbonate sediments and use a numerical model of seawater chemistry and the carbon cycle to test hypotheses for the covariation between Cenozoic seawater chemistry and climate. Records are consistent with a 2–3× increase in seawater Mg/Ca and little change in the Mg isotopic composition of seawater. These observations are best explained by a change in the cycling of Mg-silicates. We propose that Mg/Ca changes were caused by a reduction in removal of Mg from seawater in low-temperature marine clays, though an increase in the weathering of Mg-silicates cannot be excluded. We attribute the reduction in the Mg sink in marine clays to changes in ocean temperature, directly linking the major element chemistry of seawater to global climate and providing a novel explanation for the covariation of seawater Mg/Ca and climate over the Cenozoic.
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The Ca, Mg, O, and C isotopic and trace elemental compositions of marine limestones and dolostones from ODP Site 1196A, which range in depth (∼58 to 627 mbsf) and in depositional age (∼5 and 23 Ma), are presented. The objectives of the study are to explore the potential for non-traditional isotope systems to fingerprint diagenesis, to quantify the extent to which geochemical proxies are altered during diagenesis, and to investigate the importance of diagenesis within the global Ca and Mg geochemical cycles. The data suggest that Ca, which has a relatively high solid to fluid mass ratio, can be isotopically altered during diagenesis. In addition, the alteration of Ca correlates with the alteration of Mg in such a way that both can serve as useful tools for deciphering diagenesis in ancient rocks.
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The mineral dolomite and the uncertainties surrounding its origin have attracted the attention of earth scientists for over a century, The core of the dolomite "problem" is the apparent paradox posed by the paucity of dolomite in modern marine depositional environments versus its relative abundance in the sedimentary rock record, Solving this problem requires knowledge of the conditions under which the mineral forms and the rate of precipitation under those conditions, As a working hypothesis, it is suggested that the precipitation rate of dolomite may be quantified and modeled in a manner similar to other carbonate minerals through application of a rate law that represents the rate as a simple function of saturation index, r = k(Omega - 1)(n). This hypothesis is tested in a series of experiments by measuring the steady state rate of dolomite precipitation in a dolomite-seeded now reactor through analysis of reacted fluid chemistry. By varying temperature from approx 100 degrees to 200 degrees C and saturation index (Omega) from near saturation to similar to 100, sufficient data were collected to solve for the reaction order and Arrhenius rate constant (k = A exp {-(epsilon(A)/RT)}) Of this rate law, The dolomite produced in these experiments was variable in composition but typically a calcium-rich protodolomite, forming syntaxial overgrowths on the seed material. tit the highest supersaturations obtained, formation of distinct nucleation centers was observed. These experiments do confirm a strong temperature dependence for the precipitation reaction (activation energy epsilon(A) = 31.9 kcal mol(-1)) and moderate dependency on saturation index (n = 2.26, log A = 1.05). The experimental findings of this paper suggest that the abundance of dolomite in the sedimentary rock record reflects, at least in part, environmental changes in temperature and seawater chemistry over geologic time.
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Significance Abundant in the geologic record, but scarce in modern environments below 50 ° C, the mineral dolomite is used to interpret ancient fluid chemistry, paleotemperature, and is a major hydrocarbon reservoir rock. Because laboratory synthesis of abiotic dolomite had been unsuccessful, chemical mechanisms for precipitation are poorly constrained, and limit interpretations of its occurrence. Here we report the abiotic synthesis of dolomite at 25 ° C, and demonstrate that carboxylated surfaces on organic matter catalyze precipitation through complexation between carboxyl groups and Mg ²⁺ , removing water to make Mg ²⁺ available for dolomite precipitation. This mechanism is consistent with dolomite formation in depositional environments rich in organic matter. Our experimental protocol provides opportunities for calibrating conditions of low-temperature dolomite formation throughout the geologic record.
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Data collected from a series of high-temperature dolomitization experiments in which dolomite replaces calcite in Mg–Ca–Cl solutions indicate that dolomite composition (mol% MgCO3) and cation order evolve independently as a function of reaction progress. Despite a wide range of initial solution Mg/Ca (0.43–1.50), the first product to form in all experiments is disordered dolomite. Both the rate of replacement and the composition of these dolomite products are strongly dependent on the initial Mg/Ca in solution. Experimental solutions with lower Mg/Ca yield less stoichiometric (more Ca-rich) dolomite and have reactions that progress at a slower rate. Conversely, solutions with higher Mg/Ca yield more stoichiometric dolomite and are characterized by faster overall reaction rates. In all experiments, dolomite composition remains constant during most of the reaction despite a solution chemistry that continually evolves to lower Mg/Ca as magnesium is captured from solution by growing dolomite and calcium is liberated from calcite into solution.
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Dolomite is not a simple mineral; it can form as a primary precipitate, a diagenetic replacement, or as a hydrothermal/metamorphic phase, all that it requires is permeability, a mechanism that facilitates fluid flow, and a sufficient supply of magnesium. Dolomite can form in lakes, on or beneath the shallow seafloor, in zones of brine reflux, and in early to late burial settings. It may form from seawater, from continental waters, from the mixing of basinal brines, the mixing of hypersaline brine with seawater, or the mixing of seawater with meteoric water, or via the cooling of basinal brines. Bacterial metabolism may aid the process of precipitation in settings where sulfate-reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings.
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Structurally controlled hydrothermal dolomite (HTD) reservoir facies and associated productive leached limestones are major hydrocarbon producers in North America and are receiving increased exploration attention globally. They include multiple trends in the Ordovician (locally, Silurian and Devonian) of the Michigan, Appalachian, and other basins of eastern Canada and the United States, and in the Devonian and Mississippian of the Western Canada sedimentary basin. They also occur in Jurassic hosts along rifted Atlantic margins, in the Jurassic-Cretaceous of the Arabian Gulf region and elsewhere. Hydrothermal dolomitization is defined as dolomitization occurring under burial conditions, commonly at shallow depths, by fluids (typically very saline) with temperature and pressure (T and P) higher than the ambient T and P of the host formation. The latter commonly is limestone. Proof of a hydrothermal origin for HTD reservoir facies requires integration of burial-thermal history plots, fluid-inclusion temperature data, and constraints on timing of emplacement. Hydrothermal dolomite reservoir facies are part of a spectrum of hydrothermal mineral deposits that include sedimentary-exhalative lead-zinc ore bodies and HTD-hosted Mississippi Valley-type sulfide deposits. All three hydrothermal deposits show a strong structural control by extensional and/or strike-slip (wrench) faults, with fluid flow typically focused at transtensional and dilational structural sites and in the hanging wall. Transtensional sags above negative flower structures on wrench faults are favored drilling sites for HTD reservoir facies. Saddle dolomite in both replacive and void-filling modes is characteristic of HTD facies. For many reservoirs, matrix-replacive dolomite and saddle dolomite appear to have formed near-contemporaneously and from the same fluid and temperature conditions. The original host facies exerts a major influence on the lateral extent of dolomitization, resultant textures, pore type, and pore volume. Breccias, zebra fabrics, shear microfractures, and other rock characteristics record short-term shear stress and pore-fluid-pressure transients, particularly proximal to active faults. High-temperature hydrothermal pulses may alter kerogen in host limestones, a process designated "forced maturation." Basement highs, underlying sandstone (and/or carbonate?) aquifers (probably overpressured), and overlying and internal shale seals and aquitards also may constrain or influence HTD emplacement. Although many questions and uncertainties remain, particularly in terms of Mg and brine source and mass balance, recognition and active exploration of the HTD play continues to expand. Increasing use of three-dimensional seismic imagery and seismic anomaly mapping, combined with horizontal drilling oblique to linear trends defined by structural sags, helps to reduce risk.
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The Alpine Triassic units of Switzerland, Northern Italy and Western Austria offer an extensive geological archive, in which the enigmatic process of dolomite formation can be studied in a palaeoenvironmental context. Recent studies clearly demonstrate that large amounts of the Alpine Triassic dolomites are late diagenetic or hydrothermal. Nevertheless, as part of multiple generations of diagenetic overprint, some generations of fine-crystalline, Ca-rich dolomite appear strictly confined to their depositional facies and show signs of very early formation at surface temperatures in specific ancient depositional environments. In this review, three cases of Alpine Triassic dolomites are discussed, where dolomite rocks may have formed during or soon after sedimentation. The sedimentary facies indicate contrasting palaeoenvironmental conditions and, hence, document three different possible processes of dolomite formation: (i) In the Dolomite Mountains (Northern Italy), dolomite beds of the partly isolated Middle Triassic (Anisian/Ladinian) Latemar Platform are confined to the very top of shallowing-upward lagoonal facies cycles. (ii) Dolomite beds of the San Giorgio Basin (Southern Switzerland), an intraplatform basin that opened during the Anisian/Ladinian transition, are associated with organic carbon-rich shales, which were deposited in a deeper water environment under anoxic conditions. (iii) In the entirely dolomitized platform facies of the Dolomia Principale (Hauptdolomit Formation), a very early generation of fine-crystalline dolomite occurs in the shallowest part of evaporative peritidal cycles. This platform extended over thousands of square kilometres along the Tethys margin during the Late Triassic (Carnian and Norian) and large amounts of carbonate were deposited under hypersaline sabkha-like conditions. Representing three distinct depositional environments, these three different Triassic systems show features in common with several dolomitization models developed from the study of modern dolomite-forming environments; for example, the sabkha model, the evaporative lagoon/lake model, the organogenic model and the microbial model. Although these actualistic models may be applicable to reconstruct the palaeoenvironmental conditions during dolomite formation, dolomite-forming processes during the Triassic were apparently quite different from the modern world in terms of distribution and scale. Recent developments in stable-isotope geochemistry and high-resolution geochemical probing offer the possibility to make better reconstructions of Triassic palaeoceanographic conditions and suggest a non-actualistic approach to better understand dolomite formation during the Triassic.
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DOLOMITE (CaMg(CO3)(2)) is a common carbonate mineral which is found in much greater abundance in ancient rocks than in modern carbonate environments. Why this is so remains a mystery. Over the past 30 years, dolomite formation has been observed in several modern environments, and various thermodynamic, kinetic and hydrological factors have been proposed to explain its formation(1,2). But attempts to precipitate dolomite at low temperatures in the laboratory have been unsuccessful(3,4), and the &apos;dolomite problem&apos; remains a source of controversy in sedimentary geology(5-7). Here we describe experiments in which a ferroan dolomite with a fairly high degree of cation order was precipitated in the presence of sulphate-reducing bacteria from the Desulfovibrio group. We propose that the direct mediation of these anaerobes can overcome the kinetic barrier to dolomite nucleation, and that they may play an active role in the formation of this mineral in natural environments.
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La solubilité dans l'eau d'un système binaire de carbonates est régie par le produit total de solubilité, qui est la somme ∑∏ des produits partiels de solubilité des deux pôles. ∑∏, tracé en fonction du titre molaire de la phase solide et de celui dans la solution aqueuse, donne des courbes semblables aux diagrammes de phase pour la tension de vapeur de mélanges liquides. Le diagramme stable CaCO₃ — MgCO₃ — H₂O comprend deux points eutectiques stables à cause de l'intervention de la dolomite comme phase intermédiaire. La solubilité des calcites magnésiennes est caractérisée par des états métastables qui se produisent par suite de transgressions de plusieurs limites de miscibilité solide.
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Among various geochemical and petrographic approaches, dolomite crystal morphology and dolostone fabric have been widely applied in the study of ancient dolostones. It is proposed that dolomite crystal morphologies and the rock fabric may reflect the formation temperature, and thus can be used to distinguish different generations of dolomite. However, this scenario has also been challenged by some researchers. In order to test whether the dolomite crystal morphology can be used to differentiate different generations of dolomite, in this study, we measured the Mg isotopic compositions (δ26Mg) of dolomite with different crystal morphologies. δ26Mg of dolomite is controlled by a variety of factors, including temperature, magnesium isotopic composition of dolomitization fluids, and the flow rate of dolomitization fluids. If dolomite with distinct crystal morphologies were derived from different dolomitization processes, it is highly plausible that they would have different δ26Mg. Five types of dolomite with distinct crystal morphologies and rock fabric were recognized from three sampling intervals (S1, S2, and S3) in the middle Ordovician Majiagou Formation in North China. Different types of dolomite in the same sampling interval have similar δ26Mg values, suggesting that these dolomites might have derived during the same dolomitization event. Our study indicates that the crystal morphology alone may not unambiguously differentiate the generations of dolomites. We propose the following reasons: (1) the dolomite crystal morphology might be controlled by various factors rather than the formation temperature alone, or (2) the dolomite crystal morphology might be modified in diagenesis, but δ26Mg remains unchanged.
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This study presents a chemical protocol for the separation of Mg that is particularly adapted to diverse igneous rock samples, especially for high-K and low-Mg rocks. This protocol bases on a combination of three procedures: K element is first removed from the samples using precipitation procedure, followed by the separation of Fe and Ca using 2 ml AG50W-X12 cation exchange resin, finally removed Al, Ti, Na, and Fe elements using 0.5 ml AG50W-X12 cation exchange resin. Effect of acidity and concentration mismatch, and matrix effect are rigorously evaluated using Nu Plasma II MC-ICP-MS with wet plasma mode. The δ26Mg mean values of the GSB and Alfa Mg standard solutions are -2.05 ± 0.05 ‰ (2 standard deviation (2s), n = 106) and -3.91 ± 0.04 ‰ (2s, n = 51), respectively, which serve as in-house Mg standards in State Key Laboratory of Continental Dynamics (SKLCD). The average δ26Mg value of one multi-elemental Mg standard GSB-1, made by adopting GSB with matrix elements, agrees well with the recommended value after precipitation and chromatographic separation. The long-term reproducibility, assessed by repeated measurements of Mg standard solutions and reference materials (RMs), is ± 0.06 ‰. The robustness of our method is further assessed by replicated analyses of fifteen rock standards and olivine grains with large variations in MgO and K2O contents.
Article
Sedimentary dolomite plays an important role in global Mg cycling, and Mg isotopes in massive dolostones may be used to infer secular changes in seawater chemistry through geological history. However, sedimentary dolomite is generally regarded as a diagenetic product, and many details about the effects of early diagenesis on the Mg isotope composition of dolomite remain unclear. The mid-Cretaceous (Albian) Soreq and Givat Ye'arim formations near Jerusalem, Israel, contain exceptionally well-preserved massive dolostones, which provide an ideal opportunity to investigate the Mg isotope responses to early diagenesis. Dolomite samples from this section show large variations in δ ¹³ C values and Mn contents that are negatively correlated, reflecting degradation of organic matter and mineralization of organic carbon in the Mn(IV) reduction zone within soft sediment during dolomite formation. This is a rare example of a clear link between Mn(IV) reduction and dolomite precipitation based on geochemical evidence in the rock record. The dolomite samples also exhibit large variations in d(104) values and the degree of cation ordering. The latter is negatively correlated with Sr contents, implying that variable degrees of dolomite recrystallization occurred during diagenesis. δ ²⁶ Mg values of >50 dolomite samples from this section vary from −2.28‰ to −1.78‰ but do not correlate with indicators of organic matter degradation (δ ¹³ C values and Mn contents) or dolomite recrystallization (e.g., Sr contents), suggesting that Mg isotopes behave conservatively after initial dolomite (or proto-dolomite) precipitation during the very early stages of diagenesis. We propose that the Mg isotope composition of dolomite formed due to Mn(IV) reduction is buffered by seawater due to the shallowness of the Mn(IV) reduction zone in platform sediments, which is different from the dolomitization associated with bacterial sulfate reduction or methanogenesis, where Mg supply can be diffusion-limited. Furthermore, Mg isotopes in dolomite are robust to resetting by recrystallization during burial. Magnesium isotope compositions of platform dolomite that show variable and negatively correlated δ ¹³ C values and Mn contents can, therefore, be considered robust archives for reconstructing paleo-seawater Mg isotope compositions. The variation in δ ²⁶ Mg values of the dolostones in the Soreq and Givat Ye'arim formations is interpreted to reflect Rayleigh fractionation in response to dolomitization in a restricted water body. Therefore, the lowest δ ²⁶ Mg value is considered to be the Mg isotope composition of dolomite that was in equilibrium with coeval seawater in the open ocean, and thus the δ ²⁶ Mg value of Albian seawater was around −0.4‰.
Article
Mg isotopes in syndepositional dolomite have been suggested to be a potential proxy for understanding seawater chemistry. However, it is argued that the δ26Mg values of dolomite could be complicated by the effects of early diagenesis and later hydrothermal activities. Further investigations into the behaviors of Mg isotopes in dolomitization systems are needed to resolve this controversy. In the present study, we investigated early Ordovician dolostones from the Tarim Basin, including diagenetically altered dolomites, hydrothermally altered dolomites and well-preserved dolostones that precipitated in the slope, margin and interior of a carbonate platform. Different types of dolomites, bulk dolostone and limestone were sampled by microdrilling for analyses of C-O-Mg isotope compositions and REE concentrations. The dolostone δ13C values match those of coeval seawater, and the REE distribution patterns in the dolostones are comparable with those in the limestones, indicating that the dolostones originated from syndiagenetic dolomitization. The δ26Mg values of the various syndiagenetic dolomites that formed in the slope, margin and interior of the carbonate platform are similar, averaging approximately -2.06‰ ± 0.20‰. No stratigraphic variability in the dolomite Mg isotopes can be discerned, which implies that the Mg isotope compositions of the porewater were homogeneous during massive dolomitization and remained in equilibrium with seawater. Additionally, the δ26Mg values of the altered dolostone do not show correlations with diagenetic and hydrothermal signals, demonstrating that dolomite Mg isotopes are insensitive to postdepositional alteration. Given these facts, we propose that Mg isotopes in dolostones have conservative behaviors during diagenesis and late stage hydrothermal reworking.
Chapter
problems, progress and future research in dolomites and dolomitization;International Association of Sedimentologists (IAS) and Society for Sedimentary Geology (SEPM);dolomite and its origins - progress, problems and speculations;evaluation of specific hydrodynamic models and burial dolomitization;dolomite crystal evolution in modern sediments;modern dolomites and their role in understanding ancient dolomites;significance of dolomite fabrics and mineralogy;dolomite distribution and basin morphology;global factors influencing dolomite distribution
Article
Magnesium (Mg) has an atomic number of 12 and belongs to the alkaline earth element (Group II) of the Periodic Table. The pure Mg is a silvery white metal and has a melting point of 650 °C and boiling point of 1090 °C at 1 standard atmosphere (Lide 1993–1994). The electronic configuration of Mg is [Ne]3s2, with low ionization energies, which makes Mg ionic in character with a common valance state of 2+ and a typical ionic radius of 0.72 A (Shannon 1976). Magnesium is a major element and widely distributed in the silicate Earth, hydrosphere and biosphere (Fig. 1a). It is the fourth most abundant element in the Earth (after O, Fe and Si, MgO = 25.5 wt%) (McDonough and Sun 1995), the fifth most abundant element in the bulk continental crust (MgO = 4.66 wt%) (Rudnick and Gao 2003) and the second most abundant cation in seawater (after Na, Mg = 0.128 wt%) (Pilson 2013). Nonetheless, the mantle has > 99.9% of Mg in the Earth because of its high MgO content (37.8 wt%, McDonough and Sun 1995) and mass fraction. The high abundance of Mg in the silicate Earth makes it a major constituent of minerals (e.g., olivine, pyroxene, garnet, amphibole, mica, spinel, carbonate, sulfate, and clay minerals) in igneous, metamorphic and sedimentary rocks. Magnesium has three stable isotopes, with mass numbers of 24, 25 and 26, and typical abundances of 78.99%, 10.00% and 11.01%, respectively (Berglund and Wieser 2011) (Fig. 1b), and a standard atomic weight of 24.305 (CIAAW 2015). Because of the limitations in the mass spectrometry, many previous Mg isotopic studies have concentrated on either mass independent isotope anomalies to look for the radiogenic 26Mg produced by the decay of short-lived 26Al (Gray and Compston 1974; Lee and …
Article
Bedded dolomites in the Permian Basin were formed by the alteration of metastable limestones by hypersaline brines refluxing from evaporate lagoons. The hot, heavy, highly alkaline, carbon dioxide-free, magnesium-supercharged brines displaced connate waters to provide both a chemically favorable environment for magnesium-calcium exchange and a vehicle for removing displaced calcium. Fossil lagoonal brines and fillings of halite and anhydrite in the dolomite pores offer proof of the brine invasion. Sedimentary dolomites in other areas are commonly associated with evaporites, and for these dolomites a similar origin is postulated.
Article
Magnesium isotopic compositions of igneous rocks could be potentially used to trace recycling of supracrustal materials. High-δ26Mg granitoids have been previously reported and explained to reflect the involvement of surface weathered materials in their sources. Low-δ26Mg granitoids, however, have not been reported. In this study, we report high-precision Mg isotopic analyses of Cenozoic alkaline syenites and syenogranites from the Kuzigan and Zankan plutons, northwest Xinjiang, China. The Kuzigan syenites were originated from the mantle metasomatized by recycled supracrustal materials, and the syenogranites are differentiated products of the syenites. Both syenites and syenogranites have δ26Mg values (− 0.46 to − 0.26‰ and − 0.41 to − 0.17‰, respectively) significantly lighter than the mantle (− 0.25 ± 0.07‰, 2SD). No correlation of δ26Mg with either SiO2 or MgO is observed, indicating limited Mg isotope fractionation during alkaline magmatic differentiation. The low δ26Mg of the syenites and syenogranites thus reflects a light Mg isotopic source. This, combined with high 87Sr/86Sr ratios (0.70814 to 0.71105) and negative correlation between δ26Mg and δ18O, suggests that the magma source contains recycled marine carbonates. Modeling of the Mg-O-Sr isotopic data indicates that the recycled carbonate is mainly limestone with minor dolostone, suggesting that the metasomatism occurred at depths shallower than 60 to 120 km. Given that the plutons are located at the India–Eurasia collision zone, the carbonate recycling was most likely derived from the subducted Tethyan oceanic crust during the Mesozoic–Cenozoic. Our study suggests that the combined Mg, O, and Sr isotopic studies are powerful for tracing recycled carbonates and identifying their species in mantle sources.
Article
The "dolomite problem" refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record, as well as failure of laboratory inorganic dolomite precipitation at near Earth-surface temperature. Novel Mg isotope systematics provides a promising tool in resolving the "dolomite problem". Here, we develop a protocol to place constraints on the dolomitization process by using Mg isotopes. In this study, we measured Mg isotopic compositions ( δ26Mg) of two batches of partially dolomitized limestone samples from the middle Cambrian Xuzhuang Formation in North China. δ26Mg varies between -0.55‰ and -3.18‰, and shows a negative linear correlation with 1[Mg], suggesting that δ26Mg can be described by a binary mixing between the calcite and dolomite components. Mg isotopic composition of the dolomite component ( δ26Mgdol) for the lower sample set that is collected from a 4 m stratigraphic interval containing three high-frequency ribbon rock-packstone cycles is -1.6‰, while δ26Mgdol for the upper sample set (from a thick sequence of ribbon rock) is significantly higher (-0.3‰). However, neither mineralogical and elemental compositions, carbon and oxygen isotopes, nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples. δ26Mgdol of the Xuzhuang limestone is simulated by the Advective Flow (AF) and the Diffusion-Advection-Reaction (DAR) models. The AF model assumes that Mg is transported by advective fluid flows, while the DAR model simulates a contemporaneous seawater dolomitization process, in which Mg is delivered by diffusion. The AF modeling result indicates that δ26Mg of the dolomitization fluid is +0.4‰ and +1.7‰ for the lower and upper sample sets, respectively. These values are significantly higher than modern and Cenozoic seawater Mg isotopic composition, suggesting that the dolomitization fluid is not contemporaneous seawater. The AF model also predicts spatially heterogeneous δ26Mgdol with progressive enrichment in 26Mg along the fluid flow pathway. In the DAR model, both dolomite content and δ26Mgdol of the lower sample set can be simulated by using seawater Mg isotopic composition of -0.75‰, thus contemporaneous seawater dolomitization may explain δ26Mgdol of the Xuzhuang limestone. Furthermore, the DAR model demonstrates spatially homogeneous δ26Mgdol. To differentiate the AF and DAR models, samples from multiple sections are required. Nevertheless, this study implies that Mg isotope might be a useful tool in the study of dolomitization.
Article
Most current models of dolomitization are incapable of explaining the genesis of dolostones that lack evidence of supratidal origin and are not associated with evaporites. These models require sea water evaporation and a high Mg2+/Ca2 ratio in solution as essential factors for dolomitization. Calculations show that mixing meteoric ground waters with up to 30% sea water causes undersaturation with respect to calcite, whereas dolomite saturation increases continuously. Therefore, in the range of approximately 5 to 30% sea water, calcite can be replaced by dolomite. A new term "Dorag" (Persian for mixed blood or hybrid) is used for this model of dolomitization. The Dorag model satisfactorily explains the origin of dolostones along positive elements or epicontinental shelves; it is based on the effect of ionic strength on the solubility of carbonate minerals, and does not require a Mg2+/Ca2+ ratio greater than one, a ratio obtained at calcite-dolomite equilibrium. Dolomitic facies of the Middle Ordovician Series in southwestern Wisconsin and adjacent states is an example of dolostones associated with positive elements. Lithologic and paleontologic evidence, as well as primary structures, within the Mifflin Member (Platteville Formation) suggest the existence of a shallow and open-marine environment over the Wisconsin Arch at the time of deposition of the Member, whereas deeper water conditions prevailed farther from the Arch. Oxygen and carbon isotopic analyses of the Mifflin Member and chemical analyses for Sr and Na in the limestones and dolostones indicate exchange of the Mifflin carbonates with meteoric water. Because (a) evaporites, algal mats, mud cracks and other evidences of restricted environments are not found in the Mifflin Member, ( ) isotopic and chemical data suggest exchange of the Mifflin carbonates with meteoric water, and (c) the dolomite facies of this member is only associated with structural highs, the dolomitization of the Mifflin Member is best explained by mixing of sea water and ground water in the phreatic zone. Assuming Dorag dolomitization has been operative, three major lateral shifts in the dolostone-limestone boundary in the Champlainian Series can be shown to represent episodes of transgression-regression during the Middle Ordovician. Conclusions can be drawn concerning (a) paleo-sea level fluctuation, (b) paleobathymetry, and (c) configuration and position of the paleo-ground water lens.
Article
The periphery of the buildup is heavily dolomitized in a narrow zone less than 1km wide and decreases rapidly into the buildup interior. 5 types of dolomites are recognised. Petrographic observations supported by geochemical and isotope data indicate that pervasive dolomitization was a late diagenetic phenomenon postdating cementation and lithification of buildup interior deposits. Most of the dolomites probably formed by a combination of 3 processes: 1) migration of brines from adjacent basin muds undergoing compaction, 2) pressure solution, and 3) mixing of near-surface fluids with deep burial brines along fracture controlled conduits. Most of the dolomitizing waters are believed to have been derived from the dewatering of basinal strata adjacent to the buildup and from strata underlying the buildup. -from Authors
Article
Questions on dolomite were brought up at the S.E.P.M. Carbonate Symposium at St. Louis, 1954. New observations, techniques, and analyses published during the last few years provide grounds for a fresh review of the problem. Apart from evaporite basins and intertidal concentration, the only important direct source of marine MgCO3 is organic precipitation within the calcite crystal lattice (i.e., in solid solution) under warm-water conditions by the more primitive calcium-fixing plants and animals. Dolomite, the double carbonate of Ca and Mg, is not precipitated directly during sedimentation today under the warm, shallow conditions suggested by the ecologic data of ancient marine dolomites. On the contrary, it appears that as a rule it begins to form initially very early in diagenesis. This may be due to the instability of both aragonite and high-magnesium calcite, and the presence of magnesium-rich solutions (sea or connate water), under special chemical and thermodynamic conditions existing beneath the surface of newly formed sediments. Dolomitization may go on discontinuously from time to time, before and after orogeny, until all the calcareous raw material has been metasomatically replaced. From the evidence available it seems that dolomite is also formed today under deep-sea conditions and in cold water, but only in small, isolated crystals; even these may require organic, magnesium-rich calcite “starters.” Marine dolomite is thus a most useful paleoecologic indicator. Nonmarine dolomites, hydrothermal and salt-pan precipitates, etc., are not considered here.
Article
Carbonate sediments dominate the shallow waters along the arid southwest side of the Persian Gulf. In the more protected parts of the west coast of the Qatar Peninsula, the processes of near-shore sedimentation have created lagoons and embayments with high chlorinity (30–35 g/l) and reduced tidal range; they are separated from the normal Gulf waters (22–24 g/l) with their average 4-foot tides, by many miles of sea ***s than two fathoms deep. The lowest-lying parts of the lagoonal shores are fringed by salt flats—“sebkhas”—varying in width from few tens of yards to several miles. The sebkhas pass seawards into the intertidal zone, commonly via an intermediate algal flat. This is just covered by normal high tides; but only with favourable winds can occasional spring tides reach far onto the sebkha surface which is a few inches higher. Sedimentation is gradually filling the lagoons by the seaward advance of the environmental belts, so that sebkha sediment overlies stromatolitic algal laminae, and these are underlain by intertidal muddy sands composed of pelleted lime-muds, resting on lagoonal lime-muds. The chlorinity of the pore waters increases landwards and upwards in response to surface evaporation ***ses. It increases rapidly within the algal flat (50–130 g/l), where small selenite crystals form beneath the ***gher, landward parts. Together with the continuing precipitation of aragonite, this causes an increase in the Mg/Ca ratio of the pore waters from the normal marine value of 3 in the lagoon to over 10 at the sebkha edge. Within the sebkha, the ratio falls gradually to below 5, while the chlorinity continues to rise slowly to over 150 g/l. The water table is close to the sebkha surface, and beneath the uppermost layer subject to large daily temperature changes the wet sediment reaches well over 40°C. in summer time. Its pH ***low (around 6.7) and decreases downwards. These warm magnesium-rich brines cause diagenetic changes in the aragonite sebkha sediment. Dolomite appears. It occurs as a stiff, sticky, tan or tan-grey mud composed of rhombs 1–5 microns in size. Associated with it are turbid flattened crystals of gypsum up to five inches across, enclosing, displacing and placing aragonite sediment Depositional textures tend to become obscured, but macroscopic and microscopic relic structures and the changing chemistry of the pore waters make it clear that both the dolomite and the associated coarse platy gypsum are replacing aragonite. They increase in abundance away from the lagoon until they make up the bulk of the sebkha sediment. The dolomite normally appears a few inches beneath the surface, increases rapidly, and almost disappears again in a more irregular fashion within a depth of two to four feet. Carbon 14 determinations on two dolomite samples collected within 9 to 18 inches of a sebkha surface ***ve ages of 2670 and 3310 years, confirming that the dolomitisation is a penecontemporary phenomenon related to the present sedimentary environment.
Chapter
Calcium rich dolomite is in the process of formation on exposed supratidal mud flats in the Bahama Islands. On western Andros Island up to 80 percent dolomite occurs at or near the surface in an area covering hundreds of square miles. In all the Bahama localities the site of formation is inches above mean high tide level. The dolomite crystals, < 3 microns, occur in pelleted muds that are associated with laminations, stromatolites, and mud cracks. The concentration of dolomite is highest in hardened surface crusts that are cemented with Recent calcite, aragonite, and dolomite. It is believed that much of the dolomite is a penecontemporaneous replacement of calcium carbonate. The dolomite forms where tidal flooding and storm sedimentation is followed by many days of subaerial exposure. Surface evaporation during these periods of exposure increases the concentration of dissolved salts near the surface but no evaporites are preserved. Magnesium calcium ratios as high as 40 to 1 are present in the concentrated interstitial waters where dolomite is forming.
Article
Most dolomite forms as a calcium-rich and/or poorly ordered metastable phase when seawater is actively circulated through carbonate sediments. Modification of seawater by evaporation, mixing with meteoric water, and/or sulfate reduction promotes dolomitization, but is not necessary. The extensive circulation which is necessary for massive dolomitization can be caused by density or elevation head, or by convection induced by geothermal heat. Progressive stabilization of metastable dolomite (replacement by more stable phases), especially at slightly elevated temperatures in the subsurface, results in crystal enlargement and chemical modification which masks original properties.-Author
Article
Available Mg isotope data indicate that dolostones of different ages have overlapping range of Mg isotopic composition (δ26Mg) and there is no systematic difference among different types of dolomites. To further explore the Mg isotopic systematics of dolomite formation, we measured Mg isotopic compositions of Mesoproterozoic dolostones from the Wumishan Formation in North China Block, because dolomite formation in Mesoproterozoic might have been fundamentally different from the younger counterparts. Based on petrographic observations, three texturally-different dolomite phases (dolomicrite, subhedral dolomite and anhedral dolomite) are recognized in the Wumishan dolostones. Nevertheless, these three types of dolomites have similar δ26Mg values, ranging from -1.35‰ to -1.72‰, which are indistinguishable from Neoproterozoic and Phanerozoic dolostones. To explain δ26Mg values of dolostones, we simulate the Mg isotopic systematics during dolomite formation by applying the one-dimensional Diffusion-Advection-Reaction (1D-DAR) model, assuming that the contemporaneous seawater is the Mg source of dolostone. The 1D-DAR modeling results indicate that the degree of dolomitization is controlled by sedimentation rate, seawater Mg concentration, temperature, and reaction rate of dolomite formation, whereas Mg isotopic composition of dolostone is not only dependent on these factors, but also affected by δ26Mg of seawater and isotope fractionation during dolomite formation. Moreover, the 1D-DAR model predicts that dolomite formation within sediments has limited range of variation in δ26Mg with respect to limestones. Furthermore, the modeling results demonstrate that dolostone is always isotopically heavier than Ca-carbonate precipitated from seawater, explaining the systematic isotopic difference between dolostones and limestones. Finally, we can infer from the 1D-DAR model that early-formed dolostone at shallower depth of sediments is always isotopically lighter than that formed in deeper sediments, suggesting the potential application of Mg isotope as a proxy for constraining dolostone formation.
Article
Hydrothermal experiments at 220, 160, and 130°C were performed to calibrate the Mg isotope fractionation factor between dolomite and aqueous Mg. Hydrothermal experiments included synthesis of dolomite using different starting materials, as well as exchange experiments that used poorly-ordered proto-dolomite. The morphology of synthesized dolomite was dependent on starting mineralogy, suggesting that dolomite was synthesized by different pathways. Hydrothermally synthesized dolomite was initially fine-grained disordered or poorly-ordered dolomite that, with time, recrystallized to coarser-grained ordered dolomite. Isotopic exchange was monitored using 87Sr/86Sr ratios and 25Mg tracers, and these indicated near-complete isotope exchange between dolomite and aqueous solutions at the end of most hydrothermal experiments. The Mg isotope fractionation factor between dolomite and aqueous solution obtained from synthesis and exchange experiments converged with time and was independent of dolomite morphology, suggesting attainment of isotopic equilibrium. Combining results from synthesis and exchange experiments, the temperature dependent Mg isotope fractionation factor for ordered dolomite is:
Article
Dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments. Using well-established isotope systems (δ18O, δ13C, 87Sr/86Sr), fluid inclusions and elemental data, as well as a detailed sedimentological and petrographic data set, we established the precipitation environment and subsequent diagenetic pathways of a series of Proterozoic to Pleistocene syn-depositional marine evaporative (sabkha) dolomites, syn-depositional non-marine evaporative (lacustrine and palustrine) dolomites, altered marine (“mixing zone”) dolomites and late diagenetic hydrothermal dolomites. These data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios (δ26Mgdol). Dolomite δ26Mg ratios documented here range, from -2.49 to -0.45‰ (δ26Mgmean=-1.75±1.08‰, n=42). The isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites (-2.11 ± 0.54‰ 2σ, n=14). In comparing ancient compositions to modern ones, some of the variation is probably due to alteration. Altered marine (-1.41 ± 0.64‰ 2σ, n=4), and earliest diagenetic lacustrine and palustrine dolomites (-1.25 ± 0.86‰ 2σ, n=14) are less negative than sabkha dolomites but not distinct in composition. Various hydrothermal dolomites are characterized by a comparatively wide range of δ26Mg ratios, with values of -1.44 ± 1.33‰ (2σ, n=10). By using fluid inclusion data and clumped isotope thermometry (Δ47) to represent temperature of precipitation for hydrothermal dolomites, there is no correlation between fluid temperature (∼100 to 180°C) and dolomite Mg isotope signature (R2=0.14); nor is there a correlation between δ26Mgdol and δ18Odol. Magnesium-isotope values of different dolomite types are affected by a complex array of different Mg sources and sinks, dissolution/precipitation and non-equilibrium fractionation processes and overprinted during diagenetic resetting. Further progress on the use of δ26Mgdol as a proxy will require new theoretical and experimental data for Δ26Mgfluid-dol that includes dehydration effects of the free Mg aquo ion versus fluid temperature. In ancient diagenetic systems, complex variables must be considered. These include fluid chemistry and physical properties, Mg sources and sinks, temporal changes during precipitation and post-precipitation processes including open and closed system geochemical exchange with ambient fluids. All of these factors complicate the application of δ26Mgdol as proxy for their depositional or diagenetic environments. Nevertheless, the data shown here also indicate that δ26Mgdol can in principle be interpreted within a detailed framework of understanding.
Article
We present elemental and isotopic data detailing how the Mg isotope system behaves in natural and experimentally synthesized clay minerals. We show that the bulk Mg isotopic composition (δ26Mg) of a set of natural illite, montmorillonite and kaolinite spans a 2‰ range, and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable Mg. After acid leaching, these natural clays become relatively enriched in isotopically heavy Mg by between 0.2 and 1.6‰. Results of exchange experiments indicate that the Mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light Mg compared to the residual clay. The isotopic composition of this exchangeable Mg (-1.49 to -2.03‰) is characteristic of the isotopic composition of Mg found in many natural waters. Further experiments with an isotopically characterized MgCl2 solution shows that the clay minerals adsorb this exchangeable Mg with little or no isotopic fractionation, although we cannot discount the possibility that the uptake of exchangeable Mg does so with a slight preference for 24Mg. To characterize the behaviour of Mg isotopes during clay mineral formation we synthesized brucite (Mg(OH)2), which we consider to be a good analogue for the incorporation of Mg into the octahedral sheet of Mg-rich clay minerals or into the brucitic layer of clays such as chlorite. In our experiment the brucite mineral becomes enriched in the heavy isotopes of Mg while the corresponding solution is always relatively enriched in isotopically light Mg. The system reaches a steady state after 10 days with a final fractionation factor (αsolid-solution) of 1.0005 at near-neutral pH. This result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy Mg during their formation. However our results also show that exchangeable Mg is an important component within bulk clay minerals and can have an important influence over the bulk clay δ26Mg value. Modeling shows that in certain soils or sediments where the percentage of exchangeable Mg is >40% and the isotopic composition of the exchangeable Mg is around -2‰, the generation of bulk δ26Mg values of <-0.5‰ is likely. On a broader scale, Mg-rich minerals such as smectite and illite are likely to impart a stronger control over the Mg budget in clay rich sediments, and their high structural Mg component is likely to result in bulk sediment δ26Mg values that are closer in composition to the UCC. Despite this, results of modeling, together with experimental observation suggests that the uptake of exchangeable Mg into these clay rich sediments could cause a decrease in the bulk δ26Mg value by up to ∼0.3-0.4‰. This should be accounted for when assessing the δ26Mg value of sediments on a crustal scale.
Article
Dolomite is a common rock forming mineral in the geological record but its value as archive of ancient seawater δ26Mg signatures and their variations in time is at present underexplored. Unknown factors include the sensitivity of δ26Mg ratio to processes in the diagenetic and low grade metamorphic domain. This paper documents and discusses the first detailed δ26Mg data set from early diagenetic and burial dolomites. Samples come from the Upper Triassic Hauptdolomit (Dolomia Principale; The Dolomites, Italy) and include coeval dolomicrites that underwent differential burial diagenesis and low grade metamorphosis in a temperature range between about 100 and more than 350 °C. Magnesium isotope data are complemented by dolomite δ13C, δ18O and 87Sr/86Sr isotope ratios as well as Ca, Mg, Mn, Fe and Sr elemental abundances and data on the dolomite crystal structure. As indicated by dolomicrite 87Sr/86Sr ratios and petrographic evidence, sabkha calcian D1 dolomites precipitated from evaporated seawater and stabilized at an early marine pore water diagenetic stage to D2 dolomites analysed here. With increasing burial temperature, dolomite δ26Mg ratio scatter in the data set decreases with increasing Mg/Ca ratio and degree of order. Specifically, δ26Mg ratio variability is reduced from ± 0.36‰ 2σ at burial temperatures beneath 100 °C to about ± 0.14‰ 2σ at temperatures in excess of 350 °C, respectively, with mean δ26Mg values ranging constantly near − 1.9‰. This suggests that, at least for the rock buffered system investigated here, dolomicrite δ26Mg proxy data are conservative and preserve near pristine values even at elevated burial temperatures. At present, the main element of uncertainty is the Mg-isotope fractionation factor between (evaporated) seawater and dolomite. A possible solution to this problem includes the compilation of data from modern sabkha environments including pore water and calcian dolomite δ26Mg isotope signatures.
Article
Previous studies of Cambro-Ordovician dolostones from various sites in the U.S. midcontinent revealed similar textures and geochemistry, prompting similar explanations of their origin. Before these interpretations can be incorporated in a model to explain other massive dolostones they need verification in a contrasting setting. The Dupont Geohydrological Survey (DGHS) well, continuously cored through the Knox Group in west-central Tennessee, provides such a contrast, This core provides a complete sample of the Knox in the setting of a shallowly buried, flat-lying, structurally and stratigraphically simple platform sequence, which contrasts with the relatively deeply buried and structurally complex environments that dominate most prior studies. Light and luminescence petrography. XRD, SEM, EDS, XRF, ESR, and mass spectrometry were used to document the petrology and geochemistry of the Upper Knox Group (Lower Ordovician) from 456 m to 1126 m subsea (2018 ft to 4215 ft below the Kelly Bushing of the DGHS well). Of the 87 samples, 81 are dolostone, Early (E-1) dolomite forms a fine to medium crystalline replacive dolostone that lacks luminescence banding. Its geochemical signature is delta C-13 = -2.83 +/- 0.76 parts per thousand; delta O-18 = -5.85 +/- 2.01 parts per thousand; stoichiometry = 51.6 +/- 1.2 mole % Ca; MnPR = 79 +/- 27; Fe = 681 +/- 237 ppm; Mn = 79 +/- 32 ppm; and Sr = 110 +/- 63 ppm. E-2 dolomite forms a medium to coarse crystalline replacive dolostone that luminescences with a relatively dull care and brighter red rim. Its geochemical signature is delta C-13 = -2.07 +/- 0.32 parts per thousand; delta O-18 = -6.96 +/- 1.28 parts per thousand; stoichiometry = 51.0 +/- 1.3 mole % Ca; MnPR = 74 +/- 20; Fe = 880 +/- 843 ppm; Mn = 84 +/- 52 ppm; and Sr = 79 +/- 29 ppm. Late (L-1) dolomite lines vugs and fractures, Three limestone beds are unaltered remnants of the precursor to E-2 dolomite, Calcite is present both as relies in dolomite rhombs and as vug and fracture fill postdating L-1 dolomite, Three quartz arenite beds and the presence of 1% to 5% well-rounded quartz sand grains in many dolostones denote an intermittent elastic source, E-1 dolomite began as syndepositional dolomicrite deposited in ail upper intertidal to supratidal environment, and it was later modified during shallow burial. E-2 dolomite formed when subtidal grainstones and packstones were dolomitized during shallow burial. L-l is a late void fill. Texture and geochemistry of dolostone in the DGHS well is similar to that from coeval Appalachian, Ozark, and Texas-Oklahoma units. Numerous tugs, fractures. and zones of rubble occur throughout the Upper Knox core, suggesting that multiple exposures occurred during its deposition. There is no pattern of change with depth fur these features or for any other measured or observed parameter. We suggest that massive dolostone formation-involving dolomite grain growth, calcite replacement, and dolomite cement-took place in association with a succession of exposure-related events, The large volume of dolomite thus created required a large amount of magnesium, so large in fact that a volume of seawater equal to that of the entire modern ocean must have pumped through collective Upper Knox exposure surfaces.
Article
The origin of replacement dolomites in the Western Canada Sedimentary Basin (WCSB) remains controversial and problematic. In order to meet the considerable magnesium mass-balance requirements, a wide range of groundwater-flow systems, encompassing all diagenetic settings, has been invoked to explain their formation. One of the previously proposed dolomitizing flow systems is reflux circulation. Reflux circulation occurs in response to differences in fluid density controlled by variations in salinity. The restriction and evaporation of seawater can result in the generation of dense platform-top brines. These brines are potential dolomitizing fluids that can descend into underlying pore networks under the influence of gravity. We used a numerical groundwater flow model to investigate the fluid-flow constraints of reflux dolomitization in carbonate platforms. Specifically, we investigated the sensitivity of the pattern and magnitude of fluid flux to the platform-top (seawater) boundary condition, the concentration of platform-top brines and critical hydraulic parameters (permeability and permeability anisotropy). The pre-existing model of reflux employed in the WCSB has an unrealistic uniform distribution of fluid flux, which we have termed the 'constant flux' model, or CFM. In contrast to the CFM, our simulations of reflux circulation incorporate a more realistic platform-top seawater boundary condition, which is open, rather than closed, to flow. Open-topped models generate spatially dependent fluid-flux distributions and are termed here 'variable-flux' models, or VFM. Results with the VFM, for an optimal parameter configuration designed to maximize lateral flow, indicate that almost all refluxing brines discharge within 30 km of the brine's source. The spatial distribution of dolomitization times calculated using the VFM demonstrate that the CFM grossly underestimates dolomitization times for most of the platform, but overestimates dolomitization times close to the brine source where fluid flux is greatest. Calculated dolomitization times with our optimal parameter configuration indicate that a region within 15 km of the brine source could be dolomitized in the 16 Ma maximum time constraint available in the WCSB during the Devonian. Regions beyond 15 km from the brine source remain undolomitized. With our VFM it is difficult to reconcile how reflux circulation could pervasively dolomitize Devonian WCSB carbonate platforms with length scales of tens to hundreds of kilometers. The differences in the flow fields and corresponding calculated dolomitization times between the VFM and CFM models clearly demonstrate the inadequacy of the pre-existing CFM. Thus, the proposal that regional-scale reflux dolomitization accounts for most of the Devonian dolostones in the WCSB is unsubstantiated. Furthermore dolomitization times calculated using the CFM cannot be applied to other dolomites of reflux origin in the geological record.
Article
Despite intensive research over more than 200 years, the origin of dolomite, the mineral and the rock, remains subject to considerable controversy. This is partly because some of the chemical and/or hydrological conditions of dolomite formation are poorly understood, and because petrographic and geochemical data commonly permit more than one genetic interpretation. This paper is a summary and critical appraisal of the state of the art in dolomite research, highlighting its major advances and controversies, especially over the last 20–25 years. The thermodynamic conditions of dolomite formation have been known quite well since the 1970s, and the latest experimental studies essentially confirm earlier results. The kinetics of dolomite formation are still relatively poorly understood, however. The role of sulphate as an inhibitor to dolomite formation has been overrated. Sulphate appears to be an inhibitor only in relatively low-sulphate aqueous solutions, and probably only indirectly. In sulphate-rich solutions it may actually promote dolomite formation. Mass-balance calculations show that large water/rock ratios are required for extensive dolomitization and the formation of massive dolostones. This constraint necessitates advection, which is why all models for the genesis of massive dolostones are essentially hydrological models. The exceptions are environments where carbonate muds or limestones can be dolomitized via diffusion of magnesium from seawater rather than by advection. Replacement of shallow-water limestones, the most common form of dolomitization, results in a series of distinctive textures that form in a sequential manner with progressive degrees of dolomitization, i.e. matrix-selective replacement, overdolomitization, formation of vugs and moulds, emplacement of up to 20 vol% calcium sulphate in the case of seawater dolomitization, formation of two dolomite populations, and — in the case of advanced burial — formation of saddle dolomite. In addition, dolomite dissolution, including karstification, is to be expected in cases of influx of formation waters that are dilute, acidic, or both. Many dolostones, especially at greater depths, have higher porosities than limestones, and this may be the result of several processes, i.e. mole-per-mole replacement, dissolution of unreplaced calcite as part of the dolomitization process, dissolution of dolomite due to acidification of the pore waters, fluid mixing (mischungskorrosion), and thermochemical sulphate reduction. There also are several processes that destroy porosity, most commonly dolomite and calcium sulphate cementation. These processes vary in importance from place to place. For this reason, generalizations about the porosity and permeability development of dolostones are difficult, and these parameters have to be investigated on a case-by-case basis. A wide range of geochemical methods may be used to characterize dolomites and dolostones, and to decipher their origin. The most widely used methods are the analysis and interpretation of stable isotopes (O, C), Sr isotopes, trace elements, and fluid inclusions. Under favourable circumstances some of these parameters can be used to determine the direction of fluid flow during dolomitization. The extent of recrystallization in dolomites and dolostones is much disputed, yet extremely important for geochemical interpretations. Dolomites that originally form very close to the surface and from evaporitic brines tend to recrystallize with time and during burial. Those dolomites that originally form at several hundred to a few thousand metres depth commonly show little or no evidence of recrystallization. Traditionally, dolomitization models in near-surface and shallow diagenetic settings are defined and/or based on water chemistry, but on hydrology in burial diagenetic settings. In this paper, however, the various dolomite models are placed into appropriate diagenetic settings. Penecontemporaneous dolomites form almost syndepositionally as a normal consequence of the geochemical conditions prevailing in the environment of deposition. There are many such settings, and most commonly they form only a few per cent of microcrystalline dolomite(s). Many, if not most, penecontemporaneous dolomites appear to have formed through the mediation of microbes. Virtually all volumetrically large, replacive dolostone bodies are post-depositional and formed during some degree of burial. The viability of the many models for dolomitization in such settings is variable. Massive dolomitization by freshwater-seawater mixing is a myth. Mixing zones tend to form caves without or, at best, with very small amounts of dolomite. The role of coastal mixing zones with respect to dolomitization may be that of a hydrological pump for seawater dolomitization. Reflux dolomitization, most commonly by mesohaline brines that originated from seawater evaporation, is capable of pervasively dolomitizing entire carbonate platforms. However, the extent of dolomitization varies strongly with the extent and duration of evaporation and flooding, and with the subsurface permeability distribution. Complete dolomitization of carbonate platforms appears possible only under favourable circumstances. Similarly, thermal convection in open half-cells (Kohout convection), most commonly by seawater or slightly modified seawater, can form massive dolostones under favourable circumstances, whereas thermal convection in closed cells cannot. Compaction flow cannot form massive dolostones, unless it is funnelled, which may be more common than generally recognized. Neither topography driven flow nor tectonically induced (‘squeegee-type’) flow is likely to form massive dolostones, except under unusual circumstances. Hydrothermal dolomitization may occur in a variety of subsurface diagenetic settings, but has been significantly overrated. It commonly forms massive dolostones that are localized around faults, but regional or basin-wide dolomitization is not hydrothermal. The regionally extensive dolostones of the Bahamas (Cenozoic), western Canada and Ireland (Palaeozoic), and Israel (Mesozoic) probably formed from seawater that was ‘pumped’ through these sequences by thermal convection, reflux, funnelled compaction, or a combination thereof. For such platform settings flushed with seawater, geochemical data and numerical modelling suggest that most dolomites form(ed) at temperatures around 50–80 °C commensurate with depths of 500 to a maximum of 2000 m. The resulting dolostones can be classified both as seawater dolomites and as burial dolomites. This ambiguity is a consequence of the historical evolution of dolomite research.
Article
Magnesium concentrations in deep-sea sediment pore-fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust. To better characterize and differentiate these processes, we have measured magnesium isotopes in pore-fluids and sediment samples from Ocean Drilling Program sites (1082, 1086, 1012, 984, 1219, and 925) that span a range of oceanographic settings. At all sites, magnesium concentrations decrease with depth. At sites where diagenetic reactions are dominated by the respiration of organic carbon, pore-fluid δ26Mg values increase with depth by as much as 2‰. Because carbonates preferentially incorporate 24Mg (low δ26Mg), the increase in pore-fluid δ26Mg values at these sites is consistent with the removal of magnesium in Mg-carbonate (dolomite). In contrast, at sites where the respiration of organic carbon is not important and/or weatherable minerals are abundant, pore-fluid δ26Mg values decrease with depth by up to 2‰. The decline in pore-fluid δ26Mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore-fluid. The identity of this enriched magnesium sink is likely clay minerals. Using a simple 1D diffusion–advection–reaction model of pore-fluid magnesium, we estimate rates of net magnesium uptake/removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites. Independent estimates of magnesium isotope fractionation during dolomite precipitation from measured δ26Mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites, suggesting that local exchange of magnesium between sediment and pore-fluid at these sites can be neglected. Our results indicate that the magnesium incorporated in dolomite is 2.0–2.7‰ depleted in δ26Mg relative to the precipitating fluid. Assuming local exchange of magnesium is minor at the rest of the studied sites, our results suggest that magnesium incorporated into clay minerals is enriched in δ26Mg by 0‰ to +1.25‰ relative to the precipitating fluid. This work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources/sinks in low-temperature aqueous systems.
Article
The principal models in vogue today for dolomitization are the mixing zone and the sabkha models. Despite the wide acceptance of these models, there has been little critical assessment of their validity. Such an assessment is the objective of the present paper. Mixing-zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization. Contemporaneous dolomite formation in modern sabkhas is well documented, but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved. A third dolomitization model considered here is that of Baker and Kastner (1981) based on the experimental finding that sulfate ions inhibit or retard dolomitization. This model should be held in abeyance until serious contradictions are resolved. The current emphasis on mixing-zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem. Four of these avenues, each of which deemphasizes the 'special water' approach, are briefly addressed and are as follows: 1) influence of temperature and time; 2) mass transfer processes; 3) burial diagenesis of epigenetic dolomites; 4) fluid-inclusion studies. -from Author
Article
Based on present knowledge of the purely chemical controls on the kinetics of massive dolomite formation, the abundance and distribution of dolomite throughout the Phanerozoic remains an enigma, sometimes referred to as the ‘dolomite problem'. Comparing dolomite abundance to secular variation in seawater chemistry indicates that some changes in seawater chemistry are more likely to have resulted from extensive dolomitization rather than to have caused it. The recently formulated microbial dolomite model provides the opportunity to view the geological history of dolomite formation from a new perspective. A biogeochemical approach to the ‘dolomite problem' reveals a plausible connection between Phanerozoic geochemical cycles and dolomite formation. In particular, periods of more extensive dolomitization broadly correlate with diverse indicators of decreased oxygen levels in the atmosphere and oceans. Lowered oxygen levels would have fostered a more active community of anaerobic microbes, including sulphate-reducing bacteria, which in turn could have led to more extensive dolomitization of marine carbonates.
Article
Mg isotope ratios (26Mg/24Mg) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant 26Mg/24Mg ratio (expressed as δ26Mg) at −0.79 ± 0.05‰, identical to seawater δ26Mg. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a δ26Mg value of 0.11‰, potentially enriched in 26Mg by up to 0.3‰ compared to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for δ26Mg, ranging from −0.99‰ near the surface to −0.43‰ at the base of the profile. Shallow pore-waters (<1 m) have δ26Mg values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as <32%, calculated using the average Mg isotope enrichment factor between grass and rain (δ26Mggrass-δ26Mgrain) of 0.21‰. The deep pore-waters (1–15 m deep) have δ26Mg values that are intermediate between the smectite and rain, ranging from −0.76‰ to −0.43‰, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation inputs. A consistent interpretation is only possible if Mg isotope ratios are fractionated either by the precipitation of a secondary Mg bearing phase, not detected by conventional methods, or selective leaching of 24Mg from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering.
Article
The Paleocene carbonate succession in the Northeast Sirte Basin is composed of two shallowing-upward ramp cycles, where each cycle is under- and overlain by deeper-water, pelagic facies. A significant proportion of each of these two cycles is dolomitized. Petrographic study, supported by geochemical data (stoichiometry, stable isotopes, trace elements, and fluid inclusions), and integrated with broader tectono-sedimentary information, has provided the basis for interpreting these Paleocene dolomites. The use of this integrated approach in the study of dolomites suggests that, despite the much publicized uncertainties in interpreting geochemical analyses of ancient dolomites, the results of the Paleocene dolomites show that the geochemical characteristics are generally consistent with regional stratigraphic distribution and petrographic observations. Four distinct types of dolomite have been recognized in this part of the Sirte Basin. Based on the stratigraphic position and petrographic criteria, two of these types have a platformal setting and the other two are basinal. The platform varieties consist of dolomicrites and pervasive stratal dolomites. The dolomicrites, interpreted to be of syn-sedimentary origin, were probably a product of reflux of seawater, with elevated salinity, as suggested by palaeoenvironmental analysis and supported by geochemical evidence (the average S'80 value is −0.1‰ PDB; the average Sr content is 639 ppm). The pervasive dolomites were formed during the progradation of the platform sequences, and probably stabilized and augmented during shallow burial. A meteoric-marine mixing-zone is thought to have been the most likely process for the formation of these dolomites. This interpretation is supported by geochemical evidence (the average δ18O is −2.4‰ PDB; the average Sr content is 72 ppm) combined with a favourable stratigraphic position. The most characteristic feature related to both mixing-zone and reflux dolomitization is the basinward movement of the dolomitizing fluids, which suggests that the formation of these platform dolomites was related to a lowstand system tract. The two basinal varieties comprise thick (over 300 m) basinal dolomudstones and fracture-filling, sparry dolomites. The stratigraphic position of the finely crystalline basinal dolomudstones, within very thick shale successions (as a result of being very close to the depocentre of the Sirte Basin) combined with geochemical evidence (the average δ18O is −6.4‰ PDB), suggest that the dolomitizing fluids were basin-derived, with Mg2+ released from dewatering through compaction of basinal shales. The occurrence of this type of dolomite provides one of the rare examples of large-scale dolomitization of thick, basinal sequences. Late diagenetic fracture-filling dolomites exhibit a structural control on their distribution. Geochemical evidence (including fluid inclusion analysis and the lightest oxygen isotopic signature of −7.3‰ PDB) suggests that highly saline formation brines were the solutions responsible for their formation.