Cu and Zn isotope fractionation during oceanic alteration: Implications for Oceanic Cu and Zn cycles

Abstract

Abstract 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\u202f±\u202f0.05‰; 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\u202f=\u202f−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.