Chromium isotopic insights into the origin of chondrite parent bodies and the early terrestrial volatile depletion

Chondrites are meteorites from undifferentiated parent bodies that provide fundamental information about early Solar System evolution and planet formation. The element Cr is highly suitable for deciphering both the timing of formation and the origin of planetary building blocks because it records both radiogenic contributions from Mn-53-Cr-53 decay and variable nucleosynthetic contributions from the stable Cr-54 nuclide. Here, we report high-precision measurements of the massindependent Cr isotope compositions (epsilon Cr-53 and epsilon Cr-54) of chondrites (including all carbonaceous chondrites groups) and terrestrial samples using for the first time a multi-collection inductively-coupled-plasma mass-spectrometer to better understand the formation histories and genetic relationships between chondrite parent bodies. With our comprehensive dataset, the order of decreasing epsilon Cr-54 (per ten thousand deviation of the Cr-54/Cr-52 ratio relative to a terrestrial standard) values amongst the carbonaceous chondrites is updated to CI = CH >= CB >= CR >= CM approximate to CV >= CO >= CK > EC > OC. Chondrites from CO, CV, CR, CM and CB groups show intra-group epsilon Cr-54 heterogeneities that may result from sample heterogeneity and/or heterogeneous accretion of their parent bodies. Resolvable epsilon Cr-54 (with 2SE uncertainty) differences between CV and CK chondrites rule out an origin from a common parent body or reservoir as has previously been suggested. The CM and CO chondrites share common epsilon Cr-54 characteristics, which suggests their parent bodies may have accreted their components in similar proportions. The CB and CH chondrites have low-Mn/Cr ratios and similar epsilon Cr-53 values to the CI chondrites, invalidating them as anchors for a bulk Mn-53-Cr-53 isochron for carbonaceous chondrites. Bulk Earth has a epsilon Cr-53 value that is lower than the average of chondrites, including enstatite chondrites. This depletion may constrain the timing of volatile loss from the Earth or its precursors to be within the first million years of Solar System formation and is incompatible with Earth's accretion via any of the known chondrite groups as main contributors, including enstatite chondrites. (C) 2021 Elsevier Ltd. All rights reserved.