Kinetic zinc isotope fractionation in olivine phenocrysts records magma evolution history of intra-plate basalts

Zinc isotope systematics of basaltic magmas have been widely used as novel proxies for terrestrial mantle heterogeneity induced by recycled crustal materials [1-2]. The influence of mineral-melt isotope disequilibrium during magma differentiation on Zn isotopic composition of basaltic melts, nonetheless, has received limited attention. No quantitative constraint has yet been given for the diffusion-driven Zn isotope fractionation between olivine crystals and melts. Here we present high-precision zinc isotope data (δ66ZnJMC-Lyon) for a series of olivine phenocrysts separated from intra-plate basalts from the Jiaodong Peninsula in Eastern China, together with in-situ trace element analysis. Olivine phenocrysts have Zn isotopic compositions which are too light in comparison with the host basaltic melts to be explained by equilibrium isotope fractionation at magmatic temperatures. Instead, the decrease of δ66Zn values with decreasing Mg# and increasing Zn contents in olivine phenocrysts suggests diffusion-driven kinetic fractionation during olivine crystallization. The data is well-fitted with a diffusion model in which Zn diffuses from surrounding melt into olivine crystals due to the large chemical gradient with a kinetic Zn isotope fractionation factor βZn of 0.07. It is suggested that diffusion-induced isotope disequilibrium during olivine crystallization may drive Zn isotopic composition of the residual melt toward heavier values by utilizing the fractional crystallization model. Thus, kinetic effects between phenocryst and melt must be considered when applying zinc isotope systematics of any basaltic magma to probe the source heterogeneity.

[1] Liu et al., 2016, EPSL 444, 169–178. [2] Beunon et al., 2020 Earth-Sci. Rev., 103174