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

CH和CB角砾岩是不寻常的球粒陨石角砾岩，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型球粒以及分区和非分区金属颗粒的起源已被广泛研究，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可变高温下的冷凝物形成一致。目前的首选模型假定，在CAI形成后约4.5百万年，两个胚胎的巨大撞击导致了这样一个蒸汽羽的形成。另一种说法是，金属、SO和CC球粒是在太阳星云的隔离舱中从太阳气体中凝结而成的。为了更好地约束形成条件，我们提出了一个研究的原位分析，飞秒LA-MC-ICP-MS的稳定同位素的Fe和Ni的金属和Fe和Mg的球粒硅酸盐在CH和CB的方解石。我们表明，在CH和CB辉长岩的初步研究，硅酸盐和金属显示质量依赖的铁同位素分馏，与硅酸盐的同位素更重。我们还表明，CBb球粒陨石HaH 237中的未分区的金属颗粒含有铁同位素组成。化学分区的金属显示出Fe同位素的分带性，其遵循Ni的化学分带性，在核部具有轻δ 56 Fe，在边缘具有重δ 56 Fe。在高温下的快速晶粒生长可能会导致分区和观察到的Fe同位素的动力学分馏。稳定同位素分馏特征是反映凝结、蒸发和扩散过程的敏感标志。因此，我们计划对Fe同位素进行系统的研究，并在选定的颗粒上，也对Ni同位素进行系统的研究，以追踪冷凝签名并将其与扩散产生的签名区分开来。将模拟同位素和化学分区模式，以获得关于凝结或扩散时间尺度的信息。此外，我们还建议对SO球粒和CC球粒硅酸盐的Mg和Fe同位素系统进行分析，以检验现有的假说并进一步限制其成因。本研究的一个重要组成部分是对陨石进行详细的岩石学表征，包括EMP的主、次元素分析，LA-ICP-MS的硅质岩中REE、Sc等元素和金属中PGE等元素的原位分析，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。我们希望这项研究能够大大扩展我们对CH和CB的起源的了解。

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.