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Abstract: The geological implications of granitoid magmas with high Sr/Y and La/Yb are debated because these signatures can be produced by multiple processes. This study presents comprehensive major and trace element compositions and zircon SHRIMP U–Pb age data of 81 early Cretaceous granitoids and 4 mafic enclaves from the Dabie orogen to investigate partial melting of the thickened lower continental crust (LCC). On the basis of Sr/Y ratios, granitoids can be grouped into two magma series: (i) high Sr/Y granitoids (HSG) and (ii) normal granitoids with low Sr/Y. Relative to normal granitoids, HSG display the following distinct chemical features: (1) at given SiO2 and CaO contents, the HSG have significantly higher Sr than normal granitoids, defining two different trends in Sr versus SiO2, CaO diagrams; (2) highly depleted heavy rare earth element (REE) relative to middle and light REE with (Dy/Yb)N and (La/Yb)N up to 3.2 and 151, respectively; (3) variable and higher Nb/Ta; and (4) positive correlations among Sr/Y, (Dy/Yb)N, (La/Yb)N, and Nb/Ta. High Sr/Y, (La/Yb)N, (Dy/Yb)N, and Sr/CaO of HSG do not correlate with major elements (e.g., SiO2). Large variations in these ratios at a given SiO2 content indicate that these features do not reflect magma mixing or fractionation. HSG have higher Sr at a given CaO content and larger variation of (Dy/Yb)N than old crustal rocks (including exposed basement, global mafic LCC xenoliths, high Sr/Y TTG suites, and adakites in modern arcs). This precludes inheritance of the HSG chemical features from these source rocks. Instead, the chemical features of the HSG are best explained by partial melting of the thickened LCC with garnet-dominant, plagioclase-poor, and rutile-present residual lithologies. The coupled chemical features of the HSG are not observed in post-collisional granitoids younger than ca.130Ma, indicating removal of the eclogitic source and/or residuum of HSG underneath the orogen. These characteristic chemical relationships in the Dabie HSG may be applied to distinguish partial melts of thickened LCC from high Sr/Y intermediate-felsic magmatic rocks which do not show clear indications for melting depth. [Copyright &y& Elsevier]

Abstract: Adakites are commonly associated with porphyry Cu–Au ore deposits worldwide. Two groups of early Cretaceous adakites occur widely in central-eastern China but their association with mineralization contrasts sharply: adakites from the Lower Yangtze River Belt (LYRB) host one of the largest porphyry Cu–Au deposit belts in China, whereas those from the South Tan-Lu Fault (STLF), which is adjacent to the LYRB, are all ore-barren. These adakites, thus, provide a rare opportunity to explore the main factor that controls the genetic links between adakites and Cu–Au mineralization. Here we report new chronological, elemental and Sr–Nd–Pb isotopic data and present a comprehensive geochemical comparison for these two groups of adakites. At a given SiO2, the STLF adakites show lower Al2O3 and higher K2O, K2O/Na2O, MgO, Cr, Ni and Mg# than the LYRB adakites. These systematic differences may indicate a dry basaltic source for the STLF adakites and a water-enriched basaltic source for the LYRB adakites. The STLF adakites have high Sr/Y and (La/Yb)N, which are positively correlated, and low Sr/La and Ce/Pb, while the LYRB adakites show lower (La/Yb)N but higher Sr/Y, Sr/La and Ce/Pb than the STLF adakites. Furthermore, the LYRB adakites are characterized by highly radiogenic Pb isotopic compositions with 206Pb/204Pb(t) up to 18.8, which are clearly distinct from the STLF adakites with low radiogenic Pb (206Pb/204Pb(t)=15.8–16.4). Although the high Mg# of the two groups of adakites suggest reaction with mantle peridotites during magma ascent, the geochemical comparisons indicate that the STLF adakites were derived from partial melting of the delaminated eclogitic lower continental crust, while the LYRB adakites were derived from partial melting of the seawater-altered oceanic crust that was being subducted towards the LYRB during the early Cretaceous. The petrogenetic contrasts between these two groups of high-Mg adakites, therefore, indicate that the large-scale Cu–Au mineralization is associated with oceanic slab melting, not delamination or recycling of the ancient lower continental crust, as previously proposed. [Copyright &y& Elsevier]

Subduction transfers surface carbon into the Earth's interior in a main form of carbonates that influences the global carbon cycles and surface climate through geologic time. Nevertheless, whether the fate of downgoing carbonates significantly varies in past subduction zones is rarely constrained by natural observations. Marine carbonates have remarkably higher zinc isotopic ratios (expressed as δ66Zn JMC-Lyon) relative to the mantle (0.99 ± 0.24‰ vs. 0.18 ± 0.05‰), making zinc isotopes a sensitive tracer for subducting carbonates. Here we examine this issue through a comparative zinc isotope study on basalts across the North-South Gravity Lineament (NSGL) in East Asia that were genetically related to two different oceanic slabs. Together with existing data, we show that all basalts in the east of the NSGL have high δ66Zn (∼0.3–0.6‰; n = 134) that do not vary with distances to the trench and are spatially coupled with the horizontally stagnated slab in the transition zone (410 – 660 km). This indicates that subducting carbonates survived shallow dissolution and were deeply buried during westward subduction of the Paleo-Pacific slab. By contrast, basalts in the west of the NSGL display a gradual decline of δ66Zn from 0.50 ± 0.04‰ to 0.28 ± 0.03‰ (n = 35) with increasing distances to the trench. No known magmatic processes (e.g., partial melting, crystal-melt differentiation, melt-peridotite interaction, and degassing) can account for the spatial Zn isotopic variation. The role of slab-derived sulfate rich fluids is also excluded because of the mantle-like Cu isotopic compositions of these basalts. Instead, the gradual decrease of δ66Zn, together with the coupled decline of CaO/Al 2 O 3 , are best explained as the diminished amounts of dissolved carbonates in their mantle sources. Thus, substantial carbonate dissolution must have occurred during southeastward subduction of the Paleo-Asian slab, which prevents deep burial of subducting carbon. The main differences between the two large slabs include: (i) the Paleo-Asian slab has an extended longevity (∼1.1 Ga) and slow spreading rate in comparison with the Paleo-Pacific slab, leading to the main incorporation of carbonate minerals into the altered oceanic crust, and (ii) the younger Paleo-Pacific slab contains abundant deep-sea Mg-rich carbonates that were not sufficiently dissolved at shallow depths. These differences demonstrate that subduction of different oceanic slabs can lead to contrasting fates of subducting carbon in ancient subduction zones, depending on the contents and species of carbonate sediments in the oceanic crust. [ABSTRACT FROM AUTHOR]

Chromium (Cr) isotopes represent a powerful tool for tracing the redox conditions during planetary magmatic evolution. However, so far, the systematic investigation of Cr isotope variation has been only performed on ocean island basalts (OIBs) in terrestrial magmatic rocks. Therefore, the Cr stable isotope compositions (expressed as δ53Cr relative to NIST SRM 979) of other magmatic rocks, formed under different oxygen fugacity, have remained unconstrained. In this paper, we present the first Cr stable isotopic data of mid-ocean ridge basalts (MORBs) from the Eastern Pacific Ocean ridge, the Indian ridge, and the Atlantic Ocean ridge. The oxygen fugacity of such basalts is different from that of the above-mentioned OIBs. The Rhyolite-MELTS model shows that the chemical composition variations in the studied basalts are induced by varying extents of fractional crystallization of olivine, clinopyroxene, plagioclase, and spinel. The δ53Cr values of the MORBs range from −0.27 ± 0.03‰ to −0.07 ± 0.02‰ (n = 28), and two distinct groups of basalts are identified based on the correlation between δ53Cr and MgO. On the one hand, the δ53Cr of group I basalts (−0.27 ± 0.03‰ to −0.14 ± 0.03‰; n = 24) are systematically lower than the established average value of the Bulk Silicate Earth (BSE) (δ53Cr = −0.12 ± 0.04‰; 2SD), which are positively correlated with their MgO and Cr concentrations, indicating that Cr isotopes are fractionated during magmatic differentiation. Moreover, Rayleigh fractionation modelling suggests that the crystallization of olivine, clinopyroxene, and spinel gives rise to the Cr depletion and thereby decreases δ53Cr values. On the other hand, group II basalts (−0.10 ± 0.03‰ to −0.07 ± 0.02‰; n = 4) exhibit higher δ53Cr values than group I with identical MgO and Cr concentrations. This is possibly associated with the crystallization of clinopyroxene under low pressure. The average Cr isotope composition (δ53Cr = −0.16 ± 0.02‰, n = 3) of the primitive basalts (MgO > 9%) represents that of the primary MORB melt, which is lighter than the average value of BSE. Using the non-modal melting equations, the δ53Cr of the MORB mantle source is estimated to be −0.12 ± 0.02‰ (2σ), which is consistent with that of BSE. Compared with the Cr isotopic data of OIBs from Fangataufa island, we find that the equilibrium fractionation factors (Δ53Cr crystal-melt = +0.04‰ to +0.13‰) of MORBs during fractional crystallization are larger than that of Fangataufa island lavas (Δ53Cr crystal-melt = 0.010 ± 0.005‰ for low-K suite, and 0.020 ± 0.010‰ for high-K suite; Bonnand et al., 2020a), indicating that the basalts from Fangataufa island have higher oxygen fugacity than those of MORBs analyzed in this study, which is strongly supported by their higher V/Sc ratios. [ABSTRACT FROM AUTHOR]

Iron and copper isotopes are useful tools to track redox transformation and biogeochemical cycling in natural environment. To study the relationships of stable Fe and Cu isotopic variations with redox regime and biological processes during weathering and pedogenesis, we carried out Fe and Cu isotope analyses for two sets of basalt weathering profiles (South Carolina, USA and Hainan Island, China), which formed under different climatic conditions (subtropical vs. tropical). Unaltered parent rocks from both profiles have uniform δ 56 Fe and δ 65 Cu values close to the average of global basalts. In the South Carolina profile, δ 56 Fe values of saprolites vary from −0.01‰ to 0.92‰ in the lower (reduced) part and positively correlate with Fe 3+ /ΣFe ( R 2 = 0.90), whereas δ 65 Cu values are almost constant. By contrast, δ 56 Fe values are less variable and negatively correlate with Fe 3+ /ΣFe ( R 2 = 0.88) in the upper (oxidized) part, where large (4.85‰) δ 65 Cu variation is observed with most samples enriched in heavy isotopes. In the Hainan profile formed by extreme weathering under oxidized condition, δ 56 Fe values vary little (0.05–0.14‰), whereas δ 65 Cu values successively decrease from 0.32‰ to −0.12‰ with depth below 3 m and increase from −0.17‰ to 0.02‰ with depth above 3 m. Throughout the whole profile, δ 65 Cu positively correlate with Cu concentration and negatively correlate with the content of total organic carbon (TOC). Overall, the contrasting Fe isotopic patterns under different redox conditions suggest redox states play the key controls on Fe mobility and isotope fractionation. The negative correlation between δ 56 Fe and Fe 3+ /ΣFe in the oxidized part of the South Carolina profile may reflect addition of isotopically light Fe. This is demonstrated by leaching experiments, which show that Fe mineral pools extracted by 0.5 N HCl, representing poorly-crystalline Fe (hydr)-oxides, are enriched in light Fe isotopes. The systematic Cu isotopic variation in the Hainan profile reflects desorption and downward transport of isotopically heavy Cu, leaving the organically-bound Cu enriched in light isotope as supported by the negative correlation of δ 65 Cu with TOC ( R 2 = 0.88). The contrasting (mostly positive vs. negative) Cu isotopic signatures in the upper parts of these two profiles can be attributed to the different climatic conditions, e.g., high rainfall at a tropical climate in Hainan favors desorption and the development of organism, whereas relatively dry climate in South Carolina favors Cu re-precipitation from soil solutions and adsorption onto Fe (hydr)-oxides. Our results highlight the potential applications of Fe and Cu isotopes as great tracers of redox condition, ancient climate and biological cycling during chemical weathering and pedogenic translocation. [ABSTRACT FROM AUTHOR]

Serpentinite is one of the most important sources of volatiles and fluid-mobile elements in subduction zones. Attempts have been made to discriminate serpentinite-derived fluid from other sources (e.g., sediments, altered oceanic crusts) using a variety of geochemical tools, but such differentiation has proven to be changeling because fluids from different lithologies are dynamically mixed in the subduction channel. Serpentinites are essentially distinguished by high Cr contents and variable degrees of 53Cr excess. Given high Cr mobility in Cl-rich metamorphic fluids based on experimental determination, Cr elemental and isotope compositions could be potential markers for fluid-mediated mass transfer from serpentinites. Here, we test this hypothesis by analyzing Cr concentrations and isotope compositions of meta-basalts, as well as country mica schists, marbles and serpentinites from the southwestern Tianshan Orogen. The meta-basalts have highly variable Cr concentrations (30.0–625 ppm) and resolvable δ53Cr variations (−0.25‰ to −0.05‰), while mica schists and marbles have similar isotope compositions and low Cr contents (δ53Cr of −0.15‰ to −0.14‰ with Cr contents of 89.5–110 ppm for mica schists, δ53Cr of −0.21‰ to −0.17‰ with Cr contents of 4.1–8.73 ppm for marbles, respectively). Most of the investigated meta-basalts were carbonated. The positive correlations of loss on ignition (LOI), CO 2 , Sb and Sb/Ce with Cr contents, as well as with δ53Cr for these carbonated meta-basalts indicate that the metamorphic fluids are at least partially sourced from serpentinites and sedimentary carbonates. High pressure serpentinites display similar δ53Cr variations from −0.19‰ to −0.02‰, but high Cr contents ranging from 934 to 4920 ppm. Chromium isotope variations of these serpentinites could not be generated by serpentinization, due to opposite trends of Cr isotopes and serpentinization indexes compared with previous observations. Instead, dehydration of serpentinites during subduction/exhumation accounts for Cr isotope variations, generating fluids with high Cr concentrations and relatively lighter isotope compositions. According to a Rayleigh dehydration model, the estimated Cr isotope fractionation factor (α fluid-residue) between fluids and residual serpentinites is approximately 0.99995 –0.99975, implying that Cr species in the subducted serpentine derived fluid is probably as complexes of Cr3+-Cl− H 2 O(OH–). The mixing model reveals that investigated carbonated meta-basalts were sourced from initial evolved oceanic basalts metamorphosed by fluids from serpentinite dehydrations, with contributions from sedimentary carbonates. This work emphasizes that coupled Cr element and isotope systems present the potential ability to discriminate the complex fluid sources in subduction zones, particularly serpentinites. [ABSTRACT FROM AUTHOR]

Widespread Cenozoic intraplate basalts from eastern China offer the opportunity to investigate the consequences of interaction between the stagnant Pacific slab and overlying asthenosphere and chemical heterogeneity within this "big mantle wedge". We present and compile a comprehensive study of highly siderophile elements and Mg-Zn isotopes of this magmatic suite (60 samples including nephelinites, basanites, alkali basalts and tholeiites). The large-scale Mg-Zn isotopic anomalies documented in these basalts have been ascribed to mantle hybridization by recycled Mg-carbonates from the stagnant western Pacific plate. Our results reveal that the nephelinites and basanites are characterized by unfractionated platinum-group element (PGE) patterns normalized to primitive upper mantle (PUM) (e.g., Pd N /Ir N normalized to PUM = 1.1 ± 0.8, 1σ), relatively high total PGE contents (e.g., Ir = 0.25 ± 0.14 ppb) and modern mantle-like 187Os/188Os (0.142 ± 0.020). These characteristics are coupled with lighter Mg isotope (δ26Mg = −0.48 ± 0.07‰) and heavier Zn isotope (δ66Zn = +0.46 ± 0.06‰) compositions compared to the mantle values (δ26Mg: −0.25 ± 0.07‰; δ66Zn: +0.18 ± 0.05‰). Together, these data are interpreted to reflect the oxidative breakdown of low proportions of mantle sulfides in the sources of these small-degree melts, likely caused by recycled carbonates, which then release chalcophile-siderophile elements into carbonatitic melts. By contrast, the contemporaneous alkali basalts and tholeiites are characterized by highly fractionated PGE patterns (e.g., Pd N /Ir N = 4.4 ± 3.3; Ir = 0.037 ± 0.027 ppb) and radiogenic 187Os/188Os (0.279 ± 0.115) coupled with less fractionated Mg-Zn isotope compositions (δ26Mg: −0.39 ± 0.05‰; δ66Zn: +0.35 ± 0.03‰). In combination with other isotopic (e.g., Sr-Nd) and chemical (SiO 2 , Ce/Pb, Ba/Th, Fe/Mn) constraints, the alkali basalts and tholeiites were derived from higher degree melting of ancient pyroxenite-bearing mantle in addition to mixing with the aforementioned nephelinitic and basanitic melts. Collectively, we suggest that deep recycled carbonates promoted melting within the "big mantle wedge" leading to the generation of Cenozoic intraplate basalts across eastern China and the "redox freezing of carbonates" may cause the oxidation of Fe0 and S2-. This process may provide an important mechanism to oxidize mantle sulfides and transfer precious metals from deep mantle to crust. [ABSTRACT FROM AUTHOR]

Abstract Recent studies show that zinc isotopes could provide valuable clues to environmental change and biogeochemical cycle of the past oceans. This study reports a modified procedure for leaching the carbonate fractions in sedimentary rocks, a thorough evaluation of diagenetic effects, and systematic variations of Zn and Sr isotope ratios in lower part of the Ediacaran stratigraphic unit deposited in the aftermath of the Marinoan glaciation in South China. The influence of post-depositional diagenesis on Zn isotope compositions of the studied samples is assessed by comparing δ66Zn to other geochemical indexes (87Sr/86Sr, δ13C, δ18O and Mn/Sr ratios). In the five studied cap carbonate sections (Member I of the Doushantuo Formation), dolostones from four sections have δ66Zn values positively correlated with δ18O values and negatively correlated with 87Sr/86Sr ratios (0.7081–0.7204). These correlations suggest that these cap dolostone samples have been modified by post-depositional diagenesis. The light δ66Zn value (−0.02‰) suggests that initial Zn isotope ratios of cap dolostones could have been reset by hydrothermal fluids with relatively high Zn concentration and low δ66Zn values. By contrast, carbonates from Member II of the Doushantuo Formation above cap dolostones are relatively pristine based on their low 87Sr/86Sr ratios (0.7079–0.7086) being indistinct from the proposed early Ediacaran seawater 87Sr/86Sr values. Chemical and isotopic variations in these samples are interpreted to reflect primary signals that record paleo-environmental changes of the early Ediacaran ocean. A rapid increase of δ66Zn from ∼0.3‰ to 1.1‰ occurs in the middle part of Member II, accompanying by relatively invariant 87Sr/86Sr ratios that imply insignificant changes in input from continental weathering. Considering the limited change in atmospheric oxygen during this period, the rapid δ66Zn raise indicates an increase in buried organic matters, which is consistent with the coupled positive shift of δ13C carb , as well as the fossil records found in the same strata. These results provide insights into Zn cycling in the post-Marinoan oceans and facilitate the application of Zn isotopes in carbonates as a proxy for the fate of marine organic matter. [ABSTRACT FROM AUTHOR]

Significant magnesium and iron isotope fractionations were observed in an adjacent gabbro and granite profile from the Dabie Orogen, China. Chilled margin and granitic veins at the gabbro side and gabbro xenoliths in the granite indicate the two intrusions were emplaced simultaneously. The δ 26 Mg decreases from −0.28 ± 0.04‰ to −0.63 ± 0.08‰ and δ 56 Fe increases from −0.07 ± 0.03‰ to +0.25 ± 0.03‰ along a ∼16 cm traverse from the contact to the granite. Concentrations of major elements such as Al, Na, Ti and most trace elements also systematically change with distance to the contact. All the observations suggest that weathering, magma mixing, fluid exsolution, fractional crystallization and thermal diffusion are not the major processes responsible for the observed elemental and isotopic variations. Rather, the negatively correlated Mg and Fe isotopic compositions as well as co-variations of Mg and Fe isotopes with Mg# reflect Mg-Fe inter-diffusion driven isotope fractionation, with Mg diffusing from the chilled gabbro into the granitic melt and Fe oppositely. The diffusion modeling yields a characteristic diffusive transport distance of ∼6 cm. Consequently, the diffusion duration, during which the granite may have maintained a molten state, can be constrained to ∼2 My. The cooling rate of the granite is calculated to be 52–107 °C/My. Our study suggests diffusion profiles can be a powerful geospeedometry. The observed isotope fractionations also indicate that Mg-Fe inter-diffusion can produce large stable isotope fractionations at least on a decimeter scale, with implications for Mg and Fe isotope study of mantle xenoliths, mafic dikes, and inter-bedded lavas. [ABSTRACT FROM AUTHOR]

This study reports elemental and iron isotopic compositions of feldspar and its coexisting minerals from four Dabie I-type granitoids to evaluate the factors that control inter-mineral Fe isotopic fractionation in granitoids. The order of heavy iron isotope enrichment is feldspar > pyrite > magnetite > biotite ≈ hornblende. Feldspar has heavier iron isotopic compositions than its co-existing magnetite (Δ 56 Fe plagioclase–magnetite = +0.376‰ to +1.084‰, Δ 56 Fe alkali-feldspar–magnetite = +0.516‰ to +0.846‰), which can be attributed to its high Fe 3+ /Fe tot ratio and low coordination number (tetrahedrally-coordinated) of Fe 3+ . Δ 56 Fe magnetite–biotite of coexisting magnetite and biotite ranges from 0.090‰ to 0.246‰. Based on homogeneous major and iron isotopic compositions of mineral replicates, the inter-mineral fractionation in this study should reflect equilibrium fractionation. The large variations of inter-mineral fractionation among feldspar, magnetite and biotite cannot be simply explained by temperature variation, but strongly depend on mineral compositions. The Δ 56 Fe plagioclase–magnetite and Δ 56 Fe alkali-feldspar–magnetite are positively correlated with albite mode in plagioclase and orthoclase mode in alkali-feldspar, respectively. This could be explained by different Fe–O bond strength in feldspar due to different Fe 3+ /∑Fe or different crystal parameters. The Δ 56 Fe magnetite–biotite increases with decreasing Fe 3+ /∑Fe biotite and increasing mole (Na + K)/Mg biotite , indicating a decrease of β factor in low Fe 3+ /∑Fe and high (Na + K)/Mg biotite. High-silica leucosomes from Dabie migmatites with a feldspar accumulation petrogenesis have higher δ 56 Fe values (δ 56 Fe = 0.42–0.567‰) than leucosome that represents pristine partial melt (δ 56 Fe = 0.117 ± 0.016‰), indicating that accumulation of feldspar could account for high δ 56 Fe values of these rocks. High δ 56 Fe values are also predicted for other igneous rocks that are mainly composed of cumulate feldspar crystals, e.g., anorthosites. Feldspar accumulation, however, cannot explain high δ 56 Fe values of most high-silica granitoids reported in the literature, based on their low Sr, Ba contents and negative Eu anomalies. [ABSTRACT FROM AUTHOR]

Abstract: The formation of the continental crust (CC) is one of the most important processes in the evolution of the silicate Earth. Exactly how the CC formed is the subject of ongoing debate that focuses on its subchondritic Nb/Ta ratio. Nb and Ta are “geochemical identical twins,” so they usually do not fractionate from each other. Here, we show that rutile grains from hydrous rutile-bearing eclogitic layers recovered from drillcores in the Dabie-Sulu ultrahigh pressure terrain have highly variable Nb/Ta values (ranging from 5.4 to 29.1, with an average of 9.8±0.6), indicating major fractionation of Nb and Ta most likely occurred during blueschist to amphibole–eclogite transformation in the absence of rutile. It is suggested that the released fluids with subchondritic Nb/Ta were transported to, and retained by, hydrous rutile-bearing eclogite in colder regions, resulting in suprachondritic Nb/Ta ratios for drier eclogite in hotter regions. Further dehydration of hydrous rutile-bearing eclogites cannot transfer the fractionated Nb/Ta values to the CC due to the low solubility of Nb and Ta in fluids in the presence of rutile, while dehydration-melting results in a major component of the CC, the tonalite–trondhjemite–granodiorite (TTG) component, which is responsible for the low Nb/Ta of the CC. Consequently, residual eclogites have variable but overall suprachondritic Nb/Ta. [Copyright &y& Elsevier]