Moderately volatile elements in chondrites record chondrule formation, two-component mixing and redistribution on parent bodies

Abstract

Most chondrites are depleted in moderately volatile elements (MVE) relative to the bulk solar system composition represented by CI chondrites. Here we present high-precision isotope dilution data for 11 moderately volatile elements (S, Cu, Zn, Ga, Se, Ag, Cd, In, Sn, Te and Tl) together with Cd and Zn stable isotope compositions for carbonaceous, ordinary, enstatite and Rumuruti chondrites complemented by a literature compilation of MVE stable isotope compositions. Together these data allow new insights into the processes that led to MVE depletion in chondrites and their redistribution within parent bodies.

Moderately volatile element abundances in carbonaceous, ordinary and Rumuruti chondrites are best explained by two-component mixing between a chemically CI-like MVE-rich matrix and an MVE-poor refractory component dominated by chondrules. Chondrules are enriched in light MVE isotopes due to kinetic recondensation of a small vapor fraction initially lost from chondrules upon heating. Later, thermal metamorphism redistributed some MVE within chondrite parent bodies, which is evaluated here in a systematic way for different chondrite groups and plateau volatile elements based on related and comparatively large but unsystematic stable isotope fractionation. Compared to other chondrite classes, enstatite chondrites show less systematic MVE abundance patterns when the elements are plotted as a function of condensation temperatures. Type 3 and 4 enstatite chondrites are more MVE-rich than expected based on their low matrix fractions and are enriched in light Zn and Te isotopes relative to CI. The enrichment of light Zn and Te isotopes and high MVE abundances in type 3 and 4 enstatite chondrites relative to CI can be explained by recondensation of a larger MVE vapor fraction after chondrule formation than observed for other chondrite classes, which presumably occurred at comparatively high H₂ pressures. Because MVE abundances and isotope compositions are fully consistent with chondrule formation, two-component mixing and MVE redistribution on parent bodies, we refute partial condensation from a hot solar nebula as the cause for MVE depletion in chondrite formation regions of the protoplanetary disk.