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Conodont (U–Th)/He thermochronology: Initial results, potential, and problems
Abstract
We performed He diffusion experiments and (U–Th)/He age determinations on conodonts from a variety of locations to explore the potential of conodont (U–Th)/He thermochronology to constrain thermal and exhumation histories of some sedimentary-rock dominated terrains. Based on two diffusion experiments and age results from some specimens, He diffusion in conodont elements appears to be similar to that in Durango apatite fragments of similar size, and closure temperatures are approximately 60–70 °C (for cooling rates of ∼ 10 °C/m.y.). (U–Th)/He ages of conodonts from some locations yield reproducible ages consistent with regional thermal history constraints and, in at least two cases, require a closure temperature lower than ∼ 80 °C. Other samples however, yield irreproducible ages, and in one case yield ages much younger than expected based on regional geologic considerations. These irreproducible samples show inverse correlations between parent nuclides and age consistent with late-stage open-system U–Th behavior.
... Conodonts may offer a way for using the (U-Th)/He method in carbonate successions (Peppe and Reiners, 2007;Powell et al., 2018;Landman et al., 2016). These tooth-like microfossils are common in Cambrian through Triassic carbonates and shales and are preserved as biomineralized apatite with U and Th in concentrations that are comparable to magmatic sources of apatite (Trotter and Eggins, 2006;Peppe and Reiners, 2007). ...
... Conodonts may offer a way for using the (U-Th)/He method in carbonate successions (Peppe and Reiners, 2007;Powell et al., 2018;Landman et al., 2016). These tooth-like microfossils are common in Cambrian through Triassic carbonates and shales and are preserved as biomineralized apatite with U and Th in concentrations that are comparable to magmatic sources of apatite (Trotter and Eggins, 2006;Peppe and Reiners, 2007). The microfossils, referred to as elements, were part of the feeding apparatus of the host animal. ...
... Thus, conodonts are particularly attractive because successful use as a (U-Th)/He thermochronometer means that a single conodont element, when paired with other geologic data, could provide three independent time and temperature constraints-cooling age, biostratigraphic age, and maximum temperature. Peppe and Reiners (2007) was the first published study to explore the utility of conodonts as (U-Th)/He thermochronometers. Successful aspects of their study included determining that the closure temperature for conodonts is similar to magmatic apatite (60-67°C) and that the diffusion domain likely corresponds to the whole of the conodont element. ...
(U-Th)/He thermochronology is a well-established dating technique used to understand the temperature-time histories of rocks in a wide range of geologic settings. The technique is presently restricted to rocks that contain specific accessory minerals, such as apatite or zircon. Marine carbonates and shales typically lack these accessory phases in quantities and sizes practical for (U-Th)/He dating and thus present a challenge for application of the method. Here, we explore the utility of biogenic apatite from conodonts as a (U-Th)/He thermochronometer at a well-studied calibration site located in eastern Nevada and southwestern Utah. We performed (U-Th)/He thermochronometry, laser ablation inductively coupled plasma mass spectrometry, X-ray micro-computed tomography, and scanning electron microscopy (SEM) on specimens with conodont color alteration indices (CAI) of 1.5–3, extracted from carbonate rocks in the footwalls of low-angle normal faults in the Mormon Mountains, Tule Spring Hills, and Beaver Dam Mountains. Conodont (U-Th)/He (CHe) dates have high scatter; dates are commonly reproducible to 20% of sample means but can deviate up to 150%. All CAI 1.5–2.5 conodonts produce CHe dates younger than 240 Ma, consistent with thermal resetting of samples; however, most CAI 3 conodonts give ages 2–6× older than Mississippian and Permian deposition. Average U, Th, and rare earth element (REE) concentrations depend on porosity and permeability differences between albid and hyaline conodont tissue and range from <10 to 100 s of ppm in concentration. Parent isotope concentrations are especially low in CAI 3 conodonts, commonly <1 ppm, and there is an inverse relationship between these concentrations and CHe dates. The majority of parent U, Th, and Sm and REEs are concentrated within the outer 5 μm of the conodont elements and consistently show 5–10× enrichment relative to cores. Margin enrichment is also depressed with increasing CAI. SEM imaging shows a shift in the orientation of apatite microcrystallites from perpendicular to parallel to the major axes of the conodont elements at CAI 3 and corrosion and recrystallization features, likely associated with burial and diagenesis, on the surfaces of some CAI 2.5 and 3 conodonts. We propose that microstructural changes associated with increasing CAI influence CHe dates. Parent isotope loss occurs during the post-cooling stage, either in the outcrop or in the laboratory. Our hypothesis is that the double-buffered formic acid procedure for dissolving dolomitized carbonates may accelerate this loss in thermally altered, higher CAI conodonts.
... He implantation study of calcite, dolomite, magnesite, and aragonite provides evidence that typical carbonate materials have minimal He retentivity at Earth surface conditions (Cherniak et al., 2015). He diffusion study of conodont bioapatite suggests a He retentivity similar to that of magmatic apatite (Peppe and Reiners, 2007), but conodont (U-Th)/ He dating has yielded results of variable reproducibility, likely due to late-stage parent isotope loss and diagenetic processes (Peppe and Reiners, 2007;Landman et al., 2016;Bidgoli et al., 2018;Powell et al., 2018). Allanite with high Th content has been used to date Quaternary volcanic rocks in eastern California (Cox et al., 2012). ...
... He implantation study of calcite, dolomite, magnesite, and aragonite provides evidence that typical carbonate materials have minimal He retentivity at Earth surface conditions (Cherniak et al., 2015). He diffusion study of conodont bioapatite suggests a He retentivity similar to that of magmatic apatite (Peppe and Reiners, 2007), but conodont (U-Th)/ He dating has yielded results of variable reproducibility, likely due to late-stage parent isotope loss and diagenetic processes (Peppe and Reiners, 2007;Landman et al., 2016;Bidgoli et al., 2018;Powell et al., 2018). Allanite with high Th content has been used to date Quaternary volcanic rocks in eastern California (Cox et al., 2012). ...
The field of (U-Th)/He geochronology and thermochronology has grown enormously over the past ∼25 years. The tool is applicable across much of geologic time, new (U-Th)/He chronometers are under continuous development, and the method is used in a diverse array of studies. Consequently, the technique has a rapidly expanding user base, and new labs are being established worldwide. This presents both opportunities and challenges. Currently there are no universally agreedupon protocols for reporting measured (U-Th)/He data or data derivatives. Nor are there standardized practices for reporting He diffusion kinetic, 4He/3He, or continuous ramped heating data. Approaches for reporting uncertainties associated with all types of data also vary widely. Here, we address these issues. We review the fundamentals of the methods, the types of materials that can be dated, how data are acquired, the process and choices associated with data reduction, and make recommendations for data and uncertainty reporting. We advocate that both the primary measured and derived data be reported, along with statements of assumptions, appropriate references, and clear descriptions of the methods used to compute derived data from measured values. The adoption of more comprehensive and uniform approaches to data and uncertainty reporting will enable data to be re-reduced in the future with different interpretative contexts and data reduction methods, and will facilitate inter-comparison of data sets generated by different laboratories. Together, this will enhance the value, cross-disciplinary use, reliability, and ongoing development of (U-Th)/He chronology.
... Those changes are largely restricted to the conodont surface for CAI of 1-5, but above CAI of 5, internal re-crystallization may occur (Burnett 1988;Helson 1994;Nöth 1998). Notably, using conodonts as a (U-Th)/He thermochronometer has witnessed some success (Peppe and Reiners 2007;Landeman et al. 2016;Powell et al. 2018) and key advantages of conodonts over traditional thermochronometers are that conodonts can be routinely used as geothermometers and commonly occur in limestones. Although the role of CAI on the microstructural character of conodonts is clear, how those changes impact parent isotope distributions and mobility, He diffusivity, and (U-Th)/He ages is unknown. ...
... The toolbox for deciphering the thermal histories of hydrocarbon-bearing regions is growing, and advancing somewhat rapidly, particularly the numerical modeling. Several new low-temperature chronometers are being developed for application in sedimentary basins, including calcite (Copeland et al. 2007;Cros et al. 2014;Pagel et al. 2018) and fossils such as conodonts (Peppe and Reiners 2007) and crinoids (Copeland et al. 2015). These are particularly exciting since the science currently lacks reliable chronometers for carbonate-dominated basins. ...
The maturation of organic material into petroleum in a sedimentary basin is controlled by the maximum temperatures attained by the source rock and the thermal history of the basin. A cycle of continuous deposition into the basin (burial) and regional basin inversions represented by unconformities (unroofing) may complicate the simple thermal development of the basin. Applications of low-temperature thermochronology via fission-track (FT) and (U–Th)/He dating coupled with independent measurements (vitrinite reflectance, Rock-Eval) resolving the paleothermal maximum are the ideal approach to illuminate the relationship between time and temperature. In this contribution, we review the basics of low-temperature thermochronology in the context of a project workflow, from sampling to modeling, for resolving the thermal evolution of a hydrocarbon-bearing sedimentary basin. We specifically highlight the application of multi-kinetic apatite FT dating, emphasizing the usefulness of the rmr0 parameter for interpreting complex apatite age populations that are often present in sedimentary rocks. Still a rapidly advancing science, thermochronology can yield a rich and effective dataset when the minerals are carefully and properly characterized, particularly with regard to mineral chemistry and radiation damage.
... Key assumptions underlying such geochronological studies are (1) short timescales of fossilisation and early diagenetic trace element and REE uptake (<100 ka), and (2) long term closed system behaviour of fossil bones and teeth with respect to the elements under investigation. U–Pb and fission track dating as well as (U/Th)/He thermochronology of skeletal apatite, has suggested open system behaviour for these elements (Romer, 2001; Peppe and Reiners, 2007; Jolivet et al., 2008 ). However, the discussion about U–Pb remains ambiguous, as a recent study by Balter et al. (2008) interprets the observed U and Pb isotope compositions in fossil teeth to result from early diagenetic uptake, rather than linear uptake. ...
... Such features are usually attributed to complex burial histories of skeletal apatite or porous bone microstructure and cracks. Furthermore, late diagenetic open system behaviour for U (Peppe and Reiners, 2007 ) has been observed, and in a case study on a Cretaceous bone sample from Madagascar (ANE 901), a fossilisation age of at least 1 ma has been modelled (Koenig et al., 2009). Toyoda and Tokanami (1990) proposed a continuous uptake model for La in dentine of a fossil fish tooth. ...
Rare earth element (REE) patterns of fossil bones and teeth are widely used as proxies for provenance, taphonomy, and palaeoenvironment. In order to investigate if fossil bones behave as closed systems over geologic time, REE profiles were analysed by LA-ICPMS along cross sections of 54 bones from various well-characterised and well-dated settings. These include terrestrial and marine diagenetic environments, covering Early Triassic to Holocene ages. In general, all fossil bones exhibit the highest REE concentrations at the outer rim, gradually decreasing by up to four orders of magnitude toward the inner bone cortex. Intra-bone REE concentration gradients decrease significantly from Quaternary via Tertiary to Mesozoic specimens, suggesting long term REE uptake and open system behaviour of fossil bone. This view is further corroborated by 176Lu–176Hf dating of selected samples, all yielding significantly younger ages than the known chronostratigraphic ages. Hence, there is clear evidence for long term open system behaviour of fossil bones with respect to REE, which is in marked contrast to currently accepted models suggesting that REE uptake is only early diagenetic. Although unexpected, statistically significant four to seven point isochrons are observed for four fossil dinosaur bone samples and one Upper Triassic Mastodonsaurus tooth with MSWDs ranging from 0.083 to 4.5. Notably, mobility of Lu alone cannot account for the observed age patterns. Assuming constant Lu uptake rates over time, the radiometric ages should only be as low as half of the chronostratigraphic age. However, a six-point isochron defined by subsamples of a single Upper Triassic Mastodonsaurus tooth yields an age of 65.2 ± 1.1 Ma (MSWD = 0.68), much younger than half of the stratigraphic age (ca. 234 Ma). Hence, Hf must also undergo late diagenetic exchange. Likely mechanisms to account for the presence of statistically meaningful isochrons as well as for the late diagenetic exchange of both REE and Hf are diffusion, adsorption, and dissolution–reprecipitation processes.
... Compositional gradients on bone edges contrast with homogeneous REE and U in the interior of some of these same samples (Janssens et al., 1999), and together these data imply compositional homogenization during initial fossilization, with some susceptibility to overprinting on surfaces either during the late stages of fossilization, or perhaps after fossilization. Note that uptake of U millions of years after deposition is also implied in the age systematics of some materials (e.g., see Peppe and Reiners, 2007), although the location of this ''young " U is not well documented . High U concentrations have inspired numerous U-series chronologic investigations, and several studies demonstrate older U-series ages towards the external and internal surfaces of cortical bone relative to its interior (e.g., see Millard and Hedges, 1996; Pike et al., 2001 Pike et al., , 2005). ...
... For example, paleoclimate studies assume fossilization rates of tens of kyr or less for interpretation of geologic processes on timescales of hundreds of kyr to Myr (Staudigel et al., 1985; Elderfield and Pagett, 1986; Martin and Haley, 2000; Kohn and Law, 2006; MacFadden et al., 2007; Zanazzi et al., 2007). The data presented here conclusively support such assumptions, although instances of late-stage uptake do occur (e.g., see Peppe and Reiners, 2007 ). In contrast, many archeological applications focus on shorter timescales, and attempt to date initial uptake, i.e., determine the extrapolated age at the edge of a fossil based on direct geochronology across the sample. ...
Many fossils are assumed to take up trace elements by a process of combined diffusion plus adsorption (DA), yet in principle composition profiles can be explained by several different diffusion-limited processes, including diffusion plus reaction or recrystallization (DR) and double-medium diffusion (DMD). The DA and DMD models are supported by REE and U composition profiles across fossil teeth, measured by laser-ablation ICP–MS, that show error-function – like diffusion profiles into enamel from the dentine–enamel interface and concentrations in the interior of enamel that are at original biogenic levels or higher. Published composition and age profiles in some Pleistocene bones may be better explained by a DR model. All three diffusion models imply linear behavior between age and distance squared, vastly simplifying U-series dating methods for Pleistocene fossils. Modeled uptake rates for fossil teeth yield a strict minimum bound on durations of about one decade to one century. The similarity of diffusion profiles in teeth, irrespective of depositional ages ranging from ~30 ka to >30 Ma, implies that uptake occurred quickly, with a maximum duration of a few tens of kyr for typical fossil enamel; faster uptake is implied for typical fossil bone and dentine. Disparities in these uptake estimates compared to some archeological bone may reflect sampling and preservation bias for paleontological vs. archeological materials.
... A quick assessment of REE sources can be based on the Th vs ΣREE and Y/Ho vs ΣREE cross-plots (Li et al., 2017). In fact, high ΣREE (sum of all REE content) and strong LREE (light REE, i.e., sum of La and Ce) and Th enrichment characterize the pore-water contribution (McLennan, 2001;Peppe and Reiners, 2007;Shen et al., 2012;Wright and Colling, 1995); on the other hand, high Y/Ho are indicative of an hydrogen signature as, in modern ocean water, Ho is adsorbed or complexed at about twice the rate of Y (Nozaki et al., 1997;Zhang et al., 1994;Zhang and Nozaki, 1996). More difficult is the interpretation of MREE (middle REEs, i.e., sum of Pr, Nd, Sm, Eu) and HREE (heavy REEs, i.e., sum of Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) enrichments as their contribution could reflect an authigenic phosphate signal (Bright et al., 2009;Reynard et al., 1999;Sholkovitz and Shen, 1995), further complicating the interpretation of the results. ...
... Calcite, a constituent of limestone, mudstone, and fossils, has a T c of~40-80°C and common (initial) 4 He issues are mitigated by analyzing samples that have experienced significant postdepositional reheating (Copeland et al., 2007;Copeland et al., 2015;Cros et al., 2014). Conodont microfossils (bioapatite) are a target for (U-Th)/He analysis applied to carbonates and shales and the T c mirrors that of the traditional apatite system (~60°C; Landman et al., 2016;Peppe & Reiners, 2007). Ongoing research aimed at quantifying the diffusion kinetics of each of the aforementioned systems, coupled with textural and geochemical information and data comparisons with established low-temperature thermochronometry systems, will continue to expand the utility of new (U-Th)/He thermochronometers. ...
A transformative advance in Earth science is the development of low‐temperature thermochronometry to date Earth surface processes or quantify the thermal evolution of rocks through time. Grand challenges and new directions in low‐temperature thermochronometry involve pushing the boundaries of these techniques to decipher thermal histories operative over seconds to hundreds of millions of years, in recent or deep geologic time and from the perspective of atoms to mountain belts. Here we highlight innovation in bedrock and detrital fission track, (U–Th)/He, and trapped charge thermochronometry, as well as thermal history modeling that enable fresh perspectives on Earth science problems. These developments connect low‐temperature thermochronometry tools with new users across Earth science disciplines to enable transdisciplinary research. Method advances include radiation damage and crystal chemistry influences on fission track and (U–Th)/He systematics, atomistic calculations of He diffusion, measurement protocols and numerical modeling routines in trapped charge systematics, development of ⁴He/³He and new (U–Th)/He thermochronometers, and multimethod approaches. New applications leverage method developments and include quantifying landscape evolution at variable temporal scales, changes to Earth's surface in deep geologic time and connections to mantle processes, the spectrum of fault processes from paleoearthquakes to slow slip and fluid flow, and paleoclimate and past critical zone evolution. These research avenues have societal implications for modern climate change, groundwater flow paths, mineral resource and petroleum systems science, and earthquake hazards.
... unlikely that trace element leaching into enamel stopped after several hundreds of years (Peppe and Reiners, 2007). Instead, diffusion rates probably slowed down over time, causing modelled diffusion times to underestimate the actual time it took to establish diffusion fronts. ...
High resolution in situ trace element μXRF maps and profiles were measured on the enamel exposed in cross sections through archaeological human permanent molars from seven Late Neolithic/Early Chalcolithic funerary caves and megalithic graves of north-central Iberia. Changes in concentrations of Fe, Zn and Sr in inward direction into the enamel shed light on diagenetic and endogenous trace element concentrations in archaeological tooth enamel. Most of these profiles resemble sigmoid-shaped leaching profiles, suggesting that a combination of diffusion and advection processes govern the uptake of trace elements into the enamel from pore fluids on the outside of the tooth and in the more porous dentine. The present study shows how diffusion-advection (DA) models can be fitted to these trace element profiles to explain changes in trace element concentrations that happen during diagenesis. DA models explain a major part of the variation observed in leaching profiles into the enamel and can be used to reconstruct endogenous trace element concentrations, leaching times and leaching depth as well as trace element concentrations in ambient pore water during diagenesis. Models of trace element leaching together with trace element mapping reveal that Fe, Zn and Sr concentrations consistently increase during diagenesis, regardless of the type of burial site (i.e. funerary caves vs. megalithic graves). Profiles of Pb concentrations show much smaller concentration gradients, causing DA model fitting to be less accurate. Modelled leaching depths of 300–400 μm warrant a careful approach when sampling for endogenous archaeological tooth enamel for trace element and stable isotope analysis. Results also show that it is possible to reconstruct endogenous trace element concentrations from these samples, even without applying pretreatment procedures, because leaching of trace elements into the enamel often remains limited to the outer 300–400 μm of the enamel on archaeological timescales. Modelled leaching times are about ten times lower than the age of the samples, suggesting that the rate of trace element leaching into tooth enamel slows down or even halts during the burial period.
... Most REEs have a very short residence time in seawater (~300-1,000 yr) and are ultimately removed to the sediment via adsorption onto colloids, Fe-Mn oxyhydroxides, and phosphatic phases (Sholkovitz, 1992(Sholkovitz, , 1993Sholkovitz et al., 1994Sholkovitz et al., , 1999Byrne et al., 1996;Chen et al., 2015). However, in ancient marine sediments containing even small quantities of clay minerals (Al >0.5 %), release of clay-hosted REEs during diagenesis and subsequent adsorption or structural uptake by biogenic and secondary carbonate and phosphate phases makes recovery of a hydrogenous signature nearly impossible (Peppe and Reiners, 2007;Shen et al., 2012). Recovery of a hydrogenous signature is mostly likely in (bio)chemical sediments that are free (or nearly so) of detrital siliciclastics (e.g., Whittaker and Kyser, 1993;Sholkovitz and Shen, 1995;Webb and Kamber, 2000;Kamber and Webb, 2001;Nothdurft et al., 2004;Webb et al., 2009). ...
Paleoceanic environmental stresses (e.g., expanded anoxia, elevated siliciclastic fluxes) are thought to have been important factors in the latest Permian mass extinction (LPME). Here, we investigate changes in redox conditions and siliciclastic fluxes during the Permian-Triassic transition in South China through in-situ analysis of the rare earth element (REE) composition of albid crowns of single conodont elements measured using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). For comparison, we also analyzed REEs in the sediment matrix enclosing the conodonts (i.e., whole-rock samples). The study locale (Yangou) was located on a shallow carbonate platform on the eastern margin of the Yangtze Platform to minimize the influence of detrital siliciclastics and, thus, enhance the chances of recovering a hydrogenous (seawater-sourced) REE signal.
... For components preserving a seawater REE signature, cerium (Ce) anomalies can provide information about the redox conditions of the watermass (Elderfield, 1988;German and Elderfield, 1990), and REE distributions can provide information regarding fractionation in conjunction with adsorption and release of REEs onto colloids (Sholkovitz, 1992(Sholkovitz, , 1993Sholkovitz et al., 1994) and phosphatic phases (Byrne et al., 1996;Sholkovitz et al., 1999). For whole-rock samples having a REE fraction dominated by terrigenous sources, REE distributions can provide information about changes in weathering fluxes and fractionation between solid and aqueous phases (Peppe and Reiners, 2007;Shen et al., 2012aShen et al., , 2012c. Some studies have attempted to recover the REE composition of palaeo-seawater through analysis of the carbonate components of marine sediments, e.g., molluscs (Whittaker and Kyser, 1993), corals (Sholkovitz and Shen, 1995;Webb et al., 2009), and microbialites or stromatolites (Webb and Kamber, 2000;Kamber and Webb, 2001; the preserved REE distributions were acquired from seawater or from sediment porewaters containing REE of non-hydrogenous origin (Reynard et al., 1999;Shields and Stille, 2001;Shields and Webb, 2004). ...
... For components preserving a seawater REE signature, cerium (Ce) anomalies can provide information about the redox conditions of the watermass (Elderfield, 1988;German and Elderfield, 1990), and REE distributions can provide information regarding fractionation in conjunction with adsorption and release of REEs onto colloids (Sholkovitz, 1992(Sholkovitz, , 1993Sholkovitz et al., 1994) and phosphatic phases (Byrne et al., 1996;Sholkovitz et al., 1999). For whole-rock samples having a REE fraction dominated by terrigenous sources, REE distributions can provide information about changes in weathering fluxes and fractionation between solid and aqueous phases (Peppe and Reiners, 2007;Shen et al., 2012aShen et al., , 2012c. Some studies have attempted to recover the REE composition of palaeo-seawater through analysis of the carbonate components of marine sediments, e.g., molluscs (Whittaker and Kyser, 1993), corals (Sholkovitz and Shen, 1995;Webb et al., 2009), and microbialites or stromatolites (Webb and Kamber, 2000;Kamber and Webb, 2001; the preserved REE distributions were acquired from seawater or from sediment porewaters containing REE of non-hydrogenous origin (Reynard et al., 1999;Shields and Stille, 2001;Shields and Webb, 2004). ...
Rare-earth element (REE) profiles were generated from conodont bioapatite for two Permian–Triassic boundary sections in South China (Meishan and Daxiakou) in order to investigate environmental changes following the latest Permian mass extinction (LPME). REE concentrations were measured in albid crowns, the conodont histology that is densest and least susceptible to diagenetic alteration, in an effort to recover seawater REE signatures. However, an analysis of REE sources demonstrated that 80–100% of REEs in the study samples were derived from siliciclastic sources, presumably the abundant clay minerals present in the study sections. Interval I (pre-LPME) exhibited lower ΣREE concentrations and distinctly different REE distribution patterns than Intervals II (syn-LPME) and III (post-LPME) of the study sections. REE “fingerprinting” suggests that the latter two intervals contain a large fraction of REEs derived from volcanic clays, characterized by low Eu/Eu* and LaN/YbN and high Th/La ratios. The presence of volcanically derived REEs in post-LPME Interval III indicates that volcanic eruptions continued to spew ash for an extended interval following the boundary crisis or, perhaps more likely, that substantial ash deposits that fell on landmasses during the LPME were slowly eroded and transported to the marine environment. The most probable source of this volcanic ash is the Siberian Traps magmatic province. Ce/Ce* ratios of 0.8–1.0 around the LPME may reflect suboxic to anoxic seawater conditions, although it is uncertain whether Ce in the study sections is mainly of hydrogenous or detrital origin.
... Attempts to date a single Permian shark tooth and a single cancellous dinosaur bone using U-Pb isotopes yielded younger ages than the known sample age due to 222 Rn loss from the tooth (Sano and Terada, 1999;Romer, 2001). Similar problems appeared with the (U-Th)/He thermochronology of conodonts (Peppe and Reiners, 2007) and fission track dating (Jolivet et al., 2008). In all these methods, uptake rate and stability of U in bone is a major consideration and errors in recovered ages may reflect the differential behaviour and mobility of U under different redox conditions. ...
Fossil bones and teeth are potentially important repository for geochemical proxy data and a target for radiometric dating. The concentration of many trace elements in bones and teeth increases by orders of magnitude after death and it is this diagenetic incorporation that forms the basis for several areas of geochemical study. The use of bones and teeth in this context relies on two assumptions: first, that target metal ions are incorporated rapidly after death, reflecting a known environmental signal, and second, that after early incorporation, the bone or tooth remains as an essentially closed system, resistant to later diagenetic change. A wide literature has developed exploring these assumptions, but relatively little direct evidence has been used to assess the long-term diagenetic stability of trace elements within bones and teeth. In this study, we use the Lu–Hf isotope system to show that bones and teeth of Cretaceous and Triassic age from both terrestrial and marine settings experience continued, long-term diagenetic change, most likely through gradual addition of trace elements. Modelling suggests that diagenetic addition after initial recrystallisation may account for >50% of the total REE content in the sampled bones, the extent depending on initial uptake conditions. Tooth enamel and enameloid may be more resistant to late diagenetic changes, but dentine is probably altered to the same extent as bone. These results have significant implications for the use of bones and teeth as hosts of chronological, palaeoceanographic, palaeoenvironmental and taphonomic information, particularly in Mesozoic and Palaeozoic contexts.
Source-to-sink (S2S) analysis examines the effects of catchment (source) and basin (sink) dynamics on the generation, transport, composition, distribution and deposition of sediment in modern and ancient sedimentary systems. In deepwater sedimentary systems, S2S is useful for developing accurate chronostratigraphic frameworks, paleo-geographic maps and tectono-stratigraphic models of basin formation, fill and evolution. These outputs improve subsurface evaluation, reduce geologic uncertainty and guide business decisions in sectors from petroleum and critical mineral exploration to carbon sequestration. The topics presented in this chapter empower geoscientists with a working knowledge of fundamental S2S definitions and concepts, common laboratory analytical and system-modeling techniques (e.g., U-Pb-Hf geochronology and sediment mass balance studies), data interpretation approaches to quantify system processes and project framing workflows for scaling business-relevant projects. S2S is a valuable addition to any subsurface evaluation program that seeks to better characterize deepwater sedimentary systems in their area of interest.
Conodont elements are calcium phosphate (apatite structure) mineralized remains of the cephalic feeding apparatus of an extinct marine organism. Due to the high affinity of apatite for rare earth elements (REE) and other high field strength elements (HFSE), conodont elements were frequently assumed to be a reliable archive of sea-water composition and changes that had occurred during diagenesis. Likewise, the crystallinity index of bioapatite, i.e., the rate of crystallinity of biologically mediated apatite, should be generally linearly dependent on diagenetic alteration as the greater (and longer) the pressure and temperature to which a crystal is exposed, the greater the resulting crystallinity. In this study, we detected the uptake of HFSE in conodont elements recovered from a single stratigraphic horizon in the Upper Ordovician of Normandy (France). Assuming therefore that all the specimens have undergone an identical diagenetic history, we have assessed whether conodont taxonomy (and morphology) impacts HFSE uptake and crystallinity index. We found that all conodont elements are characterized by a clear diagenetic signature, with minor but significant differences among taxa. These distinctions are evidenced also by the crystallinity index values which show positive correlations with some elements and, accordingly, with diagenesis; however, correlations with the crystallinity index strongly depend on the method adopted for its calculation.
Acquisition of thermochronologic data in carbonates and shales has traditionally been elusive owing to the paucity of dateable minerals in these lithologies. Conodont apatite (U–Th)/He (CAHe) thermochronology has the potential to fill this need. We acquired 50 (U–Th)/He dates for conodonts with CAI values ≤1.5 from seven Pennsylvanian shale and limestone samples from two drillcores in the Illinois Basin. We also obtained X-Ray microcomputed tomography (MicroCT) results for 8 conodonts to evaluate the accuracy of alpha-ejection corrections. The simplified geometric corrections yield corrected dates within 5% of those derived from 3D characterization using MicroCT. Nearly all of the conodont CAHe dates are substantially younger than their depositional age, indicating that maximum post-depositional temperatures of ≤90 °C caused He loss over geologic timescales. The youngest and most reproducible dates consist of whole platform elements from shales, and may record a regional Late Cretaceous–early Tertiary cooling and erosion event. The remainder of the data exhibit strong negative date-U and date-Th correlations, characterized by higher and more variable Th/U than the conodonts with reproducible dates. These patterns are best explained by U loss, with more limited Th loss. The results suggest that whole platform elements and higher U–Th conodont materials are the most promising targets for CAHe analysis.
With the development and application of tectono-thermochronology, particularly the middle to low temperature dating system, hydrocarbon basin analysis is full of vitality. Because process of the formation and evolution of hydrocarbon basins, especially superimposed basins, accompanied oil and gas accumulation, transformation and the energy exchange, it is important to discuss and study the formation, evolution, thermal history, deposition, subsidence and restoration denudation of petroliferous basins according to the tectono-thermal process which was constrained by tectono-thermochronology. Recently, this study gradually shifts from the qualitative description to the quantitative or semi-quantitative calculation, which provides a broader platform for conduction and development of oil and gas basins analysis. This paper summarizes the thermal chronology dating systems from high to low temperature of tectono-thermochronology, and proposes their advantage applications depending on the geological significance of the mineral thermal ages and research progress of each measuring system in superimposed basins. We focused on the development process and current important application of fission track and (U-Th)/He dating systems in hydrocarbon basins whose annealing zone are similar to the oil window, and preliminarily forecast their possible development trends in the future. A variety of disciplines such as tectono-thermochronology, geochemistry, petrology and mineralogy intersects and integrates which can better serve the scientific research and production.
Conodonts have the potential to elucidate the intricacies of Palaeozoic climates, especially if delta O-18 values of single apatitic tooth-like 'elements' can be used to map evolving sea surface temperatures and differentiate oceanic water masses. Their ecological distribution as pelagic and nektobenthic organisms, high-resolution biostratigraphy, and abundance in Cambrian-Triassic rocks qualifies them as potentially robust climate archives. Previous ion microprobe conodont delta O-18 studies have proceeded directly to palaeotemperature interpretation without appreciation of inter- and intra-element variability or post-mortem artefacts. Here, ion microprobe analyses of Ordovician and Silurian conodonts establishes that: intra-element crown tissue delta O-18 typically varies by <= 1 parts per thousand (53% of conodonts analysed), is normally <= 2 parts per thousand ( 92% of analyses), and rarely varies by 2-4 parts per thousand; delta O-18 can vary across elements, suggesting a microstructural and/or diagenetic control; delta O-18 can vary between species representatives by c. 3 parts per thousand; delta O-18 of pelagic and nektobenthic taxa can be offset by 2-3%; elements processed with formic acid have highly variable delta O-18; and thermal alteration does affect delta O-18. Conodont ion microprobe delta O-18 values are comparable with those of bulk methods, but utilization of material with no consideration of geological context or processing history may introduce significant artefacts. A protocol for future conodont oxygen isotope ion microprobe studies is proposed.
The ultrastructure of 24 form-species of Ordovician conodonts has been examined with the scanning electron microscope. The conodonts represent two provincial and three subprovincial faunas and include hyaline forms, neurodonts (a subgroup of the hyalines), and cancellate forms (conodonts with white matter).
The robust neurodont elements are constructed of cone-in-cone lamellae separated by distinct interlamellar spaces. There is limited fusion between adjacent lamellae and between the long, needle-like crystallites within each lamella. Crystallites become granular in form near the base in some neurodonts. A sheet-like septum bisects the elements longitudinally, but does not pass through the central growth canal. The latter is surrounded by fused lamellae producing a strengthened wall. Minute spheres occur along crystallites in some neurodonts. In the non-neurodont hyaline conodonts, greater fusion occurs between lamellae and between crystallites, and white matter may develop along the central growth canal; no septum or spheres have been noted within this group. Increased fusion is considered to have produced stronger elements capable of acquiring lateral compression, costae, and keels. Cancellate conodonts develop white matter primarily in their cusps and denticles. White matter is finely crystalline with no lamellar structure, but with abundant circular and linear voids. There is no pattern to the occurrence of the holes, but most linear voids are oriented transversely. White matter is formed by secondary transformation from lamellar material; this change is reflected in a transitional zone of incipient white matter, where a reorientation of the hard tissue is evident. Minute spheres occur in white matter; the central growth canal is destroyed and does not penetrate beyond the transition zone. White matter is believed to provide extra strength to the element allowing marked lateral compression and sharp margins.
Although separated on structural criteria, these three conodont groups appear to occupy distinct and evolving ecosystems during the Ordovician. Three theories are proposed which, individually or in combination, may indicate the functional advantages conveyed by the development of white matter: (1) that the factors of element weight and phosphate availability were important, (2) that greater strength to cope with variable stresses was achieved, and (3) that white matter was induced by a disruption in vascular supply, possibly resulting from a partial eruption through the secreting tissues.
composed largely of detritus derived from the erosion of volcanic rocks. We were unable to separate the volcanogenic part of these rocks, and chose not to include them on our map of specifically volcanic rocks, even though in some cases, they may serve as hosts for mineral deposits. Subsurface volcanic rocks could not be assigned to assemblages, and are depicted as a single map unit. Exposed pre-Cenozoic rocks are also depicted as a single map unit, primarily to aid readability of the map. VOLCANIC ROCKS IN THE SUBSURFACE The special analysis of aeromagnetic data that is described in chapter 3 provided most of the information used to infer the subsurface extent of the volcanic rocks in the upper kilometer of the crust. While direct application of Blakely's analysis was sufficient to locate subsurface volcanic rocks in the majority of cases, two major types of complications arise in making subsurface projections of volcanic rocks. First, the magnetic methods are insensitive to age, and cannot distinguish between shallowly buried Cenozoic volcanic rocks and older (pre-Tertiary) magnetic igneous rocks. Particularly in western Nevada, where much of the basement is composed of older volcanic rocks, a generous dose of interpretation and intuition was used to delineate the subsurface Cenozoic volcanic rocks, and the map is probably least reliable in that area. Second, some Cenozoic volcanic rocks are only weakly
We report in situ ion microprobe U-Th-Pb dating of an index fossil conodont derived from Langkawi Island, northern Malaysia. Fifteen spots on five fragments of an early Silurian conodont yield 232Th-208Pb* isochron age of 429 ± 50 Ma (2σ, MSWD = 0.95). The 238U-206Pb* isochron age of 436 ± 270 Ma (2σ, MSWD = 0.47) has a large uncertainty, since the 238U abundance is significantly small, but the age is comparable to that of 232Th-208Pb*. These ages are consistent with the depositional and early diagenetic ages of the fossil in a Llandoverian sedimentary sequence (430-439 Ma). The success of the method depends on the heterogeneity of diagenetically incorpolated Th in a hundred μm size and the consequent variations in Pb isotopic compositions due to radioactive decay. Shale-normalized rare earth element (REE) abundance shows a concave pattern with middle REE enrichment. There is not a significant anomaly of Ce and Eu. These characteristics are different from those of a Carboniferous conodont, probably due to a different formation environment as suggested by other workers.
The Laramide orogeny is the Late Cretaceous to Palaeocene (80–55 Ma) orogenic event that gave rise to the Rocky Mountain fold and thrust belt in Canada, the Laramide block uplifts in the USA, and the Sierra Madre Oriental fold and thrust belt in Mexico. The leading model for driving Laramide orogenesis in the USA is flat-slab subduction, whereby stress coupling of a subhorizontal oceanic slab to the upper plate transmitted stresses eastwards, producing basement-cored block uplifts and arc magmatism in the foreland. The thermal models presented here indicate that arc magma generation at significant distances inboard of the trench (>600 km) during flat-slab subduction is problematic; this conclusion is consistent with the coincidence of volcanic gaps and flat-slab subduction at modern convergent margins. Lawsonite eclogite xenoliths erupted through the Colorado Plateau in Oligocene time are inferred to originate from the subducted Farallon slab, and indicate that the Laramide flat-slab subduction zone was characterised by a cold thermal regime. Thermal modelling indicates that this regime can be produced by flat-slab subduction of old (>∼50 Myr) oceanic lithosphere at high convergence rates. In the Canadian and Mexican portions of the Laramide orogen, the coeval development of a magmatic arc within 300 km of the trench refutes the existence of flat-slab subduction in these regions. It is proposed that subduction of an oceanic plateau/aseismic ridge may have overcome the negative buoyancy inherent in old oceanic lithosphere and resulted in a spatially restricted zone of flat-slab subduction in the USA. These findings cast doubt on the flat-slab model as a primary means of driving Laramide orogenesis along its entire length, and instead point to the need for an alternative mechanism for Cordilleran-wide Laramide orogenesis.
Recent advances in our understanding of conodont palaeobiology and functional morphology have rendered established hypotheses of element growth untenable. In order to address this problem, hard tissue histology is reviewed paying particular attention to the relationships during growth of the component hard tissues comprising conodont elements, and ignoring a priori assumptions of the homologies of these tissues. Conodont element growth is considered further in terms of the pattern of formation, of which four distinct types are described, all possibly derived from a primitive condition after heterochronic changes in the timing of various developmental stages. It is hoped that this may provide further means of unravelling conodont phylogeny. The manner in which the tissues grew is considered homologous with other vertebrate hard tissues, and the elements appear to have grown in a way similar to the growing scales and growing dentition of other vertebrates.
Acid etching combined with scanning electron microscopy (SEM) accentuates a variety of conodont microstructural patterns as compared to unetched specimens. External and internal organic layers surrounding apatitic lamellae are found with internal organic layers apparently thinner than their external counterparts. This may imply partial removal of the outermost organic layer prior to secretion of the next lamella. Etching also provides evidence of internally preserved striate ornament. Polygonal etch artifacts and zones of crystallites aligned in small areas are interpreted as possible sites for apatite biomineralization. -Authors
Trace-element studies of biogenic apatite of conodonts can be used to characterize oceanic changes during the Palaeozoic. A modified NAA technique was used to quantitatively determine REE, Th and U in biogenically precipitated apatite of conodonts and ichthyoliths. Specific REE signatures were found for contemporaneous samples from the same depositional setting. Anoxic signals, indicative of changing redox conditions in ancient seas, can be retained by fossil apatite. This is seen in Th/U and Ce measurements. Nd and Sr isotopic variations in ancient sea-waters are also recorded in the fossil apatite.-V.N.F.
Igneous fluorapatite samples from a suite of six granitic rocks from the Transantarctic Mountains have high 3He concentrations (to 5 X 109 atoms g-1) and high 3He/4He ratios (to 9 X 10-7). These values are far higher than those found in several hundred igneous apatites from elsewhere around the world and are higher than can be attributed to nuclear reactions on 6Li. This 3He is almost certainly derived from cosmic ray reactions in rocks with high exposure ages at high latitude and elevation. Several samples of fossil tooth enamel fluorapatite from the Turkana Basin of Kenya are similarly rich in 3He, with up to 1 X 107 atoms 3He g-1 and 3He/4He ratios up to 4 X 10-6. Again, this 3He is most logically attributed to cosmic ray reactions. Provided that cosmogenic 3He, like radiogenic 4He, is quantitatively retained in fluorapatite under Earth surface conditions, routine 3He exposure dating of this common phase may be possible. Based on its chemical composition, the 3He production rate in fluorapatite is about 100 atoms g-1 yr-1 at sea level and high latitude. Using this rate the apatites from the Transantarctic Mountains have apparent exposure ages of 0.5-6.2 Myr, in agreement with values elsewhere in the range. The fossil tooth enamel samples have apparent exposure ages ranging from a few up to 130 kyr. Such high exposure ages suggest some of these fossils may be lag deposits with a very long residence time at or near the Earth's surface. 3He exposure ages can provide insights to the depositional and reworking history of enamel-bearing fossils. At present the major limitations to 3He exposure dating of fluorapatite are purification of sufficient amounts of material and measurement of small amounts of 3He in the presence of large quantities of 4He. In addition, further work is necessary to establish the nucleogenic 3He background in fluorapatite.
We have explored the diffusivity characteristics of radiogenic He in titanite (sphene) and have developed analytical techniques for (U–Th)/He dating of this mineral. Results of incremental He outgassing experiments performed on titanites from a variety of geological environments suggest a thermally activated volume diffusion mechanism with an activation energy of 44.6 ± 3.4 (2σ) kcal/mol and a frequency factor of ∼60 cm2/s. Diffusivity is highly linearly correlated with the inverse square of the grain size, indicating that the He diffusion domain in titanite is the crystal itself. For typical titanite grains of 200- to 800-μm minimum dimension, the He closure temperature is in the range 191 to 218°C (for a cooling rate of 10°C/Myr). There is no indication in the titanites we studied that radiation damage plays a major role in He diffusion. (U–Th)/He ages of titanites from quickly cooled rocks yield ages (with ∼5–8% 1σ reproducibility) that are concordant with known ages, and (U-Th)/He ages of titanites from slowly cooled rocks are consistent with independently established cooling paths, supporting the closure temperature estimates. These experiments suggest that titanite (U–Th)/He ages may be useful for constraining cooling histories at temperatures near the lower limit of those accessed by feldspar 40Ar/39Ar dating but higher than apatite fission track or (U-Th)/He dating.
From our study of 6 bones of Oligocene age from different sites of one locality (the Fayum beds of Egypt) and 3 bones of Pleistocene or Pliocene age we conclude that the general applicability of uranium-helium method of dating is not possible for fossil bones or other apatite deposits. The major problem appears to be continuous helium loss rather than recent uranium gain.
High-precision stepped-heating experiments were performed to better characterize helium diffusion from apatite using Durango fluorapatite as a model system. At temperatures below 265°C, helium diffusion from this apatite is a simple, thermally activated process that is independent of the cumulative fraction of helium released and also of the heating schedule used. Across a factor of ~4 in grain size, helium diffusivity scales with the inverse square of grain radius, implying that the physical grain is the diffusion domain. Measurements on crystallographically oriented thick sections indicate that helium diffusivity in Durango apatite is nearly isotropic. The best estimate of the activation energy for He diffusion from this apatite is Ea=33+/-0.5kcal/mol, with log(D0)=1.5+/-0.6cm2/s. The implied He closure temperature for a grain of 100 mum radius is 68°C assuming a 10°C/Myr cooling rate; this figure varies by +/-5°C for grains ranging from 50 to 150 mum radius. When this apatite is heated to temperatures from 265 to 400°C, a progressive and irreversible change in He diffusion behavior occurs: Both the activation energy and frequency factor are reduced. This transition in behavior coincides closely with progressive annealing of radiation damage in Durango apatite, suggesting that defects and defect annealing play a role in the diffusivity of helium through apatite.
Constraints on the thermochronologic evolution of the New York crystalline basement are mainly restricted to the high-temperature early cooling history following 1.1 Ga Grenville metamorphism (U-Pb methods) and the late cooling history (fission track methods). The thermal history for the ~one billion years between these intervals is herein assessed using 40Ar/39Ar hornblende, muscovite, biotite and K-feldspar thermochronology. Hornblende preferred ages suggest cooling below ~450-500°C between ca. 900 and 950 Ma. Total gas ages of biotites range from ~746-1060 Ma, and their age spectra complexity is probably related to younger thermal events and/or excess argon. A single muscovite analysis yields a preferred age of 854 Ma. Argon isotope analyses of K-feldspar provide a quantitative evaluation of the thermal history between ~175 and 350°C. Multi-diffusion domain thermochronology on highly variable K-feldspar results yield internally consistent thermal histories and, along with geologic constraints, suggest thermal maxima of 275-350°C at ca. 700, 470-450 and 300 Ma. Combining all of the K-feldspar analyses provides an internally consistent thermal history for the region and allows for the following inferences. Reheating at ~700 Ma associated with Late Precambrian rifting during the formation of the Iapetus ocean apparently affected the entire Adirondack region with the highest temperatures occurring in the eastern part of New York. Local reheating in the Ordovician is inferred to have affected the eastern Adirondack Mountains and probably resulted from the combined effects of burial during the Early Paleozoic, emplacement of Taconic thrust sheets and migration of hot fluids along normal faults during the Taconic orogeny. High paleotemperatures for the Devonian section of eastern New York are related to maximum burial of the basement during the Carboniferous at ca. 300 Ma.
Igneous fluorapatite samples from a suite of six granitic rocks from the Transantarctic Mountains have high 3He concentrations (to 5×109 atoms g−1) and high 3He/4He ratios (to 9×10−7). These values are far higher than those found in several hundred igneous apatites from elsewhere around the world and are higher than can be attributed to nuclear reactions on 6Li. This 3He is almost certainly derived from cosmic ray reactions in rocks with high exposure ages at high latitude and elevation. Several samples of fossil tooth enamel fluorapatite from the Turkana Basin of Kenya are similarly rich in 3He, with up to 1×107 atoms 3He g−1 and 3He/4He ratios up to 4×10−6. Again, this 3He is most logically attributed to cosmic ray reactions. Provided that cosmogenic 3He, like radiogenic 4He, is quantitatively retained in fluorapatite under Earth surface conditions, routine 3He exposure dating of this common phase may be possible. Based on its chemical composition, the 3He production rate in fluorapatite is about 100 atoms g−1 yr−1 at sea level and high latitude. Using this rate the apatites from the Transantarctic Mountains have apparent exposure ages of 0.5–6.2 Myr, in agreement with values elsewhere in the range. The fossil tooth enamel samples have apparent exposure ages ranging from a few up to 130 kyr. Such high exposure ages suggest some of these fossils may be lag deposits with a very long residence time at or near the Earth’s surface. 3He exposure ages can provide insights to the depositional and reworking history of enamel-bearing fossils. At present the major limitations to 3He exposure dating of fluorapatite are purification of sufficient amounts of material and measurement of small amounts of 3He in the presence of large quantities of 4He. In addition, further work is necessary to establish the nucleogenic 3He background in fluorapatite.
(U-Th)/He chronometry of zircon has a wide range of potential applications including thermochronometry, provided the temperature sensitivity (e.g., closure temperature) of the system be accurately constrained. We have examined the characteristics of He loss from zircon in a series of step-heating diffusion experiments, and compared zircon (U-Th)/He ages with other thermochronometric constraints from plutonic rocks. Diffusion experiments on zircons with varying ages and U-Th contents yield Arrhenius relationships which, after about 5% He release, indicate Ea = 163–173 kJ/mol (39–41 kcal/mol), and D0 = 0.09–1.5 cm2/s, with an average Ea of 169 ± 3.8 kJ/mol (40.4 ± 0.9 kcal/mol) and average D0 of 0.46+0.87−0.30 cm2/s. The experiments also suggest a correspondence between diffusion domain size and grain size. For effective grain radius of 60 μm and cooling rate of 10°C/myr, the diffusion data yield closure temperatures, Tc, of 171–196°C, with an average of 183°C. The early stages of step heating experiments show complications in the form of decreasing apparent diffusivity with successive heating steps, but these are essentially absent in later stages, after about 5–10% He release. These effects are independent of radiation dosage and are also unlikely to be due to intracrystalline He zonation. Regardless of the physical origin, this non-Arrhenius behavior is similar to predictions based on degassing of multiple diffusion domains, with only a small proportion (
The possibility of dating minerals by the accumulation of ^4He from U and Th decay
has been recognized for many years (e.g., Strutt 1905), but in the century since the idea
was first conceived, the method has rarely been applied successfully. After several
investigations of (U-Th)/He dating of various minerals (e.g., Damon and Kulp 1957;
Fanale and Kulp 1962; Damon and Green 1963; Turekian et al. 1970; Bender 1973;
Leventhal 1975; Ferreira et al. 1975) the technique was essentially abandoned as yielding
unreliable and usually low ages, presumably as a result of diffusive He loss possibly
associated with radiation damage. In 1987, Zeitler and coworkers rekindled interest in the
method by proposing that in the case of apatite, He ages might be meaningfully
interpreted as ages of cooling through very low temperatures. Laboratory diffusion data
presented by these authors indicated a closure temperature of about 100ºC, a value
supported by more recent studies (Lippolt et al. 1994; Wolf et al. 1996b; Warnock et al.
1997). Consistent with this interpretation Wolf et al. (1996a) found that apatite He ages
increase systematically with sample elevation in a mountain range, as expected for
exhumation-induced cooling through a low closure temperature. Based on the strength of
these results and additional laboratory (Farley 2000) and natural (Warnock et al. 1997;
House et al. 1999; Stockli et al. 2000) constraints on He diffusivity, recent attention has
focused on applications of apatite He thermochronometry. There is also renewed interest
in He dating of other U- and Th-bearing minerals both for dating mineral formation and
for thermochronometry. For example, Lippolt and coworkers have undertaken detailed
studies of He diffusion and dating of various phases, most notably hematite formed in
hydrothermal systems (Lippolt and Weigel 1988; Wernicke and Lippolt 1992; Lippolt et
al. 1993; Wernicke and Lippolt 1994a,b).
Here I present an overview of recent techniques, calibrations, and applications of the
(U-Th)/He dating method; Hurley (1954) provides an excellent summary of earlier work in
this field. Much of this paper focuses on apatite, because the He behavior and requisite
analytical techniques are better established for this phase than for other target minerals,
such as zircon and titanite. Similarly, much of this paper concerns He diffusivity behavior
required for thermochronometric applications, yet recent work is also considering applications
to direct dating, for example, of young tephras (Farley et al. 2001).
Conodonts from two localities in Helena Canyon, Colorado and Tempiute Mtn., Nevada yield relatively reproducible He cooling ages (57.5 ± 1.9 Ma (2σ, n=6) and 5.81 ± 0.56 Ma (2σ, n=8), consistent with regional thermochronology and a low closure temperature (
Field and laboratory experiments show that color alteration in conodonts is directly related to the depth and duration of burial and the geothermal gradient and correlates with fixed carbon, vitrinite reflectance, palynomorph translucency, and isopach data. Five progressive and irreversible color changes ranging from pale yellow to black are discriminated. Compilation of color alteration index (CAI) maps for limestones of several ages in the Appalachian basin show: (1) a general systematic change from pale yellow (CAI = 1) to black (CAI = 5) from west to east; (2) within a structural belt, older conodonts are darkest; (3) fixed carbon values determined from coals cannot be directly applied to carbonate rocks; (4) several areas of anomalously low CAI values in windows exposing Ordovician rocks within the Blue Ridge-Piedmont terrane. Data show: (1) color alteration of conodonts is time and temperature dependent; (2) the sequence of color change from pale yellow to black found in field collections is the same as that produced by heating alone; (3) upper and lower geologic temperature limits for each CAI determined from a log time versus reciprocal of absolute temperature plot. Water in combination with confined pressure retards color alteration. The color alteration of conodonts is a valuable tool for assessing organic metamorphism because it is a rapid and inexpensive method requiring only standard laboratory techniques and a binocular microscope. The technique provides thermal cutoffs for oil, condensate, and dry gas generation. Conodont color alteration begins near the upper thermal limit for the preservation of many palynomorphs.
Conodonts are microfossils which are commonly found in marine rocks of Cambrian to Triassic age. Although their biological affinities are difficult to assess, conodonts are valuable stratigraphical indices for much of their geological range1. Recent work has also established that conodont colour alteration indices (CAI) are useful guides to diagenetic temperatures and hence burial depth2. Fission tracks3 in conodonts allow measurement of uranium concentrations and estimates of ‘age’ to be made using isotopic methods4. We report here that fission tracks counted in irradiated, thermally unaltered (as indicated by CAI) middle Palaeozoic conodonts indicate typical uranium concentrations of ~1 part in 109, with some samples higher. A single specimen of Siphonodella from the Lower Mississippian yielded an age estimate of 380±140 Myr consistent with conventional interpolations. This method may also allow the unroofing of deeply buried sediments to be dated.
We describe a simple and low-cost apparatus for in-vacuum helium diffusion measurements that reduces temperature gradients, set point overshoot, and ramping times compared with conventional resistance furnace techniques. The sample, suspended by a thin wire in a vacuum chamber, is heated by radiation from an Al-coated projector bulb passed through a sapphire viewport. Because the total mass of the sample package being heated is small (typically <100 mg), thermal gradients and thermal inertia are both small. In experiments with set points between 100 and 750 °C, this apparatus can achieve set point from room temperature in 90 s, usually with <3 °C of set point overshoot persisting for just a few seconds. Helium diffusion coefficient measurements indirectly indicate that temperatures are precise and reproducible to better than ±2 °C.
Although thermochronology cannot directly constrain paleoelevation, it can provide estimates of the form, location, and scale of paleotopographic relief, i.e., paleotopography, and its change through time. Unique thermochronologic perspectives on paleotopography come from 1) spatial patterns of surface and subsurface cooling ages or cooling histories that refl ect either the infl uence of topography on subsurface isotherm warping, or spatially focused erosion (including incision), and 2) the age-elevation relationship in paleolandscapes that may be preserved in detrital cooling age distributions. This chapter reviews the fundamental theory and results of these approaches and several example applications. Case studies show examples of both decreasing and increasing topographic relief through time, at the orogen scale and across short ridge-valley wavelengths, and signifi cant modifi cation of local topographic features in glaciated and fl uvial settings. In some cases, thermochronologic evidence for fl uvial incision at short wavelengths has also been argued to be the result of surface uplift at very long wavelengths. Although not yet used for such purposes, detrital approaches also have the potential to reconstruct paleotopographic relief and paleohypsometry in paleocatchments. In all cases, paleotopographic interpretations from thermochronology require important assumptions or case-by-case support from other lines of evidence. Central issues pertinent to thermochronologic interpretations of paleotopography are the nature of the shallow crustal thermal fi eld through which the samples cooled (including the infl uence of fl uid fl ow) and the role of rock uplift gradients in modifying simple relationships between erosion and topography.
Minor and trace element compositions of a suite of Ordovician, Silurian, and Permian conodonts have been characterised by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Continuous, high-resolution chemical depth profiles through individual conodont elements reveal systematic compositional differences between the component histologies (albid, hyaline, and basal body tissues). Comparative analyses of contemporaneous bio-apatites (ichthyoliths and inarticulate brachiopods), as well as Holocene and modern fish material, show linear relationships between their respective rare earth element, yttrium, lead, thorium, and uranium compositions, which has implications for their relative permeability and susceptibility to diagenesis. Assessment of LA-ICPMS profiles in the context of histology, general morphological structure, and post-depositional chemical exchange, suggests that conodont albid crown is the least permeable histology, whereas hyaline crown, basal tissue, ichthyoliths, and inarticulate brachiopods are strongly overprinted by the selective uptake of Y–REE–Th–U during diagenesis. Conodont albid crown is therefore more resistant than other bio-apatite histologies to postmortem uptake of Y–REE–Th–U, and trends below detection limits that more closely approach primary conodont compositions. This relationship between sample histology and geochemistry, especially in the context of diagenetic overprinting, demands careful consideration when reconstructing palaeoseawater compositions.
Apatite, titanite and olivine samples were bombarded with a ~ 150 MeV proton beam to produce ~ 10^8 atoms/mg of spallation ^3He. High-precision stepped-heating experiments were then performed in which the artificial ^3He and, for apatite and titanite, the natural radiogenic ^4He were measured to characterize the diffusive behavior of each
isotope. Helium-3 diffusion coefficients are in excellent agreement with concurrently and/or previously determined He
diffusion coefficients for each mineral. Our results indicate that proton-induced ^3He is uniformly distributed and that radiation damage associated with a proton fluence of ~ 5 x 10^(14) protons/cm^2 does not cause noticeable changes in ^4He diffusion behavior in at least apatite and titanite. Proton-induced ^3He can therefore be used to establish He diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural ^4He distribution is inhomogeneous. In addition,step-heating ^4He/^3He analysis of a mineral with a uniform synthetic ^3He concentration provides a means by which a natural ^4He distribution can be determined.
Stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4He and proton-induced 3He reveal a range in closure temperature (Tc) from ∼ 50 to 115 °C, for a cooling rate of 10 °C/Myr. There is no correlation between helium diffusion and apatite chemistry including F/Cl ratio, but the closure temperature is positively correlated with the radiogenic 4He concentration ([4He]) in each sample. We argue that [4He] is a proxy for a sample's natural exposure to actinide radioactivity below the closure temperature, and that helium diffusion in apatite is impeded by radiation-induced damage to the apatite structure. The kinetics must therefore be an evolving function of time; measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample. The effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity, including grain size.
A mathematical framework for quantitative evaluation of alpha-stopping effects on (UTh)/He ages has been developed. Alpha stopping ranges in the 238U, 235U, and 232Th chains vary between ∼10 and ∼30 μm, depending on decay energy and density/composition of the stopping medium. In the case of U- and Th-rich accessory minerals (e.g. apatite, zircon, titanite), the dominant effect of long stopping distances is alpha ejection to adjacent minerals. For grains smaller than a few hundred microns in minimum dimension, ejection effects will cause measured helium ages to substantially underestimate true ages. For example, a sphere of 100 μm radius retains only ∼82% of its alphas. For a homogeneous distribution of parent nuclides, the fraction of alphas ejected is ∼ of the mean alpha range multiplied by the crystal surface to volume ratio, independent of geometry. Removal of the outer 20 μm of a crystal prior to dating eliminates the region which has experienced alpha loss, but may lead to erroneous ages when crystals are strongly zoned with respect to uranium and thorium. By careful characterization of four sieved apatite separates from a single sample, we show that it is possible to accurately correct (UTh)/He ages for alpha ejection even when ejection exceeds 35% of total decays. Our results are useful for identifying the size and shape of grains which are best suited for (UTh)/He dating and provide the basis for correcting ages when ejection effects are significant. This work underscores that meaningful (UTh)/He ages require either large crystals, or correction of measured ages for alpha ejection.
Thesis (M.S.)--University of Wyoming, 1992. Includes bibliographical references (leaves 131-135).
Apatite fission-track ages of 168-83 Ma for 39 samples of Proterozoic crystalline rocks, three samples of Cambrian Potsdam sandstone, and one Cretaceous lamprophyre dike from the Adirondack Mountains in New York State indicate that unroofing in this region occurred from Late Jurassic through Early Cretaceous. Samples from the High Peaks section of the Adirondack massif yielded the oldest apatite fission-track ages (168-135 Ma), indicating that it was exhumed first. Unroofing along the northern, northwestern, and southwestern margins of the Adirondacks began slightly later, as shown by younger apatite fission-track ages (146-114 Ma) determined for these rocks. This delay in exhumation may have resulted from burial of the peripheral regions by sediment shed from the High Peaks. Apatite fission-track ages for samples from the southeastern Adirondacks are distinctly younger (112-83 Ma) than those determined for the rest of the Adirondack region. These younger apatite fission-track ages are from a section of the Adirondacks dissected by shear zones and post-Ordovician north-northeast-trending normal faults. Differential unroofing may have been accommodated by reactivation of the faults in a reverse sense of motion with maximum compressive stress, sigma1, oriented west-northwest. A change in the orientation of the post-Early Cretaceous paleostress field is supported by a change in the trend of Cretaceous lamprophyre dikes from east-west to west-northwest.
U–Pb isotopic age dating of Devonian conodonts; a new method for dating Paleozoic marine sedimentary deposits?
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The chemical composition and micromorphology of conodonts
- H Pietzner
- J Vahl
- H Werner
- W Ziegler
H. Pietzner, J. Vahl, H. Werner, W. Ziegler, The chemical composition
and micromorphology of conodonts, Palaeontographica, Abteilung
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Trace-element mapping of conodonts-Implications for histology and function
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J. Morrow, P. Emsbo, G. Breit, Trace-element mapping of
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Amazing displays of deformation band shear zones within structure-tectonic systems of the Colorado Plateau; from Zion to Bryce and beyond A summary of radiometric ages of Tertiary volcanic rocks in Nevada and Eastern California
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- R F Marvin
- H H Mehnert
- E H Mckee
G.H. Davis, Amazing displays of deformation band shear zones
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[27] R.F. Marvin, H.H. Mehnert, E.H. McKee, A summary of radiometric
ages of Tertiary volcanic rocks in Nevada and Eastern California.
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Paleozoic lithostratigraphic nomenclature for Minnesota: Minnesota Geological Survey Report of Investigations
- J H Mossler
J.H. Mossler, Paleozoic lithostratigraphic nomenclature for Minnesota: Minnesota Geological Survey Report of Investigations, vol. 36, 1987, 36 pp. 580 D.J. Peppe, P.W. Reiners / Earth and Planetary Science Letters 258 (2007) 569–580
Conodont apatite: A geochemical and isotopic time capsule of the paleocean?:
- Emsbo
P. Emsbo, G. Breit, A.E. Koenig, H.A. Lowers, W.R. Premo, A.G.
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Geology and Geochemistry of Tertiary Volcanic Rocks in the Northern Reveille and Southern Pancake Ranges
- K B Rash
K.B. Rash, Geology and Geochemistry of Tertiary Volcanic Rocks in
the Northern Reveille and Southern Pancake Ranges, Nye County,
Nevada, Master's, University of Nevada, Las Vegas, 1995.
An analysis of Nevada's metal-bearing mineral resources
- S D Ludington
- D P Cox
- K R Leonard
- B C Moring
S.D. Ludington, D.P. Cox, K.R. Leonard, B.C. Moring, Cenozoic
volcanic geology of Nevada, in: D.A. Singer (Ed.), An analysis of
Nevada's metal-bearing mineral resources, Nevada Bureau of
Mines and Geology Open-File Report, Reno, NV, vol. 96-2, 1996,
pp. 5-1-5-10.
The chemical composition and micromorphology of conodonts, Palaeontographica
- Pietzner