Copper Isotopic Fractionation During Seafloor Alteration: Insights From Altered Basalts in the Mariana and Yap Trenches

To further investigate copper (Cu) isotopic fractionation during seawater-oceanic crust interactions, 13 subsamples across a basalt altering section and 12 bulk-rock basalts from the southern Mariana and Yap trenches, western Pacific Ocean, were studied. We find that the delta Cu-65 values of the basalt section roughly increase from mid-ocean ridge basalt-like values (0.04% - 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 the initial dissolution of Cu-bearing sulfides releasing isotopically heavy Cu into ambient seawater, consistent with both lower Cu and sulfide contents in the altered rims. The elevated delta Cu-65 values reveal that sulfide dissolution induced a concentration-driven diffusion from the core to rims, controlling the rim-core-rim Cu content and isotopic variations on centimeter scale. By contrast, the delta Cu-65 values of bulk-rock basalts show a wide range (0.02% - 0.56%) with most samples enriched in heavy isotopes, and negatively correlate with Cu concentrations, indicating preferential adsorption of isotopically heavy Cu from seawater. We propose that chemical diffusion occurred prior to any adsorption processes, suggesting that a three-stage process involving sulfide dissolution, chemical diffusion and adsorption controls Cu isotopic fractionation during seafloor basalt alteration. Plain Language Summary Seawater-oceanic crust interactions have been known to affect the Cu-65/Cu-63 isotopic ratios of both oceans and the oceanic crust, thus playing a key role on global copper (Cu) cycling. However, the behavior of Cu during seawater-oceanic crust interactions remains poorly understood. We, for the first time, conduct a combined study on "in situ" delta Cu-65 values of 13 subsamples across a basalt altering section and whole-rock delta Cu-65 values of 12 bulk basalts from the southern Mariana and Yap trenches. The rims in the basalt section have Cu isotopic compositions lighter than mid-ocean ridge basalts, reflecting sulfide oxidation during seafloor alteration. The rim-core-rim Cu content and isotopic variation can be explained by a concentration-driven diffusion from the core to rims after sulfide oxidation. Furthermore, the whole-rock delta Cu-65 values with a wide range (0.02% - 0.56%) indicate that chemical diffusion may be followed by an absorption during seafloor alteration. The multistage Cu isotopic fractionation proposed here therefore is a step toward a more comprehensive picture of seawater-oceanic crust interactions.