Copper isotope fractionation during magma differentiation: Evidence from lavas on the East Pacific Rise at 10°30′N

Abstract

Redox-driven copper (Cu) isotope fractionation has been widely observed in low-temperature weathering and hydrothermal processes. However, how Cu isotopes may fractionate during magmatic processes remain unknown. To address this issue, we studied MORB samples from the East Pacific Rise (EPR) at 10°30′N to explore the Cu isotope behavior during magma differentiation. These samples are globally ideal because they are generated from a uniformly depleted sub-ridge mantle source but have experienced varying extents of fractional crystallization and sulfide segregation with large variations of MgO (1.76–7.38 wt%) and Cu (16.0–75.2 μg/g). Lavas with MgO ≥ 4.9 wt% involving olivine, clinopyroxene and plagioclase as the major liquidus minerals show homogeneous δ65Cu of 0.05 ± 0.03‰, suggesting little Cu isotope fractionation at this stage of magma differentiation. However, after Fe-Ti oxides appear on the liquidus, melt δ65Cu values first decrease rapidly to –0.41‰ at MgO of 3.9 wt% and then increase with the most evolved sample having δ65Cu of 0.08‰. Such a significant Cu isotope fractionation during magma differentiation has never been reported before. We suggest the large Cu isotope variation at MgO < 4.9 wt% reflect a redox change of MORB melt resulting from fractional crystallization of Fe-Ti oxides. The crystallization of ilmenite (Fe2+TiO3), which is the first Fe-Ti oxide phase during MORB differentiation, causes a sudden increase of Fe3+/∑Fe in the residual melt and drives the redox reaction Fe3+ + Cu1+ → Fe2+ + Cu2+. As a result, sulfides segregated from the MORB melts have high Cu2+ content and heavy Cu isotope compositions with Δ65CuSulfide-Silicate melt > 0 because of the preferential bonding of heavy Cu isotopes (65Cu vs. 63Cu) with Cu2+, whose fractionation rapidly decreases δ65Cu of the residual melts. The increase of Fe3+/∑Fe will quickly drive the melt to be saturated in titanomagnetite (magnetite-Ulvöspinel solid solutions), whose crystallization decreases melt Fe3+/∑Fe and drives the redox reaction Fe2+ + Cu2+ → Fe3+ + Cu1+. As a result, the segregated sulfides after titanomagnetite saturation have decreasing Cu2+ content. These sulfides are also predicted to have low Ni content and exhibit Δ65CuSulfide-Silicate melt < 0, whose segregation raises δ65Cu in the residual melts. Therefore, we suggest a significant influence of redox states of Cu and abundance of Ni in the segregated sulfides on the Cu isotope fractionation during MORB differentiation. During mantle melting for MORB at ΔFMQ < 0, Cu isotope fractionation between melt and mantle sulfide is inferred to be limited, and the upper mantle has primitive MORB-like δ65Cu of 0.06 ± 0.05‰.