The Origin of Metal and Chondrules in CH and CB Chondrites - Evidence from Fe, Ni and Mg isotopes

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CH and CB chondrites are unusual chondrite breccias that contain a range of objects that formed under different conditions and settings. In particular, the origin of chondrules of types SO and CC as well as of zoned and unzoned metal grains have been extensively studied as they carry mineralogical and chemical features consistent with a formation as condensates from a gas/melt mix or gas vapour at variable high temperatures. The preferred current model postulates a giant impact of two embryos, at about 4.5 m.y after CAI formation, to have lead to the formation of such a vapour plume. Alternatively, it was suggested that metal and SO and CC chondrules condensed from a solar gas in isolated compartments of the solar nebula. In order to better constraint the formation conditions we propose a study of insitu analyses by femtosecond LA-MC-ICP-MS of stable isotopes of Fe and Ni in metal and Fe and Mg in chondrule silicates in CH and CB chondrites. We showed in preliminary studies of CH and CB chondrites that silicates and metal display mass-dependent Fe isotopes fractionation, with silicates being isotopically heavier. We also showed that unzoned metal grains in CBb chondrite HaH 237 contain Fe isotopes of chondritic composition. Chemically zoned metal display a zonation of Fe isotopes that follows the chemical zonation of Ni, with light delta56Fe in cores and chondritic delta56Fe in rims. Rapid grain growth at high temperatures could cause the zoning and the observed kinetic fractionation of Fe isotopes. Fractionation signatures of stable isotopes are sensitive markers for condensation, evaporation and diffusion processes. Therefore, we plan a systematic study of Fe isotopes and, on selected grains, also Ni isotopes in order to trace condensation signatures and distinguish these from those generated by diffusion. Isotope and chemical zoning patterns will be modelled in order to gain information on time scales of condensation or diffusion. In addition, we propose to analyse Mg and Fe isotope systematics in silicates of SO and CC chondrules in order to test existing hypothesis and constrain their orgin further. An important part of this study is the careful petrologic characterization of the meteorites that will includes major and minor element analyses by EMP and insitu analyses of REE, Sc, etc. in siliactes and PGE, etc. elements in metal by LA-ICP-MS. These data will provide the basis for the selection of suitable grains and chondrules for stable isotope analyses. We expect this study to substantially expand our knowledge of the origin of CH and CB chondrites chondrites.