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

In the past few years, zinc isotope systematics of basaltic magmas have been widely used as novel proxies for terrestrial mantle heterogeneity induced by recycled crustal materials and for planet formation and evolution. The influence of crystal-melt isotope disequilibrium during magma differentiation on the zinc isotopic composition of basaltic melts, nonetheless, has received little attention. In addition, no quanti-tative 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 (866ZnJMC-Lyon) for a series of olivine phenocrysts separated from intra-plate alkali basalts from the Jiaodong Peninsula in Eastern China, together with in-situ chemical analysis. Olivine phenocrysts have Zn isotopic compositions which are too light in comparison with the host basaltic melts (D66Znol-melt =-0.41 parts per thousand to-0.10 parts per thousand; n = 16) to be explained by equilibrium isotope fractionation at magmatic temperatures. Instead, the decrease of 866Zn values with decreasing Mg# (i.e., Mg/[Mg + Fe2+] x 100) and increasing Zn + Fe2+ contents in olivine phenocrysts suggests diffusion-driven kinetic fractionation during olivine crystallization. The data is well-fitted with a diffusion model in which Zn together with Fe2+ diffuse from surrounding melt into oli-vine crystals and approximately equal flux of Mg diffuses into melt due to the large chemical gradient, yielding a kinetic Zn isotope fractionation factor bZn of 0.07. By utilizing the fractional crystallization model under equilibrium or disequilibrium conditions, it is suggested that diffusion-induced isotope dis-equilibrium during olivine crystallization may drive Zn isotopic composition of the residual melt toward heavier values. When using zinc isotope systematics of any basaltic magma to probe the source hetero-geneity, this impact must be taken into account if the Zn isotope disequilibrium widely exists in olivine phenocrysts. Based on the time-related quantitative diffusion model, our results show that the magma residence time for strongly alkali basaltic lavas (-20 days) is 10-60 times shorter than that for weakly alkali basaltic lavas (6 months to 4 years). This supports the generation of weakly alkali basalts via inter-action between silica-undersaturated melts and the surrounding lithospheric mantle, implying a proba-ble common mechanism in intra-plate basalts that could be comparable globally. Thus, Zn isotope disequilibrium between olivine crystals and melts could provide valuable information on the evolution history of intra-plate basaltic magmas.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

In recent years, zinc isotope systematics have been widely used to study mantle heterogeneity and planet formation and evolution in basaltic magmas. The influence of crystal-melt isotope disequilibrium on the zinc isotopic composition of basaltic melts has not received much attention. High-precision zinc isotope data for olivine phenocrysts from alkali basalts in Eastern China suggest diffusion-driven kinetic fractionation during olivine crystallization. The zinc isotope disequilibrium between olivine crystals and melts provides valuable information on the evolution history of intra-plate basaltic magmas.