Origin of metal from CB chondrites in an impact plume – A combined study of Fe and Ni isotope composition and trace element abundances

Abstract

The formation processes of the unusually metal-rich CB chondrites are a matter of debate. It is widely accepted that metal grains have formed by condensation. However, it is still debated whether they condensed directly from the solar nebula or from an impact-induced vapor plume. In this study, we present high precision Fe and Ni isotope and trace element composition of zoned and unzoned metal grains from the CBb chondrites Hammadah al Hamra 237, QUE 94411, and MAC 02675, and the CH/CBb breccia Isheyevo and unzoned metal from the CBa chondrites Bencubbin, Gujba, and NWA 4025. Data were obtained using femtosecond laser ablation (multicollector) inductively coupled plasma mass spectrometry (fs-LA-(MC)-ICP-MS). Zoned metal grains from CBb meteorites generally display parallel profiles of Ni and Fe isotope compositions with very low δ56Fe and δ60Ni, and elevated concentrations of refractory siderophile elements in their cores. These findings are consistent with dominantly kinetic isotope- and trace element fractionation during condensation from a confined and fast cooling gas reservoir. Tungsten and Mo are frequently depleted relative to other refractory elements, particularly in zoned metal grains, which is suggestive for elevated oxygen fugacities in the gas reservoir. Such conditions are indicative of the formation of these metal grains during an impact event.Compared to zoned metal, unzoned metal grains are isotopically more homogeneous and more similar to the heavier rims of the zoned metal grains. This indicates that they formed under different conditions than the zoned metals, i.e., in a more slowly cooling environment. However, several unzoned grains still display significantly variable and correlated δ56Fe and δ60Ni, suggesting that their formation was related to that of the zoned metal grains. The kinetic fractionation-dominated isotopic signatures of the zoned metal grains strongly point to their formation during fast cooling, as may be expected for the exterior envelope of an impact plume. In contrast, the more homogenous isotopic signatures of the unzoned metal grains are more consistent with dominantly equilibrium-like isotope fractionation during condensation, as may be expected for the interior of an impact plume. In this scenario, the isotopically heavier rims of the zoned grains are best explained by a depletion of the outer plume gas reservoir in refractory elements and light isotopes. Accordingly, these findings indicate that zoned and unzoned metal grains likely formed during the same event. The compositional differences among individual unzoned metal grains, but also within some of the zoned grains, indicate turbulent gas mixing, also including movement of metals during their formation, between inner and outer regions of the impact plume.