The origin of metal and chondrules in CH and CB chondrites : evidence from Fe, Ni, and Si isotopes and trace element compositions

Abstract

The formation processes of the unusually metal-rich CB and CH chondrites are highly debated. Chemically and isotopically zoned metal, unzoned metal, and cryptocrystalline (CC) chondrules from these meteorites are proposed to have formed by condensation. However, it is still unclear if they condensed directly from the solar nebula or from an impact-induced vapor plume, and how metal and chondrule formation are related. This thesis aims at unravelling the formation conditions of CH and CB chondrite constituents by laser ablation analyses of Fe, Ni, and Si isotopes in combination with trace element analyses. Parallel zoning of Fe and Ni isotopes in zoned metal from CH and CBb chondrites confirm a condensation origin of the zoning and exclude exchange diffusion as the formation process. Tungsten in zoned metal grains is depleted relative to other refractory elements which is suggestive for elevated oxygen fugacities in the gas reservoir, and thus, also for an impact event. Combined results of isotope and trace element analyses reveal that zoned metal formed in the fast-cooling shell regions, which favor kinetic fractionation while unzoned metal would have condensed from the slow-cooling interior of the plume under more equilibrium-like conditions. CC chondrules from CH and CBb chondrites were analyzed for Si isotope and trace element compositions to unravel their formation histories. Trace element abundances revealed two different populations of CC chondrules: (1) superchondritic refractory element contents and depletion in volatile elements most likely condensed from an unfractionated reservoir, and (2) with generally subchondritic element contents formed from a reservoir that was fractionated beforehand by an ultrarefractory phase. Tungsten, Mo, and Cr concentrations in metal and chondrules indicate elevated oxygen fugacities, and formation of those constituents from a closed system. Silicon isotope compositions of chondrules are heavier than BSE and the chondritic average. Thus, we propose that light Si isotopes were extracted from the gas reservoir, either due to the impact or by condensation of forsterite or melilite prior to CC chondrule formation. A relationship between CB, CH, and CR chondrites is proposed due to isotopic similarities (Cr, Ti, N, O), high metal contents, and similarities in hydrated clasts from CH and CB chondrites with CR matrix. Iron and Ni isotope results, combined with trace element contents, reveal relatively homogeneous composition of CR chondrite metal. These findings combined with textural differences, lead to the conclusion that metal from CR chondrites did not form in the same event as that of CH and CB chondrites. Bulk isotopic similarities (Cr, Ti, N, O) of the three chondrite groups may reflect (1) formation of precursor material in the same region, (2) accretion in the same region, or (3) inheritance of the isotope signature of a CR impactor by CH and CB chondrites.