Shock-induced potassium and zinc isotope fractionation in ordinary chondrites and its implications

To constrain how impacts influence the behavior of moderately volatile elements (MVEs), we report potassium (K) and zinc (Zn) contents and isotopic compositions of shock melt pockets (SMPs) and unmelted parts of three heavily shocked ordinary chondrites and bulk rocks of Chelyabinsk meteorite. All SMPs are enriched in K content and have lower isotopic values (delta K-41 = -1.99 parts per thousand, -1.22 parts per thousand and -1.40 parts per thousand) while the adjacent unmelted parts are enriched in heavy K isotopes (delta K-41 = -0.41 parts per thousand, -0.01 parts per thousand and 0.04 parts per thousand) compared to the bulk rocks of Chelyabinsk meteorite (delta K-41 = -0.77 parts per thousand and -0.73 parts per thousand). By contrast, Zn is depleted in SMPs and the isotopic compositions are heavier (delta Zn-66 = -0.19 parts per thousand, 2.42 parts per thousand, 1.74 parts per thousand) in SMPs than that in unmelted parts (delta Zn-66 = -0.65 parts per thousand, 1.76 parts per thousand, -0.97 parts per thousand). Our results indicate a decoupling between the two MVEs that Zn is lost from shock melts while K is dramatically enriched in shock melts during impacts. The isotope fractionation of Zn is probably caused by evaporation of shock melts, while K isotope fractionation is most likely caused by solid-melt diffusion which is controlled by its incompatibility. The isotopic decoupling of K from Zn during major impacts further enhances our understanding of high temperature elemental and isotopic behavior of MVEs and may shed new light on the variously heterogeneous distribution of MVEs in solar system. (c) 2023 Elsevier Ltd. All rights reserved.