| In recent years, with the development of analytical technique and improved precision on non-traditional stable isotopes, numbers of stable isotope systems have been built up and applied to diverse geologic processes. These stable isotope systems have been proved to be of great importance and application prospect in studies on both origin of the solar system, evolution of the nebula, formation and differentiation of the Earth and activities of microbial life. As a transitional metallic element, chromium is one of the most abundant elements besides major elements. So far, Cr isotopes are mainly applied to two respects: dating and tracing the evolution of the solar system in cosmochemistry and reflecting the redox-oxidation situation of palaeoenvironment and surficial environment in low temperature geological processes. Previous studies found that both the mantle of the Earth and the Moon were depleted in Cr.Experimental work displays that Cr becomes more and more siderophile with the increasing temperature and decreasing oxygen fugacity. The Cr partition coefficient can satisfy the depletion of Cr in the mantle and the demand that more than 60% of the Earth's Cr has partitioned into the core under certain conditions. At the same time,chromium is an element with multiple valence, it may present as Cr2+ and Cr3+ in the silicate and Cr0 in the metallic core. According to theoretical prediction of isotope fractionation, there is significant isotope fractionation among elements of different valence states. Thus, like Cr partitioning between silicate and metal, Cr isotopes may be used to constrain the core formation process of the Earth. The main obstacles of applying Cr isotopes to constrain the core formation of the Earth are: firstly, few study has been done to constrain the Cr isotopic composition of the Earth; secondly, no systematic investigations of Cr isotope fractionation during high temperature geologic processes (like partial melting and magmatic differentiation) has been done yet; thirdly,most of Cr in the Earth may be reserved in the core, but the Cr isotopic composition of the core has not been constrained. There was few work on the Cr isotopic fractionation between the metal and silicate that the possible Cr isotopic fractionation during core mantle segregation was unclear.To solve the problems above, systematic Cr isotopic study using a double-spike method were performed on series of well-studied mantle xenoliths from different geologic settings. According to Cr isotope analyses of diverse mantle xenoliths (spinel lherzolites, garnet peridotites, spinel harzburgites and pyroxenites. etc), the Cr isotope compositions of mantle xenoliths are extremely heterogeneous, with ?53CrNIST 979varying from -1.360.04‰(2SD) to +0.75± 0.05‰ (2SD). Of them, a rough negative correlation between ?13Cr and Al2O3 and CaO contents for most normal mantle peridotites was found, which may imply Cr isotopic fractionation during partial melting of mantle peridotites. The effects of melt extraction on the Cr isotopic composition of the residual peridotite were modeled by assuming different Cr isotopic fractionation factors (a) between residues and melts in both batch and fractional melting scenarios,however, highly variable Cr isotopic compositions of the Mongolian peridotites cannot be caused by partial melting alone, implying that kinetic Cr fractionation may also play an important role. During metasomatism, Cr diffuse from mantle peridotites to melts when the Cr concentration within them are disequilibrated, producing pyroxenite veins with extremely low ?53Cr ( down to -1.36±0.04‰) and peridotites with evaluated?53Cr(up to +0.75 ± 0.05‰). Considering that the Cr isotopic compositions of mantle xenoliths can be influenced by partial melting, metasomatism and secondary alteration,only fresh fertile or nearly fertile lherzolites were selected to determine the Cr isotopic composition of the primitive mantle, and get a value of -0.14±0.12‰ (2SD), which is in agreement with the value (-0.12±0.10‰) suggested previously.To further constrain Cr isotopic behaviors during magmatic differentiation, we studied the stable Cr isotopic compositions of well-studied Hawaiian basalts. The results show that the Cr isotopic compositions of the Mauna Kea and Koolau shield basalts are relatively homogenous and identical to each other within error bars,suggesting that late-stage alteration and incorporations of recycled altered oceanic basaltic crust and marine carbonates have not change the original Cr isotopic composition of these samples. However, the basalts from Kilauea Iki show considerable Cr isotopic variations, with ?53Cr ranging from -0.18±0.04‰(2SD) to 0±0.04‰(2SD),and their Cr isotopic compositions show positive correlations with their whole rock Cr concentrations, MgO and FeOtot contents,and negative correlations with SiO2, Al2O3 and CaO contents. These features can be interpreted as the accumulations of isotopically heavy chromite accompanying by olivine during the basaltic magmatic evolution, implying that there is small Cr isotopic fractionation during crystallization of chromite. The equilibrium Cr isotopic fractionation factor between residual melt and chromite (?53Crchr-melt) is modelled by a mixing model to account for all Kilauea Iki basalt Cr isotope data, and proves to be from 0.05‰ to 0.2‰ with a best fit value of 0.1 ‰. This result is in agreement with the situation of the lunar basalt, suggesting that there is isotopic fractionation during fractional crystallization of chromite under oxygen fugacity of both the lunar and the Earth's mantle conditions (IW-2 to FMQ or higher).Coexistence of Cr2+ and Cr3+ in the melts under this oxygen fugacity range and preference of Cr3+ in chromite might be the main reason responsible for the isotopic fractionation between chromite and melt. Given that the oxygen fugacity in the mantle of most differentiated asteroid are within the range of the lunar and the Earth's mantle conditions, similar Cr isotopic fractionation trend can be expected during the evolution of other asteroids too. Chromium isotopic fractionation during magmatic differentiation need to be taken into consideration when assessing the Cr isotopic compositions of differentiated asteroids like Vesta and Mars. The mean ?53Cr value of the Hawaiian basalts was calculated by averaging ?53Cr of samples less effected by crystallization of chromite, and we get a value of -0.17±0.05‰ (2SD),which is identical to the mean value of -0.14±0.12‰ (2SD) for mantle xenoliths, but might be systematically lighter.At the foundation of Cr isotope composition of the silicate Earth, we analyzed the Cr isotope composition of iron meteorites which may represent the core of small planetary bodies, and the Cr isotope fractionation between the metal and silicate under high temperature. It turns out that the Cr isotope composition of the iron meteorites(+0.86 ± 0.05‰, 2SD to +2.05 ± 0.04‰, 2SD),sampling the core of the asteroids,is much heavier than that of chondrites. Further, despite of significant difference in the mass-independent, the ?53Cr values of iron meteorites from the same group are indistinguishable. A series of experimental studies were conducted in piston cylinder at 1800?,1GPa, to explore the Cr isotope fractionation between the metal and silicate with varying compositions. It shows that there is small but obvious Cr isotope fractionation between fertile peridotite and Fe-Ni alloy and the Cr isotope compositions of the metal are heavier than those of the silicate, with a fractionation factor of ?53CrM-s (?53CrM-s=?53CrMetal-?53Crsilicate)=+0.08±0.04‰ (2SD). Moreover,we found that the?53CrM-s values change with the composition of the starting materials, and it seems that?53CrM-s increases with the Ni content in the metal and the oxygen fugacity of the system. In combination with the partitioning coefficient of Cr between metal and silicate which is a function of temperature, pressure and oxygen fugacity, we modelled the Cr concentration and isotope composition of the metallic core and silicate. The result shows that core-mantle segregation under relatively oxidizing conditions (IW-1.2)cannot explain the heavy Cr isotopic composition of the iron meteorites. Thus, there might be obvious Cr isotope fractionation during fractional crystallization of the metallic core. If the accretion of the Earth's core follow the reduced model, ?53Cr of the Earth's core would be identical to that of the silicate Earth; Otherwise, if the accretion of the Earth's core follow the oxidized model, ?53Cr of the Earth's core would be slightly heavier than that of the silicate Earth. Whichever accretion model of the Earth's core, ?53Cr of the silicate Earth would be identical to that of chondrites.To summarize, our studies found that there might be significant Cr isotopic fractionation during core mantle segregation, partial melting of mantle peridotite,fractional crystallization of chromite and metasomatism. Cr isotopes can play an important role in studying magmatic evolution, planetary formation and evolution.Our work provides the theoretical and data basis for the application and development of Cr isotope system in high temperature processes. |
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