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What Hf isotopes in zircon tell us about crust–mantle evolution
Abstract
The ¹⁷⁶Lu–¹⁷⁶Hf radioactive decay system has been widely used to study planetary crust–mantle differentiation. Of considerable utility in this regard is zircon, a resistant mineral that can be precisely dated by the U–Pb chronometer and record its initial Hf isotope composition due to having low Lu/Hf. Here we review zircon U–Pb age and Hf isotopic data mainly obtained over the last two decades and discuss their contributions to our current understanding of crust–mantle evolution, with emphasis on the Lu–Hf isotope composition of the bulk silicate Earth (BSE), early differentiation of the silicate Earth, and the evolution of the continental crust over geologic history. Meteorite zircon encapsulates the most primitive Hf isotope composition of our solar system, which was used to identify chondritic meteorites best representative of the BSE (¹⁷⁶Hf/¹⁷⁷Hf = 0.282793 ± 0.000011; ¹⁷⁶Lu/¹⁷⁷Hf = 0.0338 ± 0.0001). Hadean–Eoarchean detrital zircons yield highly unradiogenic Hf isotope compositions relative to the BSE, providing evidence for the development of a geochemically enriched silicate reservoir as early as 4.5 Ga. By combining the Hf and O isotope systematics, we propose that the early enriched silicate reservoir has resided at depth within the Earth rather than near the surface and may represent a fractionated residuum of a magma ocean underlying the proto-crust, like urKREEP beneath the anorthositic crust on the Moon. Detrital zircons from world major rivers potentially provide the most robust Hf isotope record of the preserved granitoid crust on a continental scale, whereas mafic rocks with various emplacement ages offer an opportunity to trace the Hf isotope evolution of juvenile continental crust (from εHf[4.5 Ga] = 0 to εHf[present] = + 13). The river zircon data as compared to the juvenile crust composition highlight that the supercontinent cycle has controlled the evolution of the continental crust by regulating the rates of crustal generation and intra-crustal reworking processes and the preservation potential of granitoid crust. We use the data to explore the timing of generation of the preserved continental crust. Taking into account the crustal residence times of continental crust recycled back into the mantle, we further propose a model of net continental growth that stable continental crust was firstly established in the Paleo- and Mesoarchean and significantly grew in the Paleoproterozoic.
... Two-stage zircon Hf model ages are commonly used to estimate when the crustal source that gave rise to the melt from which the zircon crystallised, was extracted from the mantle (Belousova et al., 2010;Dhuime et al., 2012;Iizuka et al., 2017;Wang et al., 2009;Wang et al., 2011). This approach is valid if the zircon host magma crystallises came from a single igneous lithology. ...
... Despite the limitations of zircon Hf model ages imposed by mixed end members, two encouraging lines of evidence suggest that detrital zircon Hf model ages, when applied to large data sets, give reasonable estimates of crustal growth. First, Iizuka et al. (2017) compared Hf model ages for zircons from the world's major river with Os model ages for mantle xenolith from kimberlites, for which hybrid model ages are not an issue. The major peaks for both methods are in the Paleo-to Mesoarchean and Paleo-to Mesoproterozoic periods, which they interpret to confirm that zircon Hf model ages recognize the approximate timing of major periods growth of the preserved continental crust. ...
... The generation of new crust differs from the growth of the preserved continental crust since the latter is a net result of the former and the recycling of old continental crust back into the mantle (Iizuka et al., 2017;Spencer et al., 2017). However, assuming that detrital zircons from major rivers are representative of the preserved crust, the distribution of Hf model ages of river detrital zircons provides constraints on the timing of growth of the preserved continental crust (Iizuka et al., 2013;Wang et al., 2009;Wang et al., 2011). ...
... The analyzed zircon Lu-Hf isotope data and calculated parameters of the analyzed sample JSD-C7a are given in Supplementary Table S2 and displayed in Fig. 10 Griffin, 2000). In the diagram of zircon εHf(t2) versus age (Fig. 10), the inherited zircons (2729 Ma) plot in the juvenile crust zone, and the magmatic (2522 Ma) and metamorphic (2480 Ma) zircons plot in the evolved continental crust zone (Iizuka et al., 2017). ...
... In the diagram of zircon εHf(t2) versus age (Fig. 10), the inherited zircons (2729 Ma) plot in the juvenile crust zone, and the magmatic (2522 Ma) and metamorphic (2480 Ma) zircons plot in the evolved continental crust zone (Iizuka et al., 2017). These figures confirm the existence of juvenile continental crust in the early Neoarchean in the Jiapigou terrane of the SJP. ...
... Zircon εHf (t) vs. Age (t) (afterIizuka et al., 2017).R. Yu et al. ...
Neoarchean granitic rocks are important components of Precambrian cratons, and their petrogenesis can provide constraints for understanding the evolution of continental crust. The Neoarchean potassic granites are widely distributed in southern Jilin Province, which is located in the northeastern part of the North China Craton. Newly obtained field geology observations and petrological, geochemical and geochronological data reveal that the Jiapigou potassic granitic complex is mainly composed of biotite granitic gneiss and medium- to coarse-grained granite. All the granitic gneisses and granites are high-K calcium–alkaline to shoshonite, metaluminous to peraluminous (A/CNK = 0.95–1.13; molar Al2O3/(CaO + Na2O + K2O)), enriched in LILEs and LREEs with strongly fractionated REE patterns ((La/Yb)N = 21–179), and depleted in Nb, Ta, Ti and P. The biotite granitic gneisses have low Rb/Sr ratios (∼0.06) and weak positive Eu anomalies (δEu = 1.26–1.51), while the granites have low Th, U and REEs and strong positive Eu anomalies (δEu = 7.27–16.92). The zircons of these granitic rocks generally have core–rim structures based on cathodoluminescence images and show inherited ages of 2729 ± 10 Ma, crystallization ages of 2523 ± 11 Ma to 2526 ± 15 Ma, and crystallization/metamorphism ages of 2480 ± 15 Ma to 2485 ± 9 Ma. The zircon Hf isotope results of the medium-grained monzogranite show that the εHf(t2) values vary from –2.2 to +5.3, and the two-stage model ages (TDM2) are 2.9–3.0 Ga. The magmatic zircons of the early Neoarchean (∼2.73 Ga) and the late Neoarchean (∼2.52 Ga) crystallized in high-temperature magmas (738–890 °C), whereas those of the early Paleoproterozoic (∼2.48 Ga) crystallized in relatively low-temperature (646–702 °C) magmas. These results indicate that the Jiapigou granitic rocks were products of intracrustal recycling during the late Neoarchean and the early Paleoproterozoic. The protoliths of the Jiapigou granitic gneisses were derived from the remelting of the early Neoarchean juvenile crustal rocks in the lower crust during the late Neoarchean. Subsequently, these granitic rocks underwent amphibolite-facies metamorphism and water-fluxed melting in the early Paleoproterozoic, resulting in the association of migmatic granitic gneisses and potassic granites at the shallow crust level. The cratonization in the Jiapigou terrane involved early Neoarchean (∼2.73 Ga) juvenile crust growth and late Neoarchean (∼2.53–2.52 Ga) to the early Paleoproterozoic (∼2.48 Ga) intracontinental reworking.
... Two-stage zircon Hf model ages are commonly used to estimate when the crustal source that gave rise to the melt from which the zircon crystallized, was extracted from the mantle (Belousova et al., 2010;Dhuime et al., 2012;Iizuka et al., 2017;Wang et al., 2009;Wang et al., 2011 (Zhu et al., 2020). During these periods there are expected to be more zircons that give model ages that underestimate the oldest component in the source region but overestimate the youngest component. ...
... The generation of new crust differs from the growth of the preserved continental crust since the latter is a net result of the former and the recycling of old continental crust back into the mantle (Iizuka et al., 2017;Spencer et al., 2017). However, assuming that detrital zircons from major rivers are representative of the preserved crust, the distribution of Hf model ages of river detrital zircons provides constraints on the timing of growth of the preserved continental crust (Iizuka et al., 2013;Wang et al., 2009;Wang et al., 2011). ...
Detrital zircons collected from Earth's modern rivers provide the most representative samples to study the evolution of the continental crust on a global scale. Most of the eroded material in modern rivers has experienced long-time sediment-sediment recycling, and thus they contain detrital zircons from source rocks of diverse origins, including the crustal segments that are currently inaccessible by surface geology. Zircon is a common accessory mineral that retains the isotopic composition of the host magmas from which it crystallized through multiple cycles of erosion, transport and sedimentation. A range of trace elements and isotopes in zircon can be analysed by ICP-MS and ion microprobe, which provide crucial information about history of the Earth and the evolution of its continents. The U-Pb isotopic system can be used to determine the crystallization age of the zircon, O isotope is indicative of the sedimentary contribution to the zircon parental magma, and Lu-Hf isotopes are used to infer the time elapsed since the crustal source region, which melted to form the zircon-hosting magma, was separated from the mantle. This study consists of three projects. The first is to use phosphorous and rare earth element compositions to distinguish between detrital zircons from igneous (I-type) and sedimentary (S-type) sources, in a global compilation of 6911 detrital zircons from 52 major rivers. These data give a large-scale insight into the generation and composition of felsic magmas on a global scale. The relative abundances of I-type and S-type zircons are used to assess the varying contributions of I- and S-type magmas to the continents over geological time. The second project uses the Lu content of zircons to identify those that come from the high pressure to ultra-high pressure roots that underlie high, Himalayan-type mountains. Two periods of extensive high mountain (supermountain) formation are identified. The first at 2.0-1.8 billion years ago (Ga) is associated with the formation of the Nuna supercontinent, which coincides with the emergence of the first macroscopic fossils at approximately 1.9 Ga. The second occurred 650-500 million years ago (Ma), and is associated with the formation of the Gondwana supercontinent. This event is coeval with the emergence of animal-like organisms at 575 Ma and the Cambrian Explosion at 540 Ma. We argue that these evolutionary steps were advanced by the transport of abundant bio-essential nutrients (e.g. P, Fe) into the ocean, which resulted from rapid erosion of supermountains. The third project uses the combined U-Pb, Lu-Hf and O isotopic data of detrital zircons from European rivers to identify periods of crustal growth and to calculate the growth rate of the preserved European continental crust. This is the first attempt to study the growth history of the whole European continent. The bootstrap method is introduced for calculating model age uncertainties. The results show that the growth of the preserved European continental crust started at ca. 3.5 Ga, and that about 50% of the present crustal volume had formed by the late Paleoproterozoic.
... The samples in this study are basaltic rocks with low SiO2 contents, which suggested that their primary magmas were originated from the mantle. Zircons from the hornblende gabbro sample of the Baogeqi gabbro pluton have positive εHf(t) values from +4.9 to +9.4 and displayed an evolutionary trend between the depleted mantle and chondrite ( Figure 4c), suggesting that the primary magmas were possibly originated from a relatively depleted mantle source [52,53]. However, in the chondrite-normalized REE diagrams and primitive mantle-normalized trace-element diagrams (Figure 7), the samples show LILE enrichment (e.g. ...
... The samples in this study are basaltic rocks with low SiO 2 contents, which suggested that their primary magmas were originated from the mantle. Zircons from the hornblende gabbro sample of the Baogeqi gabbro pluton have positive εHf(t) values from +4.9 to +9.4 and displayed an evolutionary trend between the depleted mantle and chondrite (Figure 4c), suggesting that the primary magmas were possibly originated from a relatively depleted mantle source [52,53]. However, in the chondrite-normalized REE diagrams and primitive mantle-normalized trace-element diagrams (Figure 7), the samples show LILE enrichment (e.g., Rb, Ba, Sr, and K) and HFSE depletion (e.g., Nb, Ta, P, Th, and Ti), which are significantly different from depleted mantle-derived magmas. ...
The final closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic settings in the northern Alxa are very important but controversial tectonic issues. The geochronology and petrogenesis of mafic igneous rocks are superior in clarifying regional tectonic settings. Here, we report on zircon U-Pb-Hf isotopes, biotite 40Ar/39Ar geochronology and whole-rock geochemical data of the hornblende gabbro from the Baogeqi gabbro pluton in the northern Alxa. The LA-ICP MS U-Pb analysis of zircon grains from the hornblende gabbro yield a weighted mean age of 262.7 ± 2.3 Ma (2σ, MSWD = 0.74), manifesting that the Baogeqi gabbro pluton emplacement was during the late Middle Permian (Capitanian). The 40Ar/39Ar dating of biotite grains from the hornblende gabbro yields a plateau age of 231.3 ± 1.6 Ma (2σ, MSWD = 0.55), indicating that the Baogeqi gabbro pluton cooled to below 300 ℃ in the Triassic. The hornblende gabbro samples are calc-alkaline with metaluminous character, and show enrichment in large ion lithophile elements (e.g., Rb, Ba, Sr, and K) but depletion in Nb, Ta, P, Th, and Ti relative to primitive mantle. Combined with the positive zircon εHf(t) values (+4.9–+9.4), we suggest that the magmas formed from the partial melting of depleted mantle were metasomatized by slab-derived fluids. Together with regional geology, these geochemical data suggest that the Baogeqi gabbro pluton was formed in an intracontinental extension setting, further indicating that the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region was in a post-collision extensional setting during the Capitanian-Late Permian. In addition, the Triassic cooling of the gabbro pluton may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean.
... While detrital zircon U-Pb-Hf-O do not necessarily provide ultimate, unequivocal clues (e.g. Payne et al., 2016), it is currently the most robust tool available for crustal evolution studies (Griffin et al., 2000;Iizuka et al., 2017;Kemp et al., 2007Kemp et al., , 2006Roberts et al., 2013;Valley et al., 2005Valley et al., , 1994. Coupling U-Pb geochronology with Hf isotopic analysis on single zircon allows evaluating the average crustal residence time of the parent magma from which the zircon crystallized, and whether this parent magma was a juvenile addition from the mantle or recycled continental crust. ...
... Such information is crucial for regional crustal evolution studies and provenance of detrital sediments (e.g. Belousova et al., 2010;Condie et al., 2005;Griffin et al., 2004;Iizuka et al., 2017Iizuka et al., , 2010Kemp et al., 2006). Zircon O isotopic composition is a key to identify supracrustal/crustal reworking components in magmas and to filter mantle-crust mixed and non-mixed Hf isotope zircon model ages (Dhuime et al., 2012;Hawkesworth and Kemp, 2006;Kirkland et al., 2013). ...
We studied detrital zircon from black sand of the Rhone River delta (southern France) which drains significant parts of Western Europe. Zircon U-Pb geochronology shows that the black sand is an extraordinary sampler of all major episodes of European magmatism peaking at 0.55 Ga (Cadomian), 0.45 Ga (Cenerian) and 0.31 Ga (Variscan). Hf in detrital zircon suggest that European magmatic episodes involved continuous crustal reworking along a crustal evolutionary array with Hf-TDM ages pointing to the reworking of an older, Mesoproterozoic (1.0–1.5 Ga) crustal reservoir. However, the coupling of O isotopes shows that the majority of the zircon possess elevated δ¹⁸O values indicating the involvement of a noteworthy sedimentary component in the genesis of European granitoids. Therefore, a preponderance of the obtained Hf-TDM ages are mixed and do not designate the reworking of Mesoproterozoic crust.
The Variscan and post-Variscan intervals include also a significant proportion of detrital zircon with mantle-like δ¹⁸O (≤6.5 ‰) while retaining Mesoproterozoic Hf model ages. Zircon of this type are uncommon in European granitoids and their origin in Rhone delta is therefore not clear, but we tend to interpret the contrasting isotopic signatures as indicating crystallization from juvenile melts whose Hf reflects source contamination.
Few ancient, pre-Neoproterozoic continental slivers are scattered in Western Europe, yet our U-Pb-Hf-O detrital zircon data do not support the widespread reworking of a pre-Neoproterozoic deep crustal reservoir in this region. The prevalent oldest rocks of Western Europe are Late Ediacaran Cadomian clastic sediments, mostly derived from the erosion of Gondwana Pan-African orogens with variable contributions from proximal Cadomian arcs. The apparent Mesoproterozoic Nd and Hf model ages of many European granitoids are likely inherited from the isotopic properties of their Late Ediacaran (and younger) precursor sediments which carry the isotopic composition of the North Gondwana crust.
... Hf isotopic signatures in zircons from the hornblende gabbro are highly positive with εHf mean values of ~ +13.8 that are close to values of the depleted mantle (Figure 7(f)) (εHf of +16 at 0 Ma; Nowell et al. 1998;Iizuka et al. 2017;Guo and Korenaga 2023). The low Th/Nb, Th/La, and La/Yb ratios (Figure 6(d,e)) from the gabbroic rocks show that there was no melting of subducted sediments in the mantle wedge or continental crust during their formation, due to the presence of these sources is often evidenced in increases of the Th and La content in the magmas (Plank 2005(Plank , 2014Turner and Langmuir 2022). ...
Lower Miocene plutons exposed in the Western Cordillera of Colombia record the reinitiation of continental arc magmatism in the Northern Andes after a period of magmatic quiescence between the Late Eocene and Early Miocene. Petrography, U-Pb zircon geochronology, whole-rock geochemistry, mineral chemistry, and zircon Hf isotope data from these plutons are used to reconstruct the Miocene magmatic evolution in the Colombian Andes and understand its relation with the major plate-tectonic reorganization experienced by the NW South American continental margin during the Lower Miocene. We examined a suite of gabbros, granodiorites and tonalites from Danubio, Pance and Tatamá plutons, formed between 21 Ma and 15 Ma. Gabbros present highly positive εHf values (+13.5 and + 11.5) with low Th/La and La/Yb ratios, and granodiorites and tonalites present lower εHf values (+14.3 to + 6.4). The results of this contribution show a major asthenospheric source with absent or minor melting of subducted sediments, and contribution of oxidized aqueous fluids. The compositional diversification of these magmas was controlled by fractional crystallization and, in the Tatamá pluton, also by local assimilation of continental crust at lower crustal levels of the upper plate. Subsequently, these magmas were emplaced in the Cretaceous to Palaeogene volcanic and sedimentary rocks in the uppermost crust. The renewed magmatic activity recorded by the studied plutons suggests a normal to step subduction of the elder Farallon Plate during the Neogene subduction re-organization in the Colombian Andes, as well as the local presence of continental crust fragments in the lower crust of the Western Cordillera of Colombia.
... It is necessary to confirm whether the Early Paleozoic fine-grained dolomite was generated by remelting of Mesoproterozoic carbonatite. Hf isotopes of zircon have been widely used to study the nature of magma source due to their extremely low Lu/Hf and high resistance to secondary Hf isotope disturbance (Iizuka et al., 2017). Therefore, zircon can preserve the source nature, even though hydrothermal alteration is widespread in Bayan Obo deposit. ...
... Detrital zircon is ubiquitous in sedimentary rocks and has remarkable advantages in detrital studies, such as (i) chemical and physical stability against weathering processes (Ewing et al. 1995), (ii) U-Pb geochronology (Bowring and Schmitz 2003), (iii) geochemical tracers (Burnham and Berry 2017;Zhu et al. 2020), and (iv) isotopic tracers (Iizuka et al. 2017;Valley 2003). The applications of detrital zircons are wideranging, such as the evaluation of global continental growth (Rino et al. 2004;Sawada et al. 2018), reconstruction of paleo-tectonic history (Cawood et al. 2012;Grimes et al. 2007), and exploration of ore deposits (Pereira and Storey 2023;Wang et al. 2021). ...
Trace element fingerprints preserved in zircons offer clues to their origin and crystallization conditions. Numerous geochemical indicators have been established to evaluate the source rock characteristics from a geochemical perspective; however, multivariate trace element data have not been sufficiently investigated statistically. As substantial amounts of zircon data from a wide range of rock types have become accessible over the past few decades, it is now essential to reassess the utility of trace elements in discriminating source rock types. We employed a new zircon trace element dataset and established classification models to distinguish eight types of source rocks: igneous (acidic, intermediate, basic, kimberlite, carbonatite, and nepheline syenite), metamorphic, and hydrothermal. Whereas a conventional decision tree analysis was unable to correctly classify the new dataset, the random forest and support vector machine algorithms achieved high-precision classifications (> 80% precision, recall, and F1 score). This work confirms that trace element composition is a helpful tool for province studies and mineral exploration using detrital zircons. However, the compiled dataset with many missing values leaves room for improving the models. Trace elements, such as P and Sc, which cannot be measured by quadrupole inductively coupled plasma mass spectrometry, are vital for more accurate classification.
... As an effective geochemical tracer, zircon Hf isotope has been widely used in the source discrimination of some important geochemical reservoirs (Dong et al., 2013). Previous study proposed that the mean Hf isotope composition of juvenile continental crust evolved constantly from ε Hf(4.5 Ga) = 0 to ε Hf(preent) = +13 (Iizuka et al., 2017). The A-type granites from this study yield an average 176 Hf/ 177 Hf value of 0.282363 (ε Hf = 0.8) with a 94% confidence interval of 0.282334 to 0.282384 (ε Hf = −0.1 to 1.7). ...
... To explain these two different half-life values several models have been proposed. These are inhomogeneous isotopic abundance distribution of Hf in the early solar system originating from stellar nucleosynthesis 37 , secondary Lu/Hf fractionation in parent bodies of meteorites 38,39 , and decay acceleration through an isomer with a half-life of 3.7 h where the isomer is excited by cosmic radiations 40,41 . At present, there are the three systematically different values (the dashed-lines in Fig. 2). ...
The ¹⁷⁶Lu-¹⁷⁶Hf nuclear decay is a powerful tool to measure the age of astrophysical and geological events and has been used as a “cosmochronometer”. However, the half-life values of ¹⁷⁶Lu measured with various experiments differ significantly. Furthermore, the half-life values evaluated from Lu-Hf isochrons in meteorites and terrestrial rocks with known ages show two different values. Here we report half-life measurements using a method that is almost independent of various uncertainties. To the best of our knowledge this is the most accurate value of ¹⁷⁶Lu half-life. We measure the total energy released from ¹⁷⁶Lu decay using a windowless 4π solid angle detector based upon bismuth germanate (BGO) scintillation crystals, where a natural Lu sample is located inside of the detector. The measured half-life of (3.719 ± 0.007) × 10¹⁰ yr corresponding to a decay constant of (1.864 ± 0.003) × 10⁻¹¹ yr⁻¹ is consistent with that obtained from the analysis of terrestrial rocks within the uncertainty.
... The average δ 18 O is 7.7‰ ± 0.8‰ (2 standard deviations [SD], n = 51) and higher than the mantle-like value of 5.3‰ ± 0.3‰ (Valley et al., 1998). The zircon ε Hf (t) value using the CHUR (chondritic uniform reservoir) composition (Iizuka et al., 2015) is 10.3‰ ± 1.7‰ (2SD, n = 13), which is identical to the range of ε Hf values for mid-ocean ridge basalt (MORB) and primitive island arc rocks (Chauvel et al., 2008;Iizuka et al., 2017). ...
Zircon geochronology has contributed to our understanding of the longevity of transcrustal magmatic systems; however, most studies focus on zircon records from felsic rocks due to the restricted occurrence of zircon in mafic-ultramafic rocks. We present U−Pb age, geochemical, and Hf−O isotope data for zircons from a hornblendite peridotite in the Hida Belt, Japan, that offers a unique opportunity to investigate the lifetime of a long-lived mafic plumbing system in an arc setting. We found two zircon U−Pb age clusters: an incompatible element-rich cluster at 196 Ma and an incompatible element-poor cluster at 186 Ma. Their homogeneous isotopic signatures (δ18O = 7.7‰ ± 0.8‰, εHf = 10.3‰ ± 1.7‰) indicate the same magma source despite the 10 m.y. age gap. These two clusters are explained by different zircon formation mechanisms that differ depending on whether or not zircon saturation requires differentiated melt with high SiO2. The enriched older zircons formed by local zircon saturation at the mafic melt-olivine interface, whereas the younger depleted zircons precipitated from the last drop of interstitial felsic melt co-existing with hornblendes. Our finding substantiates the longevity of mafic systems at lower crusts, which sustain transcrustal magma systems and crustal evolution.
... In contrast, the lower crust, which is composed mainly of meta-igneous rocks with low tungsten concentrations, is not considered a source for the formation of such deposits (Ishihara, 1977;Song et al., 2021). The zircon Lu -Hf isotope system is widely considered to provide a powerful tool for deciphering the evolution of the crust and mantle (Iizuka et al., 2017;Kemp et al., 2009;Kinny and Maas, 2003;Vervoort and Kemp, 2016;Wu et al., 2006;Yang et al., 2007). Zircon Hf isotope data compiled for granites associated with tungsten deposits in the Nanling region are characterized by ε Hf (t) values of − 8 to − 11.6 and Paleoproterozoic T DM2 (1.7-1.9 ...
Tungsten is a critical and strategic metal used widely in the aerospace, automotive, electronic, and defense in dustries. Most tungsten deposits are genetically related to granitic rocks and a small number of them are giant deposits that supply much of the World’s demand for tungsten. Understanding the genesis of these giant deposits and developing models based on this understanding that can guide exploration for them is, therefore, a matter of considerable interest. China hosts nearly 50% of the global tungsten resource, much of which is contained in a few giant deposits. This makes it an ideal location in which to study the processes that lead to the formation of giant tungsten deposits. Here, we use a compilation of the whole-rock geochemical and zircon Hf–O isotopic compositions of granitic rocks associated with tungsten deposits in China and elsewhere, in conjunction with Rayleigh fractionation modeling and Monte Carlo simulations, to quantitatively evaluate the role of the source region, oxygen fugacity, and the degree of magma differentiation in the formation of giant tungsten deposits. The zircon Hf–O isotopic compositions are very heterogeneous, indicating that there is no source either in the upper crust, the lower crust, or one representing a mixture of crust and mantle that can explain the existence of giant tungsten deposits. Our modeling also shows that changes in oxygen fugacity have little impact on the formation of giant tungsten deposits. Instead, the modeling demonstrates that a combination of a pre-enriched source and a high degree of magma differentiation are pre-requisites for forming a giant tungsten deposit.
... Large rivers erode the continental crust in the regions through which they flow, resulting in the clastic materials of various rocks being effectively mixed in the process of their transfer from the bedrock source area to the estuary Iizuka et al., 2017;Rudnick et al., 2003). Modern river sediments can therefore provide geological information on the tectono-thermal events and crustal evolution of the region through which they flowed (Goldstein et al., 1997;Iizuka et al., 2010Iizuka et al., , 2013Rino et al., 2004). ...
... Zircon is persistent against physiochemical alteration processes, has low Lu/Hf and can be independently dated through U-Pb, which simplifies the determination of its initial 176 Hf/ 177 Hf. These properties have led to a vast global zircon-based data set from intermediate to sialic rocks (see Iizuka et al., 2017). Zircon-based data sets are commonly used to constrain early evolution of Earthś mantle although zircon is rarely found in rocks derived directly from a mantle source. ...
While whole-rock Lu-Hf isotope analysis remains one of the only ways to obtain initial Hf isotope signatures of
old mafic rocks, Hf isotope analyses of more robust accessory zircon in intermediate to silicic rocks have largely
replaced whole-rock analyses during the last decade. This has led to a discrepancy in the amount of existing data
from mafic and felsic lithologies. However, especially in mafic, Si-poor rocks with a metamorphic imprint, Hf
isotope data rely on whole-rock analysis since baddeleyite, commonly used for U-Pb age analyses of mafic rocks,
is sensitive to alteration and metamorphism. Hence, to accurately evaluate the trace element and isotope signatures
of altered mafic rocks, it is important to understand the mechanisms of element mobility during metamorphism.
Here, we report whole-rock trace element compositions, Lu-Hf and Sm-Nd isotope data from variably
deformed and metamorphosed samples of a mafic intrusion in southern Sweden, the Åker metabasite. These data
suggest that trace elements were undisturbed on a whole-rock sample scale during deformation at upper
amphibolite facies (at least 1000 MPa and 600 ◦C) metamorphism under hydrated conditions. Despite redistribution
of Zr associated with the breakdown of baddeleyite and other igneous phases, the Åker metabasite has
retained its chemical and isotopic integrity since igneous crystallisation at ca. 1565 Ma. This study demonstrates
and strengthens the feasibility of whole-rock analyses of (meta-)mafic rocks for determining initial εNd and εHf
values, despite deformation and metamorphism under hydrated amphibolite-grade metamorphic conditions.
Testing the coherence of the calculated initial Nd and Hf isotope ratios by examining variably deformed and
metamorphosed varieties of a rock in a single outcrop, could be used as a model for research on more complex
Archean rocks.
... Iizuka et al. 2005;Dhuime et al. 2012), that are possibly biased by ancient crust disappearance (e.g. Iizuka et al. 2017), or a combination of detrital and igneous zircons from various ages and origins (Belousova et al. 2010), thereby distorting quantitative information. Figure 12 illustrates the use of detrital minerals to reconstruct crustal growth and the effect of above-mentioned issues. ...
Detrital heavy minerals have helped address geologically complex issues such as the nature and origin of the early terrestrial crust, the growth and evolution of the continental crust, as well as the onset of plate tectonics, together with paleogeographic reconstructions and supercontinent cycles.
With the advent of in situ analytical techniques and a more complete understanding of trace element behaviour in rock-forming and accessory minerals, we have now at our disposal a powerful suite of tools that we can apply to multiple proxies found as detrital minerals. These can be in situ dating, trace element or isotopic tracing applied to both mineral hosts and their inclusions. We opted to showcase minerals that occur as primary minerals in a wide range of rock compositions and that can provide reliable age information. Additionally, over the last decades their chemistries have been tested as proxies to understand crustal processes. These are zircon, garnet, apatite, monazite, rutile and titanite. We include an overview and provide some solutions to common biases that specifically affect these minerals. This review brings together petrological, sedimentological and geochemical considerations related to the application of these detrital minerals in crustal evolution studies, highlighting their strengths, limitations and possible future developments.
... The range of εHf 0 suggest that the sources of the magmas was relatively homogeneous ( Table 2). The highly positive εHf(t) values obtained for Charrarruca subvolcanic body, can be explained by a moderately depleted mantle component, or a strongly-depleted sub-arc mantle that was overprinted by the addition of a recycled crustal derived component, in the magma source (Belousova et al., 2010;Griffin et al., 2000;Iizuka et al., 2017). Furthermore, the great difference between the highest and lowest εHf(t) values in a single sample, attests that the analyzed zircons precipitated during the development of open-system processes that could lead to the variation of 176 Hf/ 177 Hf ratio within the melt (Kemp et al., 2007). ...
The Charrarruca porphyry-type alteration zone is located in the northern part of the Andacollo Mining District (AMD), placed in the northern Neuqu´ en Precordillera, (Argentina). Potassic, chloritic-carbonatic, sericitic, and propylitic alteration halo of ~1.5 km2 affects an andesitic/dacitic subvolcanic body. These rocks, with arc affinity and Upper Cretaceous-Paleogene age (65 ± 2.5 Ma), derived from a magma evolved under normal crust thickness conditions, allowing the correlation with the intrusive facies of the Naunauco Group. The petrogenetic characteristics of these rocks attest to indications of the fertility of this magmatism, turning into a prospective guide for the unexplored Upper Cretaceous–Eocene Neuqu´en porphyry copper metallogenic belt.
... The range of εHf 0 suggest that the sources of the magmas was relatively homogeneous ( Table 2). The highly positive εHf(t) values obtained for Charrarruca subvolcanic body, can be explained by a moderately depleted mantle component, or a strongly-depleted sub-arc mantle that was overprinted by the addition of a recycled crustal derived component, in the magma source (Belousova et al., 2010;Griffin et al., 2000;Iizuka et al., 2017). Furthermore, the great difference between the highest and lowest εHf(t) values in a single sample, attests that the analyzed zircons precipitated during the development of open-system processes that could lead to the variation of 176 Hf/ 177 Hf ratio within the melt (Kemp et al., 2007). ...
... Mantle-derived magma underplating and crust-mantle interaction are important processes involved in the genesis of granites and their geodynamic mechanism of formation (Tang et al. 2021). Hafnium isotope data for single zircon grains are particularly useful for revealing crustal-mantle evolution (Villaros et al. 2012;Iizuka et al. 2017;Spencer et al. 2020). The Hf isotopic system is immobile even when rocks have been subjected to low-to medium-grade deformation and metamorphism (Barovich et al., 1992). ...
The Nanwenhe metamorphic dome contains a rare Silurian granitic pluton that developed in the SE Yangtze Block during early Palaeozoic orogenesis, and this pluton comprises the Nanwenhe gneissic granites. The petrogenetic and structural deformation histories of the gneissic granites recorded the multi-stage interaction of the SE Yangtze Block with other blocks during the Palaeozoic and Mesozoic, including the Cathaysia, Indochina, and Paleo-Pacific blocks. Zircon U–Pb and Hf isotopes and geochemical data indicate that the Early Palaeozoic granites (433–420 Ma) are S-type granitic rocks, and they were derived from the partial melting of ancient continental crust with no contribution from mantle material. And the large variations and positive values of εHf(t) (−12.25 to +10.69) of the gneissic granites are ascribed to a heterogeneous source and disequilibrium melting. Here we compare early Palaeozoic granitoids in the SE Yangtze Block (foreland belt) with those in the orogenic core, and suggest that the SE Yangtze Block underwent limited syn-collisional crustal thickening and metamorphism, and following which post-collisional granitoids were formed through partial melting of continental crust. Structural analysis of the Nanwenhe gneissic granites allows two stages of deformation to be identified: northward detachment (D1) and northwestward thrusting (D2). ⁴⁰Ar–³⁹Ar isotope dating of muscovites from the gneissic granites yields a well-defined plateau age of 229.61 Ma, which is interpreted as the timing of Late Triassic structural overprinting associated with the formation of the Nanwenhe metamorphic dome. Integration of our new results with previous research findings from the dome and South China allows us to conclude that the detachment deformation (D1) represents Late Triassic post-collisional extension, and D2 represented Jurassic intracontinental orogeny associated with subduction of the Paleo-Pacific Block.
... As an effective geochemical tracer, zircon Hf isotope has widely been used in source discrimination of some important geochemical reservoirs (Dong et al., 2013). Previous study proposed that the mean Hf isotope composition of juvenile continental crust evolved constantly from εHf (4.5 Ga) = 0 to εHf (preent) = + 13 (Iizuka et al., 2017). The A-type granites from this study yield an average 176 Hf/ 177 Hf value of 0.282363 (εHf = 0.8) with a 94% con dence interval of 0.282334 to 0.282384 (εHf = -0.1 to 1.7). ...
Numerous Neoproterozoic granitic intrusions occur over a north-south distance of more than 700 km in western Yangtze Block. In this paper, we newly identified Shimian granitic pluton as A-type granite which was previously considered as S-type granite in the region, with presenting its zircon U-Pb age, Lu-Hf isotopic and whole-rock geochemical data. The selected fresh samples for this study are mainly porphyritic biotite monzogranites. They are weak peraluminous, A 2 -subtype granites, displaying high contents of SiO 2 and K 2 O, low Cao and Na 2 O with the characteristics of high silica, rich potassium, and low sodium. They are enriched in incompatible trace elements, but low in trace elements compatible in mafic silicates and feldspars, and chemically characterized by having high REE and Ga abundances and low Al, Ni, Ba, Sr, and Eu abundances. The major and trace element behavior suggests that the granites are formed by advanced fractional crystallization. Zircon U-Pb age shows the Shimian granitic pluton was emplaced at 728 ± 20 Ma. The studied granites yield an average ¹⁷⁶ Hf/ ¹⁷⁷ Hf value of 0.282363 ( ε Hf = 0.8) with a 94% confidence interval of 0.282334 to 0.282384 ( ε Hf = -0.1 to 1.7), indicating that they are derived from remelting of juvenile continental crust. We suggest that this newly identified A-type granite at Shimian, southwestern China was more plausibly generated in an extensional setting as a result of slab window caused by a ridge subduction, which can account not only for the formation of voluminous granitoids, but also for the basalts, boninitic pillow lavas, and SSZ-type ophiolites in western Yangtze Block.
... Currently, evidence of PECMA magmatism earlier than 60 Ma is restricted to small plutons in the Santa Marta Region Duque-Trujillo, Orozco-Esquivel et al., 2019) and antecrysts in younger, Late Paleocene plutons (Bustamante et al., 2017). These antecrysts have εHf (i) values between +1.5 and +5.9 (Figure 2a), which are characteristic of zircons formed in magmas that derived from the mantle (Iizuka et al., 2017) and were modified by assimilation of an older continental crust (Bustamante et al., 2017;Cardona et al., 2018). ...
The Paleocene‐early Eocene continental magmatic arc (PECMA) in the Northern Andes is an example of arc magmatism following a major collisional event. This arc formed after the arc‐continent collision between the Caribbean Plate and the South American continental margin at ca. 72 Ma. We used detrital zircon LA‐ICP‐MS and CA‐ID‐TIMS geochronology and geochemistry to complement the limited plutonic record of the PECMA and better characterize the PECMA's magmatic evolution. Zircon geochronology and their respective trace element geochemistry were analyzed from Paleocene‐early Eocene strata of the Bogotá Formation in the foreland region. Our results show that after the collision of the Caribbean Plate, the magmas in the PECMA differentiated under a thick continental crust with limited subduction input at ca. 66 Ma. By 62–50 Ma, scattered patterns of Hf, U, U/Yb, and Yb/Gd ratios in detrital zircons suggest the existence of contrasting magmatic inputs attributed to different depths of crustal fractionation, varied temperatures of crystallization, and significant mantle and subduction inputs. These diverse magmatic patterns reflect the evolution of the continental crust. We proposed that oblique convergence and strike‐slip tectonics favored contrasting crustal architectures along the continental margin while local lithosphere dripping from a previously thickened crust promoted the formation of hot magmas under a thick continental crust.
... Raw counts for 172 Yb, 173 Yb, 175 Lu, 176 (Lu + Yb + Hf), 177 Hf, 178 Hf, 179 Hf, 180 Hf and 182 W were collected and isobaric interference corrections for 176 Lu and 176 Yb on 176 Hf need to be precisely determined. 176 Lu was calibrated using the 175 Lu value and correction of 176 Yb on 176 Hf (Iizuka and Hirata 2005;Iizuka et al. 2017). The detailed analytical technique was described by Yuan et al. (2008). ...
The subduction of the Proto-Tethyan Ocean is still a controversial subject. Here we report zircon U-Pb ages, geochemistry, and Hf isotopic compositions of the zircon in the Mengkete quartz diorite in the Buqingshan–A’nyemaqen Tectonic Mélange Belt, which lies at the southern margin of the East Kunlun orogenic belt on the northern Tibetan Plateau, to constrain the petrogenesis of subduction-related granitoids and to reconstruct the evolution of the Buqingshan–A’nyemaqen Ocean. Zircons U-Pb dating yields coeval ages of 441–436 Ma for Mengkete quartz diorite. Mengkete plutons have variable SiO2 (56.63–65.22%) and high Al2O3 (16.09–17.79%) contents and aluminous saturation indexes (A/CNK ratios) ranging from 0.77 to 0.96, which define their metaluminous and medium-K2O calc-alkaline signatures. The plutons have a low total rare-earth element (REE) content (45.49–168.31 ppm) and slightly positive Eu anomalies (Eu/Eu* in the range of 0.96 to 1.32). They are also enriched in large-ion lithophile elements (LILEs), such as Rb, Th, and Ba, but are depleted in high-field-strength elements (HFSEs), such as Nb, Ta, Zr, Hf, and Ti. Their zircon εHf(t) values ranging from 7.79 to 13.02, and the two-stage Hf (TDM2) model ages are in the range of 1130–657 Ma. These geochemical signatures indicate that the Mengkete quartz diorite was derived from partial melting of the mafic Meso-Neoproterozoic lower crust during the northward subduction of the Proto-Tethyan Ocean. The evolution of the East Kunlun Belt can be divided into five stages from the Neoproterozoic to late Middle Triassic: (1) an opend Buqingshan‒A’nyemaqen ocean during the Neoproterozoic up to 516Ma; (2) the continuous expansion of the Buqingshan‒A’nyemaqen ocean during the period 516 to 441Ma, along with the beginning of northward subduction in the Late Cambrian; (3) further subduction from 441 to 400Ma, accompanied by the production of large volumes of arc magmatic rocks; (4) the long period during which the Buqinshan–A’nyemaqen Ocean existed (400–240 Ma); and (5) the final closure of the Buqinshan–A’nyemaqen Ocean in the late Middle Triassic.
... The Nd and Hf model ages, which represent the time that the protocrust source separated from the mantle, have long been used to constrain the growth rate of the continental crust (Jacobsen, 1988;Rudnick, 1995;Hawkesworth and Kemp, 2006;Kemp et al., 2006;Wang et al., 2009Wang et al., , 2011Dey, 2013;Hawkesworth et al., 2013Hawkesworth et al., , 2019Iizuka et al., 2013Iizuka et al., , 2017Dew et al., 2018;Petersson et al., 2018;Frieman et al., 2021;Garçon, 2021). Model ages, calculated from MORB-type depleted mantle curves (Goldstein et al., 1984;Griffin et al., 2000Griffin et al., , 2002, can be up to ~300 Ma older than those calculated from arc mantle growth curves (Dhuime et al., 2011). ...
The ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf values of depleted mid-ocean ridge basalt (MORB) mantle have long been used to calculate the Nd and Hf model ages and constrain the growth rate of the continental crust. However, because continental crust forms principally at subduction zones, the appropriate ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf for use in model age calculations are those of the mantle wedge that underlies the juvenile magmatic arcs. In this study, we compile the Nd and Hf isotopic compositions of the modern island arc volcanic rocks. The average ¹⁴³Nd/¹⁴⁴Nd value obtained for the arc mantle is 0.512959 (εNd = +6.3), with upper and lower 95% confidence uncertainties of 0.513115 (εNd = +9.3) and 0.512613 (εNd = −0.5), respectively. The average obtained for ¹⁷⁶Hf/¹⁷⁷Hf is 0.283144 (εHf = +13.2) with upper and lower limits of 0.283272 (εHf = +17.7) and 0.282927 (εHf = +5.5), respectively, at the 95% confidence level. The Nd and Hf isotopic compositions of the modern island arc volcanic rocks can be divided into three populations, the high, transitional and low ε arc mantle populations, that reflect a combination of mantle heterogeneity and additions of different slab-derived components to the mantle wedge. The high ε arc mantle (HAM) population (εNd > +5.7 and εHf > +10) and the transitional ε arc mantle (TAM) population (εNd < +5.7 and εHf > +10) were metasomatized by the transfer of unradiogenic Nd and Hf from pelagic sediments, and Fe-Mn crusts and nodules, to the mantle wedge, the HAM by fluid dominate process and the TAM by a combination of sediment melts and fluids. In contrast, melts derived from subducted sediments, particularly zircon-rich turbidites, play a more important role in the low ε arc mantle population (LAM, εNd < +5.7 and εHf < +10). The average of both Nd and Hf isotopic data for the arc mantle, together with the 95% confidence limits, are used to define the arc mantle growth curves and their uncertainties. We suggest that these arc mantle growth curves should be used to calculate Nd and Hf model ages and constrain the growth rate of the continental crust.
... This raises a critical question about the applicability of zircon Hf isotopes in tracing the composition of crustal sources. Zircons in most crustal rocks undergo multiple episodes of metamorphism, anatexis, and magmatism (Amelin et al., 2000;Belousova et al., 2010;Iizuka et al., 2017;Kemp et al., 2007;Zheng and Gao, 2021). If their Lu/Hf ratios are greatly elevated due to the decomposition of HREE-rich minerals with high Lu/Hf ratios, it can introduce a large bias when estimating the residence time of crustal sources or tracing the nature of magmatic sources Zhang et al., 2020). ...
The initial Hf isotope composition of zircon in high-grade metamorphic rocks is an important tracer for protolith origin as well as for the growth and evolution of continental crust. However, less attention has been paid to the behavior of Lu-Hf isotopes in peritectic reactions during partial melting of the continental crust. In this paper, we present a combined study of zircon U-Pb ages, Lu-Hf isotopes and trace elements as well as petrology and whole-rock geochemistry for migmatites close to the eastern Himalayan syntaxis in southeastern Tibet. The results show that peritectic zircons in the migmatites have significantly higher ¹⁷⁶Hf/¹⁷⁷Hf ratios than their protolith zircons. More interestingly, the ¹⁷⁶Lu/¹⁷⁷Hf ratios and the contents of high field strength elements (such as Nb, Ta, and Hf) and heavy rare earth elements are significantly elevated in the peritectic zircons compared to the protolith zircons. This suggests that garnets with high Lu/Hf ratios and other minerals (such as titanite, ilmenite, amphibole, and biotite) with moderate Lu/Hf ratios were decomposed during crustal anatexis, contributing to the Hf isotope composition of peritectic zircons. This is confirmed by petrographic observations that garnet and biotite occur as residues or remnants in mesosomes (biotite gneiss) and some leucosomes (felsic veins), but are absent in the melanosomes of the target migmatites. Therefore, the peritectic zircons were produced together with anatectic melts through peritectic reactions and acquired the elevated ¹⁷⁶Lu/¹⁷⁷Hf ratios compared to the inherited zircons from the protoliths if no peritectic garnet was coevally produced during the crustal anatexis. In this regard, the greatly elevated Hf isotope compositions of peritectic zircons in anatectic granites cannot faithfully reflect the Hf isotope composition of parental rocks. As such, great care must be taken when using Hf isotope ratios to trace the nature of parental rocks with respect to magmatic processes and crustal evolution.
... Such parameters, in particular , are needed to calculate the proportions of recycled and reworked crust (see Eqs. 6 and 7), and ultimately the volumes of total, juvenile, recycled, and reworked crusts through time (see Eqs. 2 and 3). In substance, this equation resumes the conclusion of Iizuka et al. (45) in their review paper on the use of Hf-U-Pb isotopic systematics of zircons to constrain continental growth through time. ...
Continents form the most accessible parts of Earth, but their complex compositions make their origin difficult to investigate. A novel approach based on a comprehensive compilation of samarium-neodymium isotopic compositions of detrital sedimentary rocks is here used to unravel continental growth through time. This record reveals that continents were as felsic as today in the past 3.7 Ga (billion years) and that their growth was not continuous but episodic. Reworking of preexisting crust was a ubiquitous process during most of Earth history, but at least six periods of continental growth can be identified every 500 to 700 Ma (million years) in the past 3.7 Ga. This recurrence could be accounted for by changes in tectonic plate velocities favoring periods of rapid subduction and enhanced production of juvenile felsic crust.
... The Lu-Hf systematics is widely applied in deciphering crust-mantle interaction and in constraining magmas source(s) (Iizuka et al., 2017;Vervoort and Kemp, 2016). Both the fine-grained biotite granite and the pegmatite shows similar and a wide range of εHf(t) values from − 8.3 to − 2.0 and− 8.2 to − 2.1, respectively. ...
This study reports a new dataset of whole-rock geochemistry, biotite chemistry, in situ zircon UPb geochronology and Hf isotope for a suite of granite and associated pegmatite samples from the Gubrunde region in the Eastern Nigeria Terrane (ENT), Nigeria. The Gubrunde granitic rocks are weakly ferroan, peraluminous and calc-alkalic to alkali-calcic in composition, and show I-type affinity. The zircon UPb geochronology gives an age of ~580 Ma for the rocks, although the presence of inherited zircons with early Pan-African ages of 696 ± 12, 647 ± 7 and 624–613 Ma are evident indicative of a complex history of their source rocks. The Gubrunde granite and the pegmatite yielded similar average Hf crustal model age TDM2 of 1.9 ± 0.1 Ga and εHf(t) values −6.2 ± 1.2, suggesting that they may have sourced from reworked old crustal rocks with minor contributions from the mantle. The granite and the pegmatite were likely to connect by fractional crystallization under low to moderate pressure (~2.2 to 3.0 kbar) and temperature (~717 °C), and low oxygen fugacity (<ΔNNO -1.14). The ca. 580 Ma magmatism may have been triggered by delamination of the lithospheric mantle as a consequence of crustal thinning during waning stage of the Pan-African orogeny.
... The contour line of εNd = + 1 with respect to the depleted mantle ( Fig. 1c) represents the boundary of juvenile and evolved crust for the following reasons. The mean Hf isotope composition of juvenile continental crust evolved constantly from εHf(4.5 Ga) = 0 to εHf(present) = + 13, and has a constant relative uncertainty of + 30%/ − 68% (Dhuime et al. 2011;Iizuka et al. 2017), with the lower limit of εHf for the Phanerozoic juvenile crust at c. 4. Given the correlation of Hf and Nd isotopes for the crustal rocks (Vervoort et al. 1999), i.e., εHf = 1.34εNd + 2.82, the lower limit of εNd for the Phanerozoic juvenile crust is ~ + 1, which accounts for choosing εNd = + 1 as the boundary of juvenile and ancient crust. The contour maps of crustal model age (T DM2 ) and residence age (T res ) represent the time passed since extraction from a depleted mantle source (Mole et al. 2019), which are provided in Fig. 2 together with major geological boundaries. ...
Porphyry Cu deposits (PCDs) are major global exploration targets for Cu, Mo, and Au, and mainly formed in accretionary orogens. This study integrates the whole-rock geochemical and Nd isotopic mapping based on ca. 5000 intermediate to felsic magmatic rocks to characterize the crustal structure and metallogeny of PCDs in the Central Asian Orogenic Belt (CAOB), the world’s largest Phanerozoic accretionary orogen. The Nd isotopic maps image clearly the extent of cratons, microcontinents, and juvenile crustal blocks, with the PCDs intimately associated with juvenile crust that is consistent with the conclusions drawn by study in other orogens such as Himalayan–Tibetan. More importantly, PCDs are mainly distributed in the juvenile crust’s margin adjacent to cratons or microcontinents, indicating that the external magmatic arcs are favorable sites for PCDs. The ore-forming intrusions of Cu-Au deposits are isotopically more depleted than those of Cu-Mo deposits, implying more input of mantle or juvenile crust materials into magmatic sources of Cu-Au deposits. Crustal thickness proxy mapping shows that Cu-Au deposits formed in thinner crust than Cu-Mo deposits, which could be used to target favorable sites for mineral exploration. The crustal thickness proxy and εNd(t) define a negative linear relationship, which quantifies the contributions of juvenile materials to PCDs in response to the changing crustal thickness, from which a newly proposed crustal maturity index was derived to distinguish the Cu-Au and Cu-Mo PCDs. The structure of the crust plays an important role in localizing PCDs; therefore, the integrated mapping of Nd isotopes and crustal thickness proxy could provide mappable exploration targets of PCDs at the orogen scale.
... Taken together, these observations indicate that the Nd -Hf decoupling in the studied samples are mainly due to zircon sorting during transport. Furthermore, the negative ΔԐ Hf (clay) values reflect the quasi-absence of clay mineral formation in the catchment (associated with radiogenic Hf isotopic composition) and would instead indicate that the distribution of Nd -Hf isotopes in these suspended particulates is dominated by the presence of primary (unweathered) minerals (Gan, 1993;Iizuka et al., 2017). Of course, we cannot rule out the possibility that the Nd-Hf decoupling of the suspended loads may be affected by incongruent weathering of the minerals (Bayon et al., 2016). ...
Strontium (Sr), neodymium (Nd), and hafnium (Hf) isotopic analyses of different size-fractions of sediments collected from the Heihe River in the North Qilian Orogen (NQO) were carried out to trace the sediment sources and to determine the relationships between the Sr–Nd–Hf isotopic behaviors, and the grain-size and hydraulic sorting effects during fluvial transport. Our results demonstrate that the sand and suspended load samples collected from the same site have different Nd isotopic compositions, while their ⁸⁷Sr/⁸⁶Sr ratios are only slightly different. These features indicate that contributions to the sediments from different sources with different grain sizes vary greatly. Coarse-grained sand may more intuitively reflect the variations in local sources than the suspended load. The suspended load samples are distributed along the Clay Array on the ԐHf vs. ԐNd diagram due to zircon sorting. A similar NdHf decoupling phenomenon is widespread in the global oceans and large river systems, indicating that the NdHf isotopic behavior depends on the hydrodynamic sorting of minerals during fluvial transport. Moreover, the crustal accretion of the NQO from the depleted mantle occurred in the Proterozoic based on the constraints provided by the TDM values of the river sediments. The Nd isotopic compositional characteristics of the river sediments indicate that the NQO has an affinity with the Yangtze Craton. The determination of the basement tectonic affinity of the NQO significantly contributes to our understanding of the Neoproterozoic evolution of the Gondwana continental margin.
... Of paramount importance for understanding the development of the central Sahara crust is geochronology. With the advent of quick, reliable, and inexpensive in situ geochronology techniques, there is an opportunity to measure a diverse and significant quantity of silicate, oxide, and phosphate minerals (e.g., zircon, baddeleyite, monazite, apatite) that can identify key periods of magmatism and metamorphism but also offers information on temperature, tectonic setting, and Hf isotopic evolution (Grimes et al., 2015;Spencer et al., 2016;Iizuka et al., 2017). Moreover, the identification of inherited zircons within igneous suites can offer insight into the nature of the Saharan crust that could possibly extend knowledge to middle or lower crust or even confirm a Palaeoarchaean or Mesoarchaean heritage of the Saharan terranes and massifs. ...
The continental crust of North-Central Africa between the Tuareg and Arabian-Nubian shields and south to the Central African Orogenic Belt is enigmatic due to the few bedrock exposures especially within the central region. The current understanding, based on a review of geochronology and isotope geochemistry, is that the central Sahara region is a large, coherent craton that was ‘highly remobilized’ during the Late Neoproterozoic amalgamation of Gondwana and referred to as the Saharan Metacraton. However, new data from the Guéra, Ouaddaï, and Mayo Kebbi massifs and the Lake Fitri inlier of Chad suggest that it may be a composite terrane of older cratonic blocks or microcontinents with intervening Mesoproterozoic to Neoproterozoic domains and referred to as the ‘Central Sahara Shield’. It is postulated that the older crust and juvenile crust were sutured together along a Pan-Gondwana collisional belt (Central Sahara Belt) that bisects the central Sahara region. The ‘Central Sahara Shield’ hypothesis suggests the Chad Lineament, a narrow arcuate gravity anomaly within central Chad, could be a collisional belt suture zone and that it may explain the existence of the relatively juvenile crust that typifies southern and eastern Chad. The new data improves upon the existing knowledge and challenges the lithotectonic paradigm of the Saharan Metacraton. Further investigations are required to fully characterize the crust of the central Sahara region and to test the contrasting hypotheses.
... Thus, understanding the origin of Archean granitoids (TTGs) has profound implications for crustal growth and patterns of crustal reworking and recycling linked to underlying geodynamic processes. One approach to addressing these unresolved issues is to carry out combined isotopic composition (such as U-Pb, Hf and O isotopes) studies on zircons from ancient granitoids because isotopic proxies can not only determine the timing of magmatism and metamorphism, source characteristics and petrogenetic processes, but also provide constraints on crustal growth (crust-mantle differentiation), reworking and recycling, and track patterns of crustal processes and underlying geodynamic regimes (Valley, 2003;Hawkesworth and Kemp, 2006;Wu et al., 2007;Dhuime et al., 2012;Naeraa et al., 2012;Payne et al., 2015;Iizuka et al., 2017). ...
When, why and how global plate tectonics were initiated, as well as secular changes of plate tectonics over time are fundamental issues in the earth sciences. In this study, we present a combined U-Pb, Hf and O isotope dataset for complex zircons from the high-grade granitic (TTG) gneisses of the Haiyangsuo complex in the Sulu orogenic belt, eastern China. This combined dataset reveals that the granitoids formed by at least ca. 3.22, 2.81, 2.70 and 2.58 Ga and recorded multiple metamorphic thermal events that occurred at ca. 3.05, 2.60–2.50 and 1.93–1.85 Ga. The 3.22 and 2.58 Ga granitoids have negative εHf(t) values (averages of −2.08 and −10.81, respectively) and were derived from remelting of evolved Eoarchean crust, in combination of 3.47 Ga inherited zircon, reflecting occurrence of matured felsic continent crust in the Paleoarchean, whereas the 2.81 and 2.70 Ga granitoids show positive εHf(t) values (averages of + 4.32 and + 2.23, respectively) and formed by remelting of juvenile crust derived from depleted mantle at ca. 3.0 Ga. The 3.22, 2.70 and 2.58 Ga granitoids have mantle-like or slightly enriched zircon δ¹⁸O values (averages of 5.53 ± 0.15, 5.41 ± 0.18 and 6.10 ± 0.22‰, respectively). However, the 2.81 Ga granitoid displays remarkably high zircon δ¹⁸O values ranging from 9.65 to 7.02‰ with an average of 8.48 ± 0.29‰ (1 SD), this demonstrates occurrences of early hydrosphere-rock interactions and intra-crustal recycling in the Mesoarchean. Synthetically, here we propose the following tectonic model for the formation of the high-δ¹⁸O Mesoarchean granitoids: (1) Generation of voluminous high-density lower crust along with the formation of early felsic continent dominated by TTGs; (2) gravity-driven subduction of denser oceanic crust relative to continental crust induced by delamination of high-density lower crust together with lithospheric mantle; and (3) subsequently partial melting of altered high-δ¹⁸O hydrated oceanic crust caused by delamination-induced hot asthenospheric mantle upwelling to form the high-δ¹⁸O Mesoarchean granitoid. Furthermore, we argue that gravity-driven subduction occurred in the Archean, and subduction may be responsible for the trend towards increasing δ¹⁸O values within zircons since the Mesoarchean.
... Studies from Jack Hills, Australia, have demonstrated that detrital zircons preserve ages (~4.40 Ga) much older than the oldest (~4.03 Ga) exposed bedrock lithologies (Bowring and Williams 1999;Wilde et al. 2001;Reimink et al. 2016). The U-Pb ages from zircon provide clues on crystallisation ages while hafnium and oxygen isotopes provide crucial information on mantle extraction time and sedimentary versus magmatic origin of zircons (Valley et al. 2005;Kemp et al. 2007;Iizuka et al. 2017). Therefore, detrital zircons have been extensively used to understand various geological processes like crustal evolution (Hawkesworth et al. 2010;Spencer 2020), provenance (Hietpas et al. 2011b;Sun et al. 2018), orogenic and tectonothermal events (Park et al. 2010;Sorcar et al. 2020), tectonic settings (Cawood et al. 2012;Shao et al. 2020), maximum depositional age (Dickinson and Gehrels 2009;Sharman and Malkowski 2020), paleogeographic and paleoclimate reconstruction (Cawood and Nemchin 2001;Dickinson and Gehrels 2008;Pullen et al. 2011;Zhao et al. 2020), paleo-drainage (Hurtig et al. 2020), denudation, exhumation and thermal histories (Sircombe and Freeman 1999;Welke et al. 2016;Chai et al. 2020). ...
Over the past three decades, the advent of sophisticated imaging, in-situ measurements as well as data deconvolution techniques led to remarkable progress in the field of heavy mineral research. The prevalence of zircon in a wide range of igneous, sedimentary and metamorphic rocks has been used frequently in estimating provenance, depositional age, tectonic settings, drainage evolution, and crustal evolution studies. However, the biased age spectra (induced by hydrodynamic fractionation, sampling and measurement protocol and inheritance) yielded by detrital zircons reinvigorated the need to utilise other heavy mineral phases (monazite, apatite, titanite and rutile) to address a range of geological processes. Different heavy minerals are moderate to highly durable and provide variable responses to magmatic and metamorphic events thereby providing clues that may be missed by single detrital grain analysis. Thus, emphasising the multi-mineral detrital approach as an indispensable method to investigate several geological processes. The present review highlights the role of detrital zircon and the associated limitations in using a single heavy mineral approach in geological studies. This review further emphasizes the advantages of using multi-mineral/proxy studies and discussed the scope of heavy mineral research.
Progress in laser physics has enabled us to produce various particles such as electrons, photons, and ions from high field plasma generated from the interaction between high power laser and materials. Neutrons could be generated by nuclear reactions on the second target behind the first target. At present, it is possible to generate neutrons with a flux of up to 10 ¹¹ neutrons/shot. These neutrons are suitable for the study of nuclear astrophysics. We present the perspective and the present activity for the application to the nuclear astrophysics.
Mafic enclaves in granites are generally considered to represent coeval mafic melts that derived from metasomatized mantle, which can provide valuable information about crust–mantle interaction. Exploring the genetic links between the mafic enclaves and their host monzogranite from the Triassic Zhashui Pluton, Qinling orogenic belt. The enclaves (220 ± 4.6 Ma) and the monzogranite (220 ± 2.8 Ma) display identical zircon U‐Pb ages, and they also share similar trace element and zircon Lu‐Hf isotopes, indicating a cognate source. The monzogranite displays zircon ε Hf ( t ) values of –0.99 to +1.98, while the mafic enclaves show similar values of –0.45 to +3.26; however, differences in mineral chemistry reveal different crystallization conditions. The amphibole from the mafic enclaves has higher temperature and pressure (757°C; 2.65 kbar) compared to those of the host monzogranite (733°C; 1.96 kbar), suggesting that mafic minerals in the enclaves crystallized at an early stage. Moreover, apatite in the mafic enclaves displays slightly higher volatile contents (0.72 wt%) than those of the monzogranite (0.66 wt%), indicating a volatile‐rich condition. These results suggest that the mafic enclaves represent early hydrous mafic cumulates in the granitic chamber, and subsequent magma convection would have led to the formation of the mafic enclaves.
This work delves into the intricate Paleoproterozoic geological setting of Uruguay, focusing on the Rio de la Plata Craton (RPC) and its prominent constituent Piedra Alta Terrane (PAT). The PAT constitutes the core of the RPC, characterized as a complex geological entity that encompasses a granite-migmatite basement, metamorphic belts, granite intrusions, and Proterozoic magmatism. To the west, it interfaces with the Transbrasiliano-Kandi tectonic corridor, bounded by the Late-Neoproterozoic Pampia Terrane. The PAT s southeastern boundary extends to Sierra de Tandil and perhaps to Sierra de La Ventana, demarcating its limits with Phanerozoic rocks of the North Patagonian Massif. The Sarandí del Yí shear zone defines the eastern limit of the PAT. Despite varied interpretations of the Piedra Alta Terrane’s significance, it is important to note that consensus prevails regarding its four major associations: a granite-gneiss-migmatitic basement complex, three volcanosedimentary belts of low to medium metamorphic grade, diverse late igneous intrusions, and extensional Statherian magmatism (1.79 Ga), represented by tholeiitic gabbroic dikes. In this study, a novel tectonic model for the PAT evolution emerges, supported by an extensive zircon dataset comprising new U-Pb ages, Hf isotopes, and trace element geochemistry. The analytical core involves U-Pb analyses on zircon for dating igneous rocks and establishing the provenance and maximum age of deposition of metavolcanosedimentary sequences. Lu-Hf isotopic analyses on the same zircon crystals provide petrogenetic insights, revealing episodes of crustal growth and minor recycling. Trace element geochemistry and zircon εHf(t) data further accentuate these findings, hinting at mantle source enrichment due to subduction within an intraoceanic magmatic-arc arc setting.
In this contribution, we present new early middle Devonian igneous and metaigneous units with a major juvenile magmatic source input in the North Patagonian Massif, which were discovered through U‐Pb and Lu‐Hf zircon analyses. Afterward, we assessed their tectonic implications for northwestern Patagonia and then for southern South America, combining our results with available database information consisting of igneous crystallization ages and isotopic data of the Devonian to early Carboniferous magmatic units, tectonic‐metamorphic analyses, and thermochronologic record. This study allows for distinguishing retreating and advancing subduction switching in northwestern Patagonia (38°30′ to 44°S) and a contrasting coetaneous evolution for basement outcrops exposed further north (27°30′ and 37°30′S). The early middle Devonian (400–380 Ma) northwestern Patagonian magmatism is characterized by widespread magmatism and positive εHf–εNd linked to forearc and backarc magmatism that evolved within a retreating subduction stage. A tectonic switching toward advancing orogeny stage began in the late Devonian, evidenced by a lull in magmatic activity with a negative εHf–εNd trend, possibly contemporaneous with the first tectonic‐metamorphic event in western Patagonia. An early Carboniferous magmatic gap, followed by the subsequent development of the main foliation in the basement during the Carboniferous‐Permian period, denotes the acme of this contractional stage. In contrast, the Devonian period in the northern segment is characterized by mostly negative εHf–εNd values, reverse shear zone activity in the foreland, and an inboard magmatism migration, evidencing a compressive tectonic setting that changed to an extensional configuration in the early Carboniferous with widespread arc magmatism development.
Reported data for Lu, Hf mass fractions and ¹⁷⁶ Hf/ ¹⁷⁷ Hf isotopic ratios of mafic‐ultramafic rock reference materials (RMs) are rare at present, hampering the comparison of data quality between difference laboratories. Herein, we measured the Lu and Hf mass fractions and the ¹⁷⁶ Hf/ ¹⁷⁷ Hf isotopic ratios of ten mafic‐ultramafic rock RMs, including OKUM (komatiite), WPR‐1 (serpentinised peridotite), NIM‐N (norite), NIM‐P (pyroxenite), UB‐N (serpentinised peridotite), JP‐1 (peridotite), NIM‐D (dunite), MUH‐1 (serpentinised harzburgite), HARZ01 (harzburgite), DTS‐2b (dunite), which contain extremely low amounts of Lu and Hf (Lu, 2–150 ng g ⁻¹ ; Hf, 5–500 ng g ⁻¹ ). All measurements were conducted on a Neptune Plus multi‐collector inductively coupled plasma‐mass spectrometer employing an Aridus II desolvator inlet system and Jet sample/X skimmer cone configuration. The following RMs, OKUM, WPR‐1, UB‐N, JP‐1, NIM‐D, HARZ01 and DTS‐2b, have ¹⁷⁶ Hf/ ¹⁷⁷ Hf precision of less than 60 ppm (2 s ) at test portions of 0.06 to 0.3 g, and are good ultramafic RMs for Lu‐Hf systems. In contrast, significant Hf mass fraction and isotopic variability were observed in NIM‐N, NIM‐P and MUH‐1 at test portion masses of 0.1 to 0.3 g. Collectively, our study provides a new database that can be used by the geochemical community for evaluating the Lu‐Hf system of mafic‐ultramafic rocks.
The variations in source rocks and melting conditions of granites can provide essential clues for the crustal magmatic response in orogenic process. Based on geochronology, whole‐rock and mineral chemistry, this paper reveals two different granites in the Northern Qinling migmatite complex, which reveal obvious differences in source region and melting condition. The older granodiorite (402±0.8Ma) displays typical Na‐rich adakite affinity, i.e, high Na2O/K2O (2.04 to 2.64) and Sr/Y (96 to 117) ratios, they have relative evolved isotopic compositions (ɛNd (t) = −0.04 to −0.52; zircon ɛHf(t) = −0.06 to +7.78). The younger leucogranite (371±2 Ma) displays higher SiO2 (72.32% to 73.45%), lower (FeOT+MgO+CaO+TiO2) contents (<2%) and depleted Sr‐Nd‐Hf isotopic compositions (i.e., ɛNd(t)=+2.6 to +3.0; zircon ɛHf (t) = +5.94 to +14.12), as well as high 10000*Ga/Al and FeOT/MgO ratios, indicating that they represent highly fractionated I‐type granites that derived from melting of juvenile crust. The variations in source rocks and melting condition of the two granites indicating a tectonic switch from compression to extension in 400 to 370Ma, this switch is later than that in the eastern section of the North Qinling, indicating a scissor collision process between the South Qinling and NCC in Devonian era.
Crustal zircon xenocrysts from mantle-derived magmatic rocks have the potential to probe the deep crust. Here we present integrated U–Pb dating and Hf-isotope analyses of zircons from a Paleozoic diamondiferous kimberlite dike in the Maping area of Zhenyuan County (southeastern Guizhou Province), with implications for the tectonothermal evolution of unexposed continental crust beneath the southern Yangtze Block, South China. All zircons (n = 236) show a wide range of U–Pb ages between Mesoarchean and middle Carboniferous. Among them, 96 zircons with 90–110% concordance yield concordant ages from 2942 ± 8 Ma to 342 ± 2 Ma, and form major age peaks at ∼2.9 Ga, ∼2.6 Ga and ∼2.0 Ga. The overwhelming majority of zircons are dominated by Mesoarchean–Paleoproterozoic U–Pb ages regardless of their concordance degrees. The zircon populations mainly consist of magmatic zircons, with minor metamorphic grains (Th/U < 0.10). The youngest magmatic zircon (Th/U = 0.43) with a well concordant ²⁰⁶Pb/²³⁸U age of 342 ± 2 Ma (M1-16) is interpreted as the maximum emplacement age of the Maping diamondiferous kimberlites. Most zircons with pre-eruption ages are considered to be xenocrysts, and they may be derived from the deep-seated continental crust through which kimberlite host magmas have passed. Their U–Pb ages and Hf isotopic compositions suggest the possible existence of a highly evolved Archean basement beneath the southern Yangtze Block, South China, which is much older than its known surface rocks. A lot of magmatic zircon xenocrysts reveal complex Precambrian crustal evolution in the southern Yangtze Block. These processes involved the important growth of continental crust at 2.6–2.5 Ga, and in the meantime, crustal reworking could be intermittently proceeding at 3.0–2.6 Ga. In addition, a group of xenocrystic zircons are identified to be of metamorphic origin, indicating that the proposed Archean basement beneath the southern Yangtze Block likely experienced a metamorphic event around 2.0 Ga. This geologically significant episode is consistent with the well-developed coeval metamorphism in other places of the Yangtze Block (e.g., Kongling Terrane), which has been considered to link to the assembly of the Paleoproterozoic Nuna/Columbia supercontinent. Our zircon data implies that the unexposed Archean basement beneath the southern Yangtze Block was affected by multiple thermal activities.
Subduction of oceanic plates beneath continental lithosphere is critical for understanding tectonic evolution and evaluation of prospecting and exploration. Within the Yidun Terrane (YDT) of southwestern China, a number of Mesozoic to Cenozoic granitoid intrusions are exposed and they are useful for investigating the tectonic evolution of the Paleo-Tethys system. However, Mesozoic magmatism of the northern portion of the YDT, remains ambiguous regarding to their magmatic spatial–temporal evolution and their mineralization potential. As the largest pluton in the northern YDT, the Cuojiaoma batholith mainly consists of monzogranite and granodiorite. In this study, we present new zircon U–Pb and molybdenite Re–Os ages, whole-rock geochemical, and zircon Hf–O isotopic data for the Cuojiaoma batholith. LA-ICP-MS zircon U-Pb dating of granodiorite and monzogranite exhibit ages of 221.8 ± 1.4 Ma (n = 22, MSWD = 2.4) and 216.7 ± 2.2 Ma (n = 14, MSWD = 0.03), respectively. A total of 9 molybdenite samples differing Re-Os model ages of 205.4 ± 3.3 and 220.6 ± 5.5 Ma, yield a robust weighted mean model age of 209.9 ± 1.8 Ma (MSWD=2.4, n=9) representing the depositional age of molybdenite. The monzogranite and granodiorite’s mineralogical and geochemical characteristics indicate they are classified as (medium-) high-K calc-alkaline and metaluminous to weakly peraluminous I-type granite. Geochemically, they are enriched in large-ion lithophile elements (LILEs, e.g., Rb, U, K) and light rare earth elements (LREEs), and depleted in high-field-strength elements (HFSEs, e.g., Nb, Ta, Ti and P) and heavy rare earth elements (HREEs), and contain distinctly or slightly negative Eu anomalies and no significant Ce anomalies, indicating an affinity to classical island arc magma. Combined with their negative zircon εHf(t) values (−16.24 to −2.49 and −16.41 to −1.43) and two-stage Hf model ages (2019–1255 Ma and 2027–1200 Ma), plus their zircon δ¹⁸O values, which range from 5.96 to 8.01 and from 5.05 to 7.61 for monzogranite and granodiorite, respectively, these geochemical indexes indicate that the Cuojiaoma batholith shares similar petrogenesis to other intrusions within the YDT. The formation of the early granodiorite may be genetically related to the slab subduction and is most likely formed by mixture of lower crustal melts and mafic magma derived from partial melting of mantle wedge induced by the influx of slab derived melt (fluid). Subsequent slab break-off and the upwelling asthenosphere at ∼216 Ma to 210 Ma led to high heat flow and extensive melting of the overlying mantle wedge, followed by the highly crystallization differentiation, which finally contributed to the monzogranite and the subsequential disseminated molybdenite in a post-subduction extension setting. The northern YDT possesses high Mo metallogenic prospectivity, especially for the magmatic activity that dominantly by lower crustal melt and genetically related to the slab break-off occurred at ∼ 216 Ma and represented by the highly fractionated granite derived from the large Cuojiaoma granitic batholith.
The Mesoproterozoic Baoban Group is the oldest basement in Hainan Island and has played an important role in the Columbia (Nuna) supercontinent reconstructions. The Mesoproterozoic granitic intrusions in the Baoban Group are the most widely exposed Precambrian magmatic rocks and are the key to understand the tectonic settings of Hainan Island and its relationship with the South China Block and the Columbia supercontinent. Our new LA‐ICP‐MS zircon U‐Pb dating on three mylonitic granite samples from the Tuwaishan and Baoban areas yielded ages ranging from 1447 Ma to 1437 Ma, representing the absolute timing of the emplacement of the granitic intrusions. Combined with previously published geochronological data for rocks from the Baoban Group and regional mafic intrusions, we conclude that the Baoban Group formed at 1460–1430 Ma, coeval with the emplacement of the granitic and mafic intrusions. New in‐situ zircon Lu‐Hf isotope analyses for the three mylonitic granite samples yielded positive εHf(t) values ranging from +0.49 to +8.27, and model ages () ranging from 2181 Ma to 1687 Ma, suggesting that the granitic intrusions were originated from a mixed source of Paleoproterozoic crust with juvenile crust. New zircon trace element data show characteristics of high Th/U values of 0.24–1.50, steep slope from LREE to HREE and negative Pr, Eu anomalies with positive Ce, Sm anomalies, representing typical magmatic zircons formed in continental crust. Compared with available magmatic and detrital zircon ages from Precambrian rocks in Cathaysia Block, Yangtze Block and western Laurentia, it is inferred that Hainan Island was separated from both Cathaysia Block and Yangtze Block and was connected with western Laurentia in Columbia supercontinent. Considering the decreasing tendency of basin deposition time along the western margin of Laurentia, we propose that Hainan Island was located to the north or northwest of the Belt‐Purcell Supergroup along the western margin of Laurentia during the breakup of the Columbia supercontinent.
A comprehensive analysis of U-Pb geochronology and Hf-O isotopes of zircons from granulite xenoliths hosted in the Cretaceous basalt and zircon xenocrysts in the Jurassic andesitic volcanic rocks of Western Liaoning within the North China Craton (NCC) provides an opportunity to understand the evolution of the lower crust in the region. Zircons from these granulites yield ²⁰⁷Pb/²⁰⁶Pb ages of 1678–2646 Ma and consistent upper intercept ages of 2514–2544 Ma. In contrast, zircon xenocrysts yield two periods of ²⁰⁷Pb/²⁰⁶Pb ages of Neoarchean (2500–2591 Ma) and Paleoproterozoic (1733–2499 Ma), which produce consistent upper intercept ages of 2543–2549 Ma and lower intercept ages of 1761–1789 Ma, respectively. The Neoarchean zircons from the granulites and xenocrysts generally have similar positive εHf(t) values resulting in Hf depleted mantle model ages (TDM) of 2.5–2.9 Ga with a mode of 2.7 Ga and variable δ¹⁸O (5.05‰ to 8.81‰) values. However, the δ¹⁸O values of zircons from the xenocrysts are a little bit higher than those of zircons from granulites (peak of 8.00‰ for the xenocrysts and 6.00‰ for the xenoliths). Some Paleoproterozoic zircon xenocrysts have negative εHf(t) values (−12.8 to −9.39) and similar ¹⁷⁶Hf/¹⁷⁷Hf ratios and δ¹⁸O values to those of the Neoarchean zircons, whereas the other Paleoproterozoic zircons show chondritic εHf(t) (−4.34 to 1.16) and higher δ¹⁸O (7.70‰ to 10.19‰) values. All these data demonstrate that the primordial lower crust beneath Western Liaoning originally formed in the Neoarchean, perhaps around 2.6–2.7 Ga. The ubiquitous presence of ca. 2.5 Ga magmatic zircons illustrates that an important melting event indeed happened in the lower crust beneath Western Liaoning, similar to the thermal event throughout the entire NCC. In addition, the Neoarchean lower crust underwent another important thermal event during the Paleoproterozoic, which was considered to be associated with the final amalgamation of the Western and Eastern blocks of the NCC. Thus, we can conclude that there indeed existed an Archean lower crust beneath the Western Liaoning until to the Late Mesozoic, and this Archean lower crust underwent a similar multiple-stage (Neoarchean and Paleoproterozoic) evolutionary history as those throughout the NCC.
Zircon age-Hf isotopic data on the Archean Singhbhum and Dharwar cratons and the Archean-Proterozoic Southern Granulite Terrain (SGT) obtained at the CSIR-NGRI and by others elsewhere are in focus here. These data are used to decipher episodes of juvenile crust formation in the protracted (collectively spanning ∼3.7 billion years) geologic history of the three terranes in the light of their regional geology, structure and deep-crustal architecture based on recent geophysical experiments as well as current perspectives on early Earth crust forming processes and geodynamics. Our important observations and inferences include: (1) the Hf-isotopic compositions of the Hadean-Eoarchean aged (ca. 4.2–3.6 Ga) zircon grains from the Singhbhum craton have distinctly unradiogenic Hf-isotopic compositions quite similar to the Jack Hills Hadean-Eoarchean detrital zircons, suggesting derivation from TTG-like melts generated by the internal reworking of a long-lived, geochemically enriched mafic reservoir formed around ca. 4.5 Ga; (2) a shift to strongly radiogenic zircon Hf isotope compositions during the early Paleoarchean around ca. 3.6–3.5 Ga (Singhbhum craton) and ca. 3.5–3.4 Ga (Western Dharwar craton) is conspicuous. This may relate to the time of development of depleted mantle reservoirs, the source of the voluminous Paleo-Mesoarchean juvenile felsic magmatism and crust formation events that extended for ca. 400–300 million years; (3) in the entire Dharwar craton and the northern parts of the SGT there is clear evidence for widespread juvenile magmatic episodes during the Neoarchean, around ca. 2.7 Ga and ca. 2.55 Ga, the latter being predominant and widespread; (4) in the southernmost part of the SGT, prominent juvenile magmatic episodes are also evident during the Paleoproterozoic (ca. 2.0 Ga, Trivandrum block) and early Neoproterozoic (ca. 1.0–0.9 Ga, in parts of the Madurai block); (5) onset of plate tectonic processes in the Singhbhum and Western Dharwar cratons during early Paleoarchean (ca. 3.6–3.5 Ga) cannot be ruled out, but there is clear evidence for the operation of plate tectonics, significant crustal growth and terrane amalgamation only after ∼3.0 Ga in the Dharwar craton and the SGT and (6) regional dome and basin structural pattern of the pre-3.0 Ga crust attests to the role of internal differentiation processes (Rayleigh-Taylor Inversions) and vertical tectonics for the Paleo-Mesoarchean crust of the Singhbhum and Dharwar cratons. Together with other lines of evidence; changes in bulk crustal composition, deep crustal architecture, zircon age-Hf isotope distribution etc., we infer a transition to plate tectonics around 3.0 Ga in the Singhbhum and Dharwar cratons.
The east Pilbara Craton provides a complete geological record of crustal evolution through the Paleoarchean and Mesoarchean. Based on almost 50 years of research on the best exposed eastern part of the craton, this comprehensive review describes and interprets the stratigraphy, structure, geochemistry, geochronology, and mineralization of this section of Earth's crust.
Zircon is the most abundantly used mineral for dating igneous and metamorphic events and for tracing source characteristics. Understanding the geochemical behavior of the U-Pb-Hf-O isotope systems during high-grade metamorphism is therefore important for accurate interpretation of the isotopic information. We report zircon U-Pb-Hf-O isotopes and trace elements of retrograded eclogites and host gneisses from Chicheng, North China Craton, with the aim to obtain new insights into the fidelity of U-Pb-Hf-O isotopes in zircon as recorders of high-grade metamorphism. U-Pb dating suggested that the Chicheng mélange experienced eclogite facies metamorphism at ~1.84 Ga, and then exhumed to amphibolite facies at 320–300 Ma. Zircons with Paleoproterozoic ages formed in metamorphic melts-derived from the gneiss during the eclogite facies metamorphism. Zircons with ages of 300–320 Ma formed by recrystallization of peak metamorphic zircons during fluid-assisted amphibolite-facies retrograde metamorphism. This process led to the near-complete resetting not only of U-Pb ages but also of Hf-O isotopic compositions of the peak metamorphic zircons, while preserve REE patterns. These results contrast with the sluggish Hf diffusion rate predicted from experimental studies, and support findings that isotopic data from metamorphic zircons in retrograded high-grade metamorphic rocks need not be faithful recorders of their sources.
Western South America provides an outstanding laboratory for studies of magmatism and crustal evolution because it contains Archean-Paleoproterozoic cratons that amalgamated during Neoproterozoic supercontinent assembly, as well as a long history of Andean magmatism that records crustal growth and reworking in an accretionary orogen. We have attempted to reconstruct the growth and evolution of western South America through U-Pb geochronologic and Hf isotopic analyses of detrital zircons from 59 samples of sand mainly from modern rivers. Results from 5524 new U-Pb ages and 1199 new Hf isotope determinations are reported. Our data are integrated with previously published geochronologic and Hf isotopic information, yielding a combined record that includes >42,000 ages and >1900 Hf isotope analyses. These large data sets yield five main conclusions: (1) South America has an age distribution that is similar to most other continents, presumably reflecting processes of crustal generation and/or preservation related to the supercontinent cycle, with age maxima at 2.2-1.8 Ga, 1.6-0.9 Ga, 700-400 Ma, and 360-200 Ma; (2) <200 Ma magmatism in western South America has age maxima at ca. 183, 166, 149, 125, 110, 88, 65, 35, 21, and 4 Ma (with significant north-south and east-west variations), yielding an average cyclicity of ~33 m.y.; (3) for the past 200 m.y., no correlation exists between magmatism and the velocity of convergence between central South America and Pacific oceanic plates, the age of the downgoing plate, or the absolute motion of South America; (4) Hf isotopes record reworking of older crustal materials during most time periods, with incorporation of juvenile crust at ca. 1.6-1.0 Ga, 500-300 Ma, and ca. 175-35 Ma; and (5) the Hf isotopic signature of <200 Ma magmatism is apparently controlled by the generation of evolved crust during crustal thickening and eastward arc migration, versus juvenile magmas during extensional tectonism and westward and/or outboard migration of arc magmatism.
Studies of Lu–Hf isotope systematics in meteorites have produced apparent " ages " that are older than Pb–Pb ages and older than the estimated age of our solar system. One proposed explanation for this discrepancy is that irradiation by cosmic rays caused excitation of 176 Lu to its short-lived isomer that then underwent rapid decay to 176 Hf. This explanation can account for apparent excesses in 176 Hf that correlate with Lu/Hf ratio. Mass balance requires that samples with measurable excess in 176 Hf should also have measurable deficiencies in 176 Lu on the order of 1‰–3‰. To unambiguously test the accelerated decay hypothesis, we have measured the 176 Lu/ 175 Lu ratio in terrestrial materials and achondrites to search for evidence of depletion in 176 Lu. To a precision of 0.1‰ terrestrial standards, cumulate and basaltic eucrites and angrites all have the same 176 Lu/ 175 Lu ratio. Barring a subsequent mass-dependent fractionation event, these results suggest that the apparent excesses in 176 Hf are not caused by accelerated decay of 176 Lu, and so another hypothesis is required to explain apparently old Lu–Hf ages.
In-situ electron-microprobe dating of monazite holds the promise of being an effective technique for obtaining chronologic data. Our research focuses on I- and S-type granitoids of the Lachlan Fold Belt, Australia, whose petrology and zircon chronology have been thoroughly characterized. This study documents the textural relationships, morphology, zoning, and ages of monazite in these granitoid rocks. The I-type granitoids that lack monazite usually contain other mineral phases enriched in rare earth elements, such as allanite, titanite, and bastnasite. Only silica-rich, highly evolved I-type granitoids contain monazite. This preference in I-type rocks for phases other than monazite to host the REE and Th limit the applicability of monazite dating for this group of igneous rocks. On the other hand, monazite is ubiquitous in S-type rocks, both as interstitial and included grains. High-resolution X-ray maps of individual monazites reveal complex patterns of chemical zoning. Weighted averages of multiple analyses for individual chemical domains show small but systematic differences in age. These weighted averages of the chemical domains are considered the best estimate of the age of the monazite. Monazites from 8 different S-type samples range in age from 405 to 759 Ma, with the majority being 490 Ma and older. These are premagmatic ages for these granitoids, which have crystallization ages of 400 to 430 Ma. These premagmatic ages are similar to Cambro-Ordovician ages obtained from inherited zircon cores in the same granitoids, indicating that monazite can survive anatexis in peraluminous rocks. Thus, monazite dating in peraluminous rocks may illuminate characteristics (composition and age) of source rocks and anatectic processes.
The rate of growth of the continental crust is controversial. We present an evaluation of time-constrained analyses of oxygen isotopes in zircon grains and incompatible element (Zr, Th) concentrations in magmatic rocks to test for variations in the degree of crustal recycling through geological time. The data indicate a rise in these geochemical proxies from ca. 3.0 Ga to a statistically significant peak at 1.2-1.1 Ga during the amalgamation of supercontinent Rodinia, and a decrease thereafter. When combined with other geological and geophysical observations, the data are interpreted as a consequence of an unprecedented level of crustal recycling and sediment subduction during Rodinia assembly, arising from a "Goldilocks" (i.e., just right) combination of larger, thicker plates on a warmer Earth with more rapid continental drift relative to modern Earth. The subsequent decrease in delta O-18, Zr, and Th measurements is interpreted to reflect decreasing drift rates on a cooling Earth.
Significance
The radioactive decay of lutetium-176 to hafnium-176 has been used to study Earth’s crust−mantle differentiation that is the primary agent of the chemical and thermal evolution of the silicate Earth. Yet the data interpretation requires a well-defined hafnium isotope growth curve of the bulk Earth, which is notoriously difficult to reconstruct from the variable bulk compositions of undifferentiated chondrite meteorites. Here we use lutetium–hafnium systematics of meteorite zircon crystals to define the initial hafnium isotope composition of the Solar System and further to identify pristine chondrites that are the best representative of the lutetium–hafnium system of the bulk Earth. The established bulk Earth growth curve provides evidence for Earth’s crust−mantle differentiation as early as 4.5 billion years ago.
The strong resilience of the mineral zircon and its ability to host a wealth of isotopic information make it the best deep-time archive of Earth's continental crust. Zircon is found in most felsic igneous rocks, can be precisely dated and can fingerprint magmatic sources; thus, it has been widely used to document the formation and evolution of continental crust, from pluton- to global-scale. Here, we present a review of major contributions that zircon studies have made in terms of understanding key questions involving the formation of the continents. These include the conditions of continent formation on early Earth, the onset of plate tectonics and subduction, the rate of crustal growth through time and the governing balance of continental addition v. continental loss, and the role of preservation bias in the zircon record.
Supplementary material
A compilation used in this study of previously published detrital zircon U-Pb-Hf isotope data are available at http://www.geolsoc.org.uk/SUP18791
A global U-Pb and δ18O zircon database shows temporal changes in the magmatic record related to changes in the degree of crustal reworking. The δ18O composition of bulk sediment remains relatively constant through geologic time, with a mean value of 14.9‰. In contrast, the δ18O values in magmatic zircons vary from relatively low values averaging ∼6‰ in the Archean to increasingly higher and scattered values defining a series of peaks and troughs in post-Archean data. The degree of crustal reworking increases at times of supercontinent assembly. Therefore we attribute the pattern of post-Archean δ18O values recorded by magmatic zircons to a significant increase in the incorporation of high δ18O sediment in response to enhanced crustal thickening and reworking associated with the onset of collisional tectonics, especially during formation of supercontinents.
It is unclear how the earliest continental crust formed on an Earth that was probably originally surfaced with oceanic crust. Continental crust may have first formed in an ocean island-like setting, where upwelling mantle generates magmas that crystallize to form new crust. Of the oceanic plateaux, Iceland is closest in character to continental crust, because its crust is anomalously thick1 and contains a relatively high proportion of silica-rich (sialic) rocks2. Iceland has therefore been considered a suitable analogue for the generation of Earth's earliest continental crust3. However, the geochemical signature of sialic rocks from Iceland4-7 is distinct from the typical 3.9- to 2.5-billion-year-old Archaean rocks discovered so far8. Here we report the discovery of an exceptionally well-preserved, 4.02-billion-year-old tonalitic gneiss rock unit within the Acasta Gneiss Complex in Canada. We use geochemical analyses to show that this rock unit is characterized by iron enrichment, negative Europium anomalies, unfractionated rare-earth-element patterns, and magmatic zircons with low oxygen isotope ratios. These geochemical characteristics are unlike typical Archaean igneous rocks, but are strikingly similar to those of the sialic rocks from Iceland and imply that this ancient rock unit was formed by shallow-level magmatic processes that include assimilation of rocks previously altered by surface waters. Our data provide direct evidence that Earth's earliest continental crust formed in a tectonic setting comparable to modern Iceland.
Zircons are crucial to understanding the first 500 Myr of crustal evolution of Earth. Very few zircons of this age (>4050 Ma) have been found other than from a ~300 km diameter domain of the Yilgarn Craton, Western Australia. Here we report SIMS U-Pb and O isotope ratios and trace element analyses for two ~4100 Ma detrital zircons from a Paleozoic quartzite at the Longquan area of the Cathaysia Block. One zircon ((207)Pb/(206)Pb age of 4127 ± 4 Ma) shows normal oscillatory zonation and constant oxygen isotope ratios (δ(18)O = 5.8 to 6.0‰). The other zircon grain has a ~4100 Ma magmatic core surrounded by a ~4070 Ma metamorphic mantle. The magmatic core has elevated δ(18)O (7.2 ± 0.2‰), high titanium concentration (53 ± 3.4 ppm) and a positive cerium anomaly, yielding anomalously high calculated oxygen fugacity (FMQ + 5) and a high crystallization temperature (910°C). These results are unique among Hadean zircons and suggest a granitoid source generated from dry remelting of partly oxidizing supracrustal sediments altered by surface waters. The ~4100 Ma dry melting and subsequent ~4070 Ma metamorphism provide new evidence for the diversity of the Earth's earliest crust.
A long-standing paradigm assumes that the chemical and isotopic compositions of many elements in the bulk silicate Earth are the same as in chondrites. However, the accessible Earth has a greater (142)Nd/(144)Nd ratio than do chondrites. Because (142)Nd is the decay product of the now-extinct (146)Sm (which has a half-life of 103 million years), this (142)Nd difference seems to require a higher-than-chondritic Sm/Nd ratio for the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation and implies the formation of a complementary (142)Nd-depleted reservoir that either is hidden in the deep Earth, or lost to space by impact erosion. Whether this complementary reservoir existed, and whether or not it has been lost from Earth, is a matter of debate, and has implications for determining the bulk composition of Earth, its heat content and structure, as well as for constraining the modes and timescales of its geodynamical evolution. Here we show that, compared with chondrites, Earth's precursor bodies were enriched in neodymium that was produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher (142)Nd/(144)Nd ratios; after correction for this effect, the (142)Nd/(144)Nd ratios of chondrites and the accessible Earth are indistinguishable within five parts per million. The (142)Nd offset between the accessible silicate Earth and chondrites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. As such, our results obviate the need for hidden-reservoir or super-chondritic Earth models and imply a chondritic Sm/Nd ratio for the bulk Earth. Although chondrites formed at greater heliocentric distances and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth's bulk chemical composition.
The early evolution of planetesimals and planets can be constrained using variations in the abundance of neodymium-142 ((142)Nd), which arise from the initial distribution of (142)Nd within the protoplanetary disk and the radioactive decay of the short-lived samarium-146 isotope ((146)Sm). The apparent offset in (142)Nd abundance found previously between chondritic meteorites and Earth has been interpreted either as a possible consequence of nucleosynthetic variations within the protoplanetary disk or as a function of the differentiation of Earth very early in its history. Here we report high-precision Sm and Nd stable and radiogenic isotopic compositions of four calcium-aluminium-rich refractory inclusions (CAIs) from three CV-type carbonaceous chondrites, and of three whole-rock samples of unequilibrated enstatite chondrites. The CAIs, which are the first solids formed by condensation from the nebular gas, provide the best constraints for the isotopic evolution of the early Solar System. Using the mineral isochron method for individual CAIs, we find that CAIs without isotopic anomalies in Nd compared to the terrestrial composition share a (146)Sm/(144)Sm-(142)Nd/(144)Nd isotopic evolution with Earth. The average (142)Nd/(144)Nd composition for pristine enstatite chondrites that we calculate coincides with that of the accessible silicate layers of Earth. This relationship between CAIs, enstatite chondrites and Earth can only be a result of Earth having inherited the same initial abundance of (142)Nd and chondritic proportions of Sm and Nd. Consequently, (142)Nd isotopic heterogeneities found in other CAIs and among chondrite groups may arise from extrasolar grains that were present in the disk and incorporated in different proportions into these planetary objects. Our finding supports a chondritic Sm/Nd ratio for the bulk silicate Earth and, as a consequence, chondritic abundances for other refractory elements. It also removes the need for a hidden reservoir or for collisional erosion scenarios to explain the (142)Nd/(144)Nd composition of Earth.
The African continent consists of several cratons rimmed by orogenic belts, which were mainly formed by collisions of the cratons during the Gondwana supercontinent assembly. Understanding of the timing and nature of the Pan-African Orogeny is essential to decipher the development of the African continent and Gondwana supercontinent. We carried out U–Pb dating and geochemical analyses of approximately 500 detrital monazites from the Nile, Niger, Congo, Zambezi and Orange Rivers. The U–Pb dating defined age peaks at ∼600 Ma for the Nile, ∼580 Ma for the Niger, ∼630, ∼610 and ∼550 Ma for the Congo, ∼500 Ma for the Zambezi and 1200–1000 Ma for the Orange. All but the Orange age peaks correspond to the period of the Pan-African Orogeny. In the Niger, Congo and Zambezi, the Pan-African age peaks are 20–40 Myr younger than those defined by the detrital zircons from the same rivers. In the Nile, however, the detrital monazite and zircon age peaks are contemporaneous. The geochemical analyses revealed significant variations in Eu/Eu∗, Gd/Lu and Th/U ratios among the detrital monazites, which can be attributed to elemental partitioning with the co-existing feldspars, garnet, and zircon/rutile, respectively. Considering that typical abundances of these minerals vary among igneous and metamorphic rocks, these geochemical data can be potentially linked to the genesis of the detrital monazites. The geochemical systematics, together with the discrepancy between the detrital monazite and zircon ages, suggest that the Pan-African monazite age peaks essentially reflect the timing of syn- to post-collisional metamorphic events in the source terranes, whereas the detrital zircons provide a more representative record of magmatic events. The results imply that the Pan-African Orogeny was promoted by pre- to syn-collisional felsic magmatism during the early stages and, 20–40 Myr later, it was dominated by syn- and/or post-collisional metamorphism.
Isotopes isolated after impact
Details about how Earth formed are gleaned from the daughter products of certain short-lived radioactive isotopes found in rocks. Rizo et al. describe subtle tungsten isotope variations in rocks from the very deep mantle in Baffin Island and the Ontong Java Plateau (see the Perspective by Dahl). The results suggest that portions of Earth have remained unmixed since it formed. The unmixed deep mantle rocks also imply that Earth's core formed from several large impact events.
Science , this issue p. 809 ; see also p. 768
The subducted continental crust material will be gravitationally trapped in the deep mantle after having been transported to depths of greater than ∼250–300 km (the “depth of no return”). However, little is known about the status of this trapped continental material as well as its contribution to the mantle heterogeneous after achieving thermal equilibrium with the surrounding mantle. Here, we conduct an experimental study over pressure and temperature ranges of 9-16 GPa and 1300-1800 °C to constrain the fate of these trapped upper continental crust (UCC). The experimental results show that partial melting will occur in the subducted UCC along normal mantle geotherm to produce K-rich melt. The residual phases composed of coesite/stishovite + clinopyroxene + kyanite in the upper mantle, and stishovite + clinopyroxene + K–Hollandite + garnet + CAS–phase in the mantle transition zone (MTZ). The residual phases achieve densities greater than the surrounding mantle, which provides a driving force for descent across the 410-km seismic discontinuity into the MTZ. However, this density relationship is reversed at the base of the MTZ, leaving the descended residues to be accumulated above the 660-km seismic discontinuity, which may contribute to the “second continent”. The melt is ∼0.6–0.7 g/cm3 less dense than the surrounding mantle, which provides a buoyancy force for ascent of melt to shallow depths. The ascending melt, which preserves a significant portion of the bulk–rock rare earth elements (REE), large ion lithophile elements (LILE), and high-filed strength elements (HFSE), may react with the surrounding mantle. Re-melting of the metasomatized mantle may contribute to the origin of the “enriched mantle sources” (EM–sources). Therefore, the deep subdcuted continental crust may create geochemical/geophysical heterogeneity in Earth’s interior though subduction, stagnation, partial melting and melt segregation.
The robust nature of the mineral zircon, combined with our analytical ability to readily acquire in-situ uranium-lead (U-Pb), lutetium-hafnium (Lu-Hf) and oxygen (O) isotopic data, has resulted in a rapid rise in the use of zircon isotopic datasets for studying both the generation of continental crust and its growth through Earth history. In such studies there has been a strong focus on developing methods to determine the timing and/or proportion of juvenile magmatic addition to the continental crust. One widespread approach to determine the timing of crustal growth has been the construction or fitting of ‘reworking arrays’ to regional Hf isotopic datasets. Simple stochastic models are presented which highlight that in many cases apparent reworking arrays are much more likely to represent a process of on-going dilution and refertilisation of ancient crust, consistent with “Hot Zone” models of granitoid generation and the need to refertilise lower crustal reservoirs to maintain magmatism. A new compilation of magmatic rock zircon Lu-Hf and O isotope data is used to demonstrate that the use of mantle-like O isotope data as a screening tool for “meaningful” Hf model ages is also unlikely to be reliable, with independently constrained data indicating that as few as 14 % of Hf model ages provide a meaningful indicator of the timing of crustal growth. The limitations of Hf model ages are discussed with regard to existing approaches for continental growth and we demonstrate that popular inverse modelling approaches suffer from a bias created by both the use of model ages and numerical artefacts. In an effort to address some of the limitations within existing models, we develop stochastic models based on joint calibration of multiple datasets which allow for more unique solutions.
Zircon has played a critically important role in our understanding of the growth and evolution of the Earth. The U–Pb isotope system as preserved in zircon, more than any other mineral or method, has provided the most precise geochronological constraints for timing of geological events and processes on the Earth. More recently, technological advances have allowed for the precise determination of the Hf isotope composition of zircon, a geochemical tracer that has provided important details on the Earth's chemical evolution, in particular the evolution of the crust–mantle system. When combined, U–Pb ages and Hf isotopes in zircons hold the promise of providing unprecedented resolution in the timing and processes of planetary differentiation. Nowhere is this more true than for the early history of the Earth, where younger tectonothermal processes have compromised the isotope information in bulk rock samples. With the promise of this integrated technique, however, lies numerous potential pitfalls in the acquisition and interpretation of these data. In this paper we review several important issues related to unraveling the complexities of integrated U–Pb age and Hf isotope datasets, especially with respect to understanding crust–mantle evolution. In particular, we address the potential difficulty of assigning accurate initial Hf isotope compositions as well as some of the inherent problems associated with so-called “depleted-mantle model ages”. Finally, we make some suggestions regarding the optimum analytical approach and presentation of the Hf (and Nd) isotope data to obtain the clearest record of Earth's chemical evolution.
New crustal clues from old rocks
The ghost of continental crust long eroded away may exist in certain element ratios found in Archean rocks. Tang et al. used Ni/Co and Cr/Zn ratios as a proxy for the magnesium oxide that long ago weathered away in Earth's oldest rocks. This allowed a reconstruction of rock composition, which appears to be very different from that of the crust today. The shift to contemporary crust composition occurred after the Archean era, suggesting the onset of plate tectonics.
Science , this issue p. 372
Chemical weathering of rocks is a spontaneous (i.e., irreversible) thermodynamic process leading to a more stable state for natural materials under a given set of conditions (e.g., temperature and pressure). It results from the reaction of aqueous, acidic, and oxidizing solutions with the minerals in rocks and soils. There is no doubt that the increasing number of studies dealing with chemical weathering during recent decades is related to increasing concern about global climate change. This chapter will consider these questions. The objective is to estimate chemical weathering rates of silicates, and to define which parameters control these rates at a global scale on the basis of the chemical composition of rivers draining both small and large watersheds. The importance of parameters controlling chemical weathering rates should be evaluated and included in climate models.
Crystallization and chemical differentiation of the early Earth was governed by a combination of various processes. Within the uncertainties of physical parameters, the two end-member models, equilibrium crystallization and fractional crystallization, are both possible. In terms of crystal size, the boundary between these models is ∼1mm. Analysis of nucleation and crystal growth in the convecting magma ocean suggests that the crystals in the magma ocean are approximately this size. The equilibrium model is preferred because it seems to satisfy better the geochemical constraints. According to this model, at the early stages crystallization proceeds from bottom up without any substantial crystal-melt segregation. In a vigorously convecting magma ocean the thermal profile is approximately adiabatic and the crystal fraction decreases gradually with depth. The basic physical reason for equilibrium crystallization is a fast convective cooling during which the solid and the liquid phases had a very limited time window for crystal-melt segregation, about 1000 years. Chemical differentiation due to crystal-melt segregation is more significant at low pressures corresponding - with large uncertainties - to the upper mantle. It is caused by several factors. When the temperature drops below liquidus everywhere the nucleation-growth-dissolution cycle of crystals changes to continuous crystal growth. This increases the crystal size to ∼1cm indicating the beginning of crystal-melt segregation. When the crystal fraction increases to about 60% near the surface, the convective heat transport is controlled by solid-state creep which is many orders of magnitude slower. Crystal-melt segregation occurs via melt percolation, which is consistent with the geochemical constraints. Crystallization of the remaining partially molten layers takes about 107-109 years. This stage merges with the subsequent planetary evolution controlled by mantle convection and radiogenic heating. The long lifetime of the shallow magma ocean suggests that this might be the place where liquid iron equilibrates with the mantle before it sinks down to the core.
The fate and amount of granitic materials subducted into the deep mantle are still under debate. The density
, elastic property, and phase stability of granitic materials in the mantle pressures are key to clarifying them. Here, we modeled high-pressure properties of the granitic assemblage
by using ab initio mineral physics data of grossular
garnet, K-hollandite, jadeite, stishovite, and calcium ferrite (CF)-type phase. We find that the ongoing subducting granitic assemblage, such as sediments
and average upper crust
rocks, is much denser than pyrolite
in the pressure range from 9 GPa (~270 km), at which coesite
undergoes a phase transition to stishovite, to around 27 GPa (~740 km). Above this pressure, granitic material becomes less dense than a pyrolite. This indicates that the granitic assemblage becomes gravitationally stable at the base of the mantle transition zone
(MTZ). Results suggest a possibility that the granitic materials could accumulate around the 740 km depth if carried into the depth deeper than 270 km and segregated at some depth. Comparison of the velocities between granitic and pyrolitic materials shows that granitic materials can produce substantial velocity anomalies in the MTZ and the uppermost
lower mantle
(LM). Seismic observations such as anomalously fast velocities, especially for the shear wave, around the 660-km discontinuity, the complexity of 660-km discontinuity, and the scatterers in the uppermost LM could be associated with the subducted granitic materials.
Angrites are a group of basaltic achondrites with distinctive mineralogic and geochemical characteristics that have the potential to provide insights into processes occurring on planetesimals in the early Solar System. These achondrites have been used as anchors linking the relative age information obtained from short-lived, extinct chronometers (e.g., Al-Mg, Hf-W, and Mn-Cr) with absolute chronometers (e.g., U-Pb). Angrites provide excellent examples of early differentiation processes, such as core formation and silicate differentiation, on protoplanetary bodies. The significant increase in the number of known angrite samples in recent years has offered the opportunity to compare several short- and long-lived isotopic systems in samples with different petrogenetic histories that formed on the same parent body. To this end, the 147Sm-143Nd, 146Sm-142Nd, 176Lu-176Hf, and 87Rb-87Sr isotope systematics have been investigated in a suite of plutonic, coarse-grained (NWA 4590, NWA 4801, and NWA 2999) and quenched, fine-grained (D'Orbigny) angrites. The coupled 147,146Sm-143,142Nd systematics indicate possible isotopic disturbances in two angrites (D'Orbigny and NWA 2999) resulting from post-crystallization processes. The internal 146Sm-142Nd isochrons of two coarse-grained angrites (NWA 4590 and NWA 4801) provide an updated best estimate of the initial Solar System 146Sm/144Sm ratio (i.e., at 4568 Ma) of 0.0084 ± 0.0003. The 176Lu-176Hf isotope systematics in these angrites do not provide evidence of a previously proposed intense irradiation event in the early Solar System. The internal 176Lu-176Hf isochrons for the NWA 4590 and D'Orbigny angrites provide an estimate for the Solar System initial 176Hf/177Hf ratio of 0.279775 ± 0.000031 (2σ) that agrees within uncertainty with the value of average chondrites reported by Bouvier et al. (2008). Finally, the calculated initial 87Sr/86Sr ratios based on the measured Sr-isotopic composition of plagioclase in these angrites yield an estimated initial 87Sr/86Sr ratio of 0.698980 ± 0.000011 for the angrite parent body. This is indistinguishable from a recently determined value for the Solar System initial 87Sr/86Sr based on values measured in calcium-aluminum-rich inclusions (CAIs) after correcting for nucleosynthetic effects in the CAIs. The low initial 87Sr/86Sr of the angrite parent body implies that it acquired its volatile element depleted characteristic within 1.8 Ma of Solar System formation, likely because it accreted from volatile depleted planetesimals that formed in the hot inner nebula. These integrated isotopic systematics suggest a complex history for the angrite parent body not previously inferred from short-lived chronometers and provide new estimates for the initial isotopic composition of the early Solar System.
The continental crust is the principal record of conditions on the Earth during the past 4.4 billion years. However, how the continental crust formed and evolved through time remains highly controversial. In particular, the composition and thickness of juvenile continental crust are unknown. Here we show that Rb/Sr ratios can be used as a proxy for both the silica content and the thickness of the continental crust. We calculate Rb/Sr ratios of the juvenile crust for over 13,000 samples, with Nd model ages ranging from the Hadean to Phanerozoic. The ratios were calculated based on the evolution of Sr isotopes in the period between the T DM Nd model age and the crystallization of the samples analysed. We find that the juvenile crust had a low silica content and was largely mafic in composition during the first 1.5 billion years of Earthâ €™ s evolution, consistent with magmatism on a pre-plate tectonics planet. About 3 billion years ago, the Rb/Sr ratios of the juvenile continental crust increased, indicating that the newly formed crust became more silica-rich and probably thicker. This transition is in turn linked to the onset of plate tectonics and an increase of continental detritus into the oceans.
Detrital igneous zircons from Jack Hills, Western Australia, range in age from ~3.0 to nearly 4.4 Ga and contain an inclusion assemblage dominated by quartz and muscovite, cited as evidence of their derivation from peraluminous granitoids. However, some phosphate inclusions in these zircons are known to be secondary from their post-depositional U–Pb ages and manifest mineralization along cracks. We undertook a survey of mineral inclusions in 4.3–3.0 Ga Jack Hills zircons with particular emphasis on their relationship to possible alteration features (e.g., cracks, disturbed internal zonation, and visual turbidity). Mineral inclusions revealed at polished surfaces show variations in modal mineralogy, mostly corresponding to their relationship with cracks. Muscovite is common both on and away from cracks, although the chemistry of muscovite inclusions shows little relationship with other potential alteration features. Inclusions filling cracks (secondary) and inclusions isolated from cracks differ in their modal mineralogy, although both suites are rich in muscovite and quartz. The higher incidence of crack-intersecting inclusions among younger zircons may reflect effects of the (generally larger) inclusion size among younger zircons. Mismatches between the isolated and crack-intersecting populations indicate selective loss of certain phases (e.g., feldspar, apatite) and over-representation of quartz and muscovite along cracks likely due to the effects of larger inclusion size and varying degrees of overpressure following zircon cooling and decompression. Inclusions not associated with cracks in magmatically zoned versus regions with disturbed zoning have similar phase proportions. This indicates only minor inclusion replacement away from cracks (i.e., the isolated assemblage is likely primary). This holds true also for inclusions within visually turbid versus clear volumes of zircon. Phase proportions within the inclusion assemblages differ with age indicating a provenance shift toward fewer mafic phases and apatite in b 3.6 Ga relative to Hadean granitoid sources.