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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]

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]