Seawater-oceanic crust interactions have been known to affect the ⁶⁵Cu/⁶³Cu isotopic ratios of both oceans and the oceanic crust, thus playing a key role on global Cu cycling. However, geochemical behavior of Cu during seawater-oceanic crust interactions remains poorly understood. In this study, we report “in situ” δ⁶⁵Cu values of 13 subsamples across a basalt altering section and whole-rock δ⁶⁵Cu values of 12 bulk basalts from the southern Mariana and Yap trenches. We find that the δ⁶⁵Cu values of the basalt section roughly increase from MORB-like values (0.04 to 0.10‰) in the core to higher values (up to 0.20‰) near the core-rim interface, and then lower values (down to -0.17‰) at the rims. Isotopically light rims reflect that the initial dissolution of Cu-bearing sulfides released isotopically heavy Cu into ambient seawater, as suggested by both lower Cu and sulfide contents in the altered rims. The elevated δ⁶⁵Cu values reveal that sulfide dissolution induced a concentration-driven diffusion from the core to rims, controlling the Cu content and isotopic variations on the basalt section. By contrast, the δ⁶⁵Cu values of bulk-rock basalts show a wide range (0.02 to 0.56‰) with most samples enriched in heavy isotopes, and negatively correlate with Cu concentrations, indicating preferential adsorption of isotopically heavy Cu from seawater. Therefore, we propose that Cu isotopic fractionation may occur in oceanic crust basalts through a three-stage process involving sulfide dissolution, chemical diffusion and adsorption during seafloor alteration, which have significant influence on the Cu cycling at hadal zones.

Copper isotopes,seafloor alteration,Mariana trenches,Oceanic Cu cycling