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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. Plain Language Summary: 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 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" 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. 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 δ65Cu 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. Key Points: High‐precision copper (Cu) isotopic profile across a basalt altering section was reportedSystematic Cu and δ65Cu variations were produced by oxidative sulfide breakdown, diffusion and adsorptionMultistage Cu isotopic fractionation occurred during seafloor basaltic alteration [ABSTRACT FROM AUTHOR]

Black shales represent one of the main reservoirs of metals released to hydrosphere via chemical weathering and play an important role in geochemical cycling of metals in the ocean. The stable isotope systematics of transitional metals (e.g., Cu and Zn) may be used as a proxy for evaluating their geochemical cycling. To investigate the behaviors of Cu and Zn isotopes during metal enrichment of black shales and the migration during weathering, in this study we reported Cu and Zn concentration and isotope data for unweathered and weathered metalliferous shales and siliceous interbeds from the Maokou Formation in central China. The unweathered shales and cherts have moderately enriched Cu and Zn concentrations with silicate-like δ 65 Cu (+ 0.14 ± 0.09‰, 1σ) but heavy δ 66 Zn (0.51 ± 0.11‰, 1σ). The elevated δ 66 Zn values reflect an important contribution from seawater via sulfide precipitation and/or organic matter (OM) adsorption. The Zn isotopic compositions of these metalliferous shales are different from those of the ‘normal’ shales, highlighting the potential of Zn isotopes as a tracer for metal enrichment in natural systems. The weathered shales and cherts have an extreme δ 65 Cu range from − 6.42‰ to + 19.73‰ and a modest δ 66 Zn range of + 0.25‰ to + 0.78‰. The strongly weathered samples have lower Cu and Zn concentrations and lighter isotopic compositions compared to the weakly weathered samples. The leaching of Cu- and Zn-rich sulfides in shallow depths and their downward transport and refixation by Fe-sulfide account for the Cu and Zn isotope fractionation, with the huge Cu isotope variation generated by multistage redox leaching. In general, δ 66 Zn values of the weathered shales shift towards light values compared to the unweathered protoliths, suggesting that shale weathering releases Zn which is isotopically heavier than igneous rocks and the global riverine average (+ 0.33‰). Our results therefore indicate that Cu isotopes can be extremely fractionated during weathering of Cu-rich shales and both heavy Cu and Zn isotopes are preferentially released into fluids during shale weathering. These results should be considered when evaluating geochemical cycling of Cu and Zn in the modern or past oceans. [ABSTRACT FROM AUTHOR]

Zinc isotopes have been increasingly utilized as a proxy for (bio)geochemical cycles of nutrient Zn in the ecosystem like soils, but the behavior and mechanism of Zn isotope fractionation during weathering and soil formation remain to be constrained. Here we reported zinc contents and isotope data for saprolites and laterites from two well-characterized mafic-rock weathering profiles, formed under a subtropical climate (South Carolina, USA) and a tropical climate (Hainan Island, China), respectively. Leaching experiments with basalt and andesite as the starting materials were also performed for comparison. The leaching results revealed no resolvable Zn isotope fractionation during dissolution of silicate minerals in the absence of organic ligands in solutions. The Hainan laterites show strong Zn depletion and have lower δ66Zn Lyon values (0.10‰ to 0.27‰) relative to the parent rock (0.31 ± 0.04‰), which are coupled with light copper isotope compositions in the same samples. These observations suggest that Zn mobilization was achieved via metal-organic complexation and Fe-colloids transportation. By contrast, saprolites from the South Carolina are extremely Zn-rich (ten-fold higher concentration than the parent rock) and most samples have higher δ66Zn values (0.10‰ to 0.65‰) compared with the parent rock (0.24 ± 0.04‰). Zinc isotope ratios of the South Carolina profile are positively correlated with iron isotope ratios (δ56Fe) and Fe3+/Fe Total , the latter of which is related to secondary Fe (hydr)oxide formation. These relationships link the high Zn contents and δ66Zn values of saprolites from the South Carolina profile to the substantial adsorption of Zn onto or isomorphic substitution into Fe (hydr)oxides that prefers heavier Zn isotopes. Our study highlights that Fe (hydr)oxides exert a strong influence on the re-distribution of nutrient Zn in natural soils and the (bio)geochemical cycling in natural soil systems. [ABSTRACT FROM AUTHOR]

The behaviours of Cu, Zn and their isotopes during seafloor fluid-peridotite interaction and its influence on oceanic Cu-Zn cycles were investigated by analyzing a series of unaltered to strongly altered oceanic peridotites. Most of the altered peridotites have δ66Zn values indistinguishable from those of the unaltered ones (0.19 ± 0.05‰; 2SD), suggesting that seafloor alteration generally induces a small Zn isotope fractionation. A few altered samples that seem to have gained Zn have elevated δ66Zn, probably due to adsorption by ubiquitous secondary phases (e.g. goethite). Almost all altered peridotites show Cu loss (up to 90%) and have Cu isotope ratios that are shifted toward heavy values relative to the unaltered ones. Given that Cu is organically-complexed in fluids over a wide range of temperature and such fluids have strong preferences for the heavy isotopes, alteration and weathering alone are unlikely to account for the substantial loss of Cu coupled with elevated δ65Cu. An alternative mechanism is that initial Cu in peridotites was partly lost by alteration and weathering, and then Cu was added via adsorption by secondary mineral phases preferring the heavy Cu isotopes. This is confirmed by leaching experiments that were designed to extract the adsorbed Cu fractions in secondary phases. The results show that the leachates and the residua have, respectively, higher and lower δ65Cu values in comparison with the bulk peridotites, demonstrating that adsorption is an important mechanism for generating heavy copper isotope enrichments in the altered peridotites. In any case, mass balance considerations indicate that oceanic alteration must have resulted in a net input of isotopically light Cu from oceanic peridotites into the ocean. Recent estimates found that the Cu isotopic composition of the modern ocean is imbalanced, because all characterized sinks are isotopically light relative to characterized input fluxes, which requires an additional, isotopically light input. Our results indicate that seafloor alteration of abyssal peridotites is an isotopically light input (δ65Cu = −0.4‰ to −0.1‰) to the modern ocean, up to ∼1‰ lighter than the riverine input (∼0.7‰). First-order flux estimates suggest that this process exerts an important control on the marine budget of Cu. Most of Zn released during seafloor alteration has δ66Zn similar to that of mantle peridotites and this input is thus slightly lighter than the riverine input (∼0.33‰). Current estimates of modern oceanic Zn isotope cycles require consideration of this isotopically light source. [ABSTRACT FROM AUTHOR]