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

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 mu g/g). Lavas with MgO >= 4.9 wt% involving olivine, clinopyroxene and plagioclase as the major liquidus minerals show homogeneous delta Cu-65 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 delta Cu-65 values first decrease rapidly to -0.41% at MgO of 3.9 wt% and then increase with the most evolved sample having delta Cu-65 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+/Sigma 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 Delta 65Cu(Sulfide- Silicate melt) > 0 because of the preferential bonding of heavy Cu isotopes (65Cu vs. 63Cu) with Cu2+, whose fractionation rapidly decreases delta Cu-65 of the residual melts. The increase of Fe3+/Sigma Fe will quickly drive the melt to be saturated in titanomagnetite (magnetite-Ulv ospinel solid solutions), whose crystallization decreases melt Fe3+/Sigma 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 Delta 65Cu(Sulfide- Silicate melt) < 0, whose segregation raises delta Cu-65 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.FMQ < 0, Cu isotope fractionation between melt and mantle sulfide is inferred to be limited, and the upper mantle has primitive MORB-like delta Cu-65 of 0.06 +/- 0.05%.

Automated summary

Copper isotope fractionation during magmatic processes has been observed for the first time, with significant fractionation occurring during magma differentiation. The study also found that the fractionation of copper isotopes in magma differentiation is influenced by the redox state and sulfide segregation.