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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]

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]