Abstract
Beyond its economic value, copper (Cu) serves as a valuable tracer of deep magmatic processes due to its close relationship with magmatic sulfide evolution and sensitivity to oxygen fugacity (fO(2)). However, determining Cu's oxidation state (+ 1 or + 2) in silicate melts, crucial for interpreting its behavior and reconstructing fO(2) in the Earth's interior, has long been a challenge. This study utilizes X-ray Absorption Near Edge Structure spectroscopy to investigate the Cu oxidation state in hydrous mafic silicate melts equilibrated under diverse fO(2) (- 1.8 to 3.1 log units relative to the Fayalite-Magnetite-Quartz buffer), temperature (1150-1300 °C), and pressure (1.0-2.5 GPa) conditions. Our results reveal that Cu predominantly exists as Cu(+) across all fO(2) conditions, with a minor Cu(2+) component. This dominance of Cu(+) persists even in relatively oxidized melts, highlighting its limited sensitivity to fO(2) under upper mantle conditions. This significantly constrains the utility of Cu as an oxybarometer in hydrous silicate melts in the deep Earth. However, our findings suggest that Cu isotopes primarily reflect the interplay of sulfide segregation/accumulation during magmatic differentiation, shedding light on these fundamental processes in Earth's interior.