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-Cr decay and variable nucleosynthetic contributions from the stable Cr nuclide. Here, we report high-precision measurements of the massindependent Cr isotope compositions (εCr and εCr) 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 εCr (per ten thousand deviation of the Cr/Cr ratio relative to a terrestrial standard) values amongst the carbonaceous chondrites is updated to CI = CH ≥ CB ≥ CR ≥ CM ≈ CV ≈ CO ≥ CK > EC > OC. Chondrites from CO, CV, CR, CM and CB groups show intra-group εCr heterogeneities that may result from sample heterogeneity and/or heterogeneous accretion of their parent bodies. Resolvable εCr (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 εCr 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 εCr values to the CI chondrites, invalidating them as anchors for a bulk Mn-Cr isochron for carbonaceous chondrites. Bulk Earth has a εCr 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.