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

CH和CB球粒陨石是不同寻常的球粒陨石角砾岩，其中包含在不同条件和环境下形成的一系列物体。特别是，SO和CC型球粒以及分带和无分带金属颗粒的起源已经得到了广泛的研究，因为它们携带的矿物学和化学特征与气体/熔体混合物或气体蒸汽在变高温下的冷凝物形成一致。目前的首选模型假设，在CAI形成后大约4.5英里处，两个胚胎的巨大撞击导致了这种蒸汽羽流的形成。另一种说法是，金属、SO和CC球粒在太阳星云的孤立隔间中从太阳气体中凝聚而成。为了更好地约束形成条件，我们提出了用飞秒LA-MC-ICP-MS原位分析金属中Fe和Ni以及CH和CB球粒硅酸盐中Fe和Mg稳定同位素的研究。我们在CH和CB球粒陨石的初步研究中表明，硅酸盐和金属显示出质量依赖的铁同位素分馏，硅酸盐的同位素更重。我们还发现，在CBb球粒陨石HaH 237中，未分带的金属颗粒含有球粒组成的铁同位素。化学分带金属显示铁同位素的分带遵循Ni的化学分带，在核中有轻的delta56Fe，在边缘有球粒状的delta56Fe。高温下晶粒的快速生长导致了铁同位素的分带和动力学分异。稳定同位素的分馏特征是凝结、蒸发和扩散过程的敏感标志。因此，我们计划对铁同位素进行系统的研究，并在选定的颗粒上，也对镍同位素进行系统的研究，以追踪冷凝特征，并将这些特征与扩散产生的特征区分开来。为了获得凝结或扩散的时间尺度信息，将模拟同位素和化学分区模式。此外，我们建议分析SO和CC球粒硅酸盐中的Mg和Fe同位素系统，以验证现有的假设并进一步约束它们的起源。本研究的一个重要组成部分是对陨石进行细致的岩石学表征，包括用EMP进行主、次元素分析，用LA-ICP-MS对硅酸中REE、Sc等元素和金属中PGE等元素进行原位分析。这些数据将为选择合适的颗粒和球粒进行稳定同位素分析提供依据。我们期望这项研究能够大大扩展我们对CH和CB球粒陨石起源的认识。

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.