Chromium Stable Isotope Panorama of Chondrites and Implications for Earth Early Accretion
Ke Zhu (朱柯), Frédéric Moynier, Conel M. O’D. Alexander, Jemma Davidson, Devin L. Schrader, Jian-Ming Zhu, Guang-Liang Wu, Martin Schiller, Martin Bizzarro, and Harry Becker
The Astrophysical Journal, Volume 923, Number 1
“We investigated the stable isotope fractionation of chromium (Cr) for a panorama of chondrites, including EH and EL enstatite chondrites and their chondrules and different phases (by acid leaching). We observed that chondrites have heterogeneous δ53Cr values (per mil deviation of the 53Cr/52Cr from the NIST SRM 979 standard), which we suggest reflect different physical conditions in the different chondrite accretion regions. Chondrules from a primitive EH3 chondrite (SAH 97096) possess isotopically heavier Cr relative to their host bulk chondrite, which may be caused by Cr evaporation in a reduced chondrule-forming region of the protoplanetary disk. Enstatite chondrites show a range of bulk δ53Cr values that likely result from variable mixing of isotopically different sulfide-silicate-metal phases. The bulk silicate Earth (δ53Cr = –0.12 ± 0.02‰, 2SE) has a lighter Cr stable isotope composition compared to the average δ53Cr value of enstatite chondrites (–0.05 ± 0.02‰, 2SE, when two samples out of 19 are excluded). If the bulk Earth originally had a Cr isotopic composition that was similar to the average enstatite chondrites, this Cr isotope difference may be caused by evaporation under equilibrium conditions from magma oceans on Earth or its planetesimal building blocks, as previously suggested to explain the magnesium and silicon isotope differences between Earth and enstatite chondrites. Alternatively, chemical differences between Earth and enstatite chondrite can result from thermal processes in the solar nebula and the enstatite chondrite-Earth, which would also have changed the Cr isotopic composition of Earth and enstatite chondrite parent body precursors.”
