Ruthenium isotope fractionation in protoplanetary cores

Abstract

Mass-dependent Ru isotope variations (δ102/99Ru) and Ru concentrations were determined for 35 magmatic iron meteorites from the five major chemical groups (IIAB, IID, IIIAB, IVA, IVB). In addition, four equilibrated ordinary chondrites were analyzed. The IIAB, IIIAB and IVB iron meteorites display increasingly heavier Ru isotopic compositions with decreasing Ru content. Modeling demonstrates that the trends for these three iron groups can be reproduced by the incremental extraction of isotopically lighter Ru into solids, which leads to progressively heavier δ102/99Ru in the remaining melt. The modeling further shows that the Ru isotopic variations of the IIAB and IIIAB irons are consistent with derivation from parental melts with an ordinary chondrite-like δ102/99Ru, whereas the IVB irons more likely derive from a melt with heavier δ102/99Ru. This heavy Ru isotopic composition of the IVB parental melt probably results from high-temperature processing of the IVB precursor material. The Ru isotope systematics of the IID and IVA irons are more complex and show no correlation between δ102/99Ru and Ru content. Although most samples exhibit heavy Ru isotopic compositions, especially the late-crystallized irons of these groups deviate from the expected fractional crystallization trends. This deviation most likely results from mixing and re-equilibration of early-crystallized solids and late-stage liquids, followed by further fractional crystallization. The mixing might be related to the migration of liquids through a complex network of dendrites or to the overturn of a cumulate inner core, and bears testimony to the complex solidification history of at least some protoplanetary cores.