Iron and zinc isotope fractionation during sulfide-silicate melt immiscibility: insights from clinopyroxene megacrysts

This dataset includes major and trace element compositions of clinopyroxene megacrysts, and iron and zinc isotope compositions of clinopyroxene megacrysts, sulfide inclusions, and host basalts in eastern China. To understand how iron (Fe) and zinc (Zn) isotopes behave during sulfide-silicate melt immiscibility, we analyzed clinopyroxene megacrysts and their encapsulated sulfide inclusions from Cenozoic basalts in eastern continental China. These megacrysts, known to have co-precipitated with garnet alongside immiscible segregation of sulfide melts from basaltic magmas, show increasing δ56Fe (from 0.06‰ to 0.22‰) with decreasing Mg# in response to high-pressure magma differentiation. Sulfide inclusions have markedly lighter Fe isotopes (δ56Fe from –0.38‰ to 0.13‰), resulting in large and variable Fe isotope fractionation between clinopyroxene and sulfide (∆56FeClinopyroxene-Sulfide = 0.04‰-0.55‰). In contrast, Zn isotope fractionation is more uniform (∆66ZnClinopyroxene-Sulfide = ~0.18‰) between clinopyroxene (δ66Zn = 0.39 ± 0.05‰) and sulfide (δ66Zn = 0.21 ± 0.05‰).
We attribute the variable Fe isotope fractionation to the compositional variations of sulfide melts. Early-stage Cu-rich sulfides with more chalcopyrite (Cu1+Fe3+S2) component have heavy Fe isotope compositions (δ56Fe ≈ 0.07‰) and a limited isotope fractionation with co-crystallizing clinopyroxene (∆56FeClinopyroxene-Sulfide ≈ 0.10‰). In contrast, late-stage Cu-poor sulfides are isotopically light (δ56Fe ≈ –0.38‰), resulting in a greater ∆56FeClinopyroxene-Sulfide of ~0.55‰. Quantitative modeling shows that while sulfides host isotopically light Fe and Zn, their segregation is volumetrically minor. As a result, fractional crystallization of silicate minerals (e.g., clinopyroxene and garnet), instead of the segregation of immiscible sulfides, dominates Fe-Zn isotope evolution during basaltic magma differentiation. This finding requires revision of models that invoke extensive sulfide segregation to explain heavy Fe isotope signatures in evolved magmas.