Contrasting zinc isotopic fractionation in two mafic-rock weathering profiles induced by adsorption onto Fe (hydr)oxides

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 δ66ZnLyon 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+/FeTotal, 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.