Hematite records palaeo-seawater Zn isotopic compositions: Evidence from Zn isotopic fractionation during adsorption onto and incorporation into Fe (hydr)oxides

Abstract

The Zn isotopic compositions of Fe (hydr)oxides have significant potential for reconstructing seawater Zn isotopic compositions in the geological past. However, the Zn isotopic fractionations between adsorbed/structural and aqueous Zn species during the formation of ferric hydroxides and their subsequent transformation into hematite remain poorly constrained. In this study, experiments were conducted to quantify the Zn isotopic fractionation during adsorption onto hematite, coprecipitation with ferrihydrite, and transformation of ferrihydrite into hematite. Adsorption experiments onto hematite indicate that heavy Zn isotopes are adsorbed preferentially onto the solid phase by equilibrium fractionation, with the fractionation (Δ⁶⁶Zn_{adsorbed‒aqueous}) of 0.29‰ ± 0.03‰ being independent of the Zn concentrations and pH. In Zn‒Fe coprecipitation experiments, preferential enrichment of heavy Zn isotopes into the ferrihydrite structure resulted in an average fractionation (Δ⁶⁶Zn_{structural‒aqueous}) of 0.22‰ ± 0.07‰, which is attributed to indirect incorporation into the structure after adsorption onto the surface. The ferrihydrite–hematite transformation experiments show that adsorbed and structural Zn are released during ferrihydrite dissolution and subsequently incorporated into newly formed hematite or re-adsorbed onto its surface. Heavy Zn isotopes that were previously adsorbed onto ferrihydrite or partially incorporated into its structure are released back into solution during the transformation, whereas the isotopic signature of structural Zn in the ferrihydrite is partially preserved in the resulting hematite. The Zn isotopic fractionation between solids and solutions is better described by equilibrium fractionation at low f_solid (<80%) and by Rayleigh fractionation at higher f_solid, yielding an average isotopic fractionation (Δ⁶⁶Zn_{structural‒aqueous}) of 0.03‰ ± 0.03‰ at low f_solid (<80%). This suggests that no isotopic fractionation occurs between structural and aqueous Zn species during the transformation of ferrihydrite into hematite, regardless of the molar Zn/Fe ratio and pH at low f_solid (<80%). These results provide new insights into the Zn isotopic systematics of Fe (hydr)oxides in archives such as Fe formations and ferromanganese crusts, and highlights their utility as reliable proxies for reconstructing ancient seawater Zn isotopic compositions and constraining Zn biogeochemical cycling and marine primary productivity.