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

Abstract

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 δ65Cu 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 δ65Cu 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 δ65Cu 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.

Heavy Copper Isotopes in Arc‐Related Lavas From Cold Subduction Zones Uncover a Sub‐Arc Mantle Metasomatized by Serpentinite‐Derived Sulfate‐Rich Fluids

“…Guo et al. (2022) also proposed that the elevated δ 65 Cu values of the altered basalts from the Mariana and Yap trenches were due to the preferential adsorption of heavy Cu isotopes onto secondary minerals in the altered basalts (Busigny et al., 2018; Liu et al., 2019). Nevertheless, backscattered electron images of the studied lavas indicate that they do not contain secondary minerals (Figure S4 in Supporting Information S1).…”

“…Recently, Guo et al. (2022) suggested that the elevated δ 65 Cu values in an altered basalt section from the southern Mariana trench were due to concentration‐driven diffusion induced by the dissolution of sulfide in the altered rims during alteration. However, our whole‐rock samples were collected from the core sections of each fresh sample (Figure S3 in Supporting Information S1).…”

“…The influence of post‐magmatic alteration or weathering must be assessed as this process can produce considerable Cu isotope fractionation (e.g., Guo et al., 2022; Huang, Liu, Gao, et al., 2016; Li et al., 2009; Little et al., 2019; Liu et al., 2014, 2019; Mathur et al., 2005). Several lines of evidence indicate that the seafloor alteration did not affect the Cu isotopic compositions of the studied lavas.…”

“…In particular, the intergrowth texture of mss and iss (Figure 3b and Figure S5 in Supporting Information S1) is further indicative of a magmatic origin (Patten et al, 2012(Patten et al, , 2013Zelenski et al, 2018). Guo et al (2022) also proposed that the elevated δ 65 Cu values of the altered basalts from the Mariana and Yap trenches were due to the preferential adsorption of heavy Cu isotopes onto secondary minerals in the altered basalts (Busigny et al, 2018;Liu et al, 2019). Nevertheless, backscattered electron images of the studied lavas indicate that they do not contain secondary minerals (Figure S4 in Supporting Information S1).…”

“…Third, 65 Cu is preferentially released to hydrothermal fluids during alteration, thereby producing low δ 65 Cu values in the altered rocks (Fernandez & Borrok, 2009;Fujii et al, 2013;Mathur et al, 2009Mathur et al, , 2015Sherman, 2013), which contrasts with the high δ 65 Cu values observed in the studied lavas (Figure 2). Recently, Guo et al (2022) suggested that the elevated δ 65 Cu values in an altered basalt section from the southern Mariana trench were due to concentration-driven diffusion induced by the dissolution of sulfide in the altered rims during alteration. However, our whole-rock samples were collected from the core sections of each fresh sample (Figure S3 in Supporting Information S1).…”