CR球粒陨石金属元素地球化学
Trace element geochemistry of CR chondrite metal
摘要：
我们报告的微量元素分析激光烧蚀电感耦合等离子体质谱法的金属颗粒从9个不同的CR晶体，区分颗粒球粒内部（"内部晶粒"），球粒表面壳（"边缘晶粒"）和矩阵（"孤立的晶粒"）。除了少数异常颗粒，镍标准化的微量元素模式是相似的所有三个岩相设置，主要是未分馏的难熔亲铁元素和贫化挥发性金，铜，银，硫。所有类型的颗粒被解释为来自一个共同的前体近似的最不熔化，细粒度的对象在CR陨石。这也排除了金属蒸气的再凝结作为大量边缘颗粒的来源。金属前体可能是由不完全冷凝形成的，有证据表明，在与较低温度的铂族元素（PGE）贫化冷凝物混合之前，高温隔离了富铂族元素（PGE）的冷凝物。富含Ni的内部晶粒的圆形形状表明在缓慢冷却（1 - 100 K/h）时熔融并与硅酸盐平衡，主要是通过Fe的氧化/蒸发。我们建议，镍贫，变形边缘颗粒，通常包括在富含辉石的外围常见的I型球粒，结果不太激烈的处理的边缘增生到球粒后，由内部记录的熔融事件。这意味着要么有两个单独的加热事件，分别形成橄榄石/内部颗粒和辉石/边缘颗粒，在这两个事件之间，尘埃被堆积在球粒周围，要么有一个单一的高温事件，其中球粒边缘记录了一个晚期的“淬火阶段”，在这种情况下，尘埃堆积在球粒上，而它们是熔融的。在后一种情况下，球粒形成区的尘埃浓度很高。
英文摘要：
We report trace element analyses by laser ablation inductively coupled plasma mass spectrometry of metal grains from 9 different CR chondrites, distinguishing grains from chondrule interior ("interior grains"), chondrule surficial shells ("margin grains") and the matrix ("isolated grains"). Save for a few anomalous grains, Ni-normalized trace element patterns are similar for all three petrographical settings, with largely unfractionated refractory siderophile elements and depleted volatile Au, Cu, Ag, S. All types of grains are interpreted to derive from a common precursor approximated by the least melted, fine-grained objects in CR chondrites. This also excludes recondensation of metal vapor as the origin of the bulk of margin grains. The metal precursors presumably formed by incomplete condensation, with evidence for high-temperature isolation of refractory platinum-group-element (PGE)-rich condensates before mixing with lower temperature PGE-depleted condensates. The rounded shape of the Ni-rich, interior grains indicates melting and equilibration with silicates upon slow cooling (1-100 K/h), largely by oxidation/evaporation of Fe. We propose that Ni-poorer, amoeboid margin grains, often included in the pyroxene-rich periphery common to type I chondrules, result from less intense processing of a rim accreted onto the chondrule subsequent to the melting event recorded by the interior. This means either that there were two separate heating events, which formed olivine/interior grains and pyroxene/margin grains, respectively, between which dust was accreted around the chondrule, or there was a single high-temperature event, of which the chondrule margin record a late "quenching phase", in which case dust accreted onto chondrules while they were molten. In the latter case, high dust concentrations in the chondrule-forming region are indicated.
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