首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >Highly siderophile elements and ~(187)Re-~(187)Os isotopic systematics of the Allende meteorite: Evidence for primary nebular processes and late-stage alteration
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Highly siderophile elements and ~(187)Re-~(187)Os isotopic systematics of the Allende meteorite: Evidence for primary nebular processes and late-stage alteration

机译:阿连德陨石的高度亲铁元素和〜(187)Re-〜(187)Os同位素系统:原发性星状突和后期蚀变的证据

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摘要

The abundances of highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, and Pd, as well as ~(187)Re-~(187)Os isotopic systematics were determined for calcium-aluminum-rich inclusions (CAIs), chondrules, and matrix, separated from the CV3 carbonaceous chondrite Allende. Consistent with prior studies, CAIs are characterized by significant depletions in Pd relative to the other HSE, while the other HSE are in generally bulk chondritic relative abundances. The depletions in Pd can be linked with initial formation of CAIs via condensation, or subsequent processing by evaporative processes. Chondrules generally have relative HSE patterns similar to CAIs, although they have lower absolute abundances. Palladium depletions in chondrules may reflect solid metal-liquid metal fractionation at the time of formation, or alternatively, be the result of processes that acted on precursor materials. Matrix samples have nearly chondritic absolute abundances of all HSE measured. Consequently, matrix is the only major chondritic component examined here that shows no relative depletion in Pd. Mass balance suggests the existence of an unidentified Pd-rich carrier, although it is possible that the dataset presented here is too limited to represent typical HSE abundances of some chondritic components (e.g., chondrules). The ~(187)Re-~(187)Os isotopic systematics of only six out of twenty-four Allende chondritic components analyzed plot within uncertainties of a 4568 Ma primordial reference isochron. The deviations from the expected isochron most likely reflect late-stage, open-system behavior within the last 2 billion years, and, in some cases, could even have resulted from terrestrial alteration. The open-system behavior is most readily observed in small, millimeter-size sub-samples of Allende, consistent with Re and/or Os mobility on that scale.
机译:确定了富含铁-铝夹杂物(CAI)的高度亲铁元素(HSE)Re,Os,Ir,Ru,Pt和Pd的丰度以及〜(187)Re-〜(187)Os同位素系统,球粒状和基质,与CV3碳质球粒状阿连德分离。与以前的研究一致,CAIs的特征是钯相对于其他HSE的消耗显着减少,而其他HSE的总体呈软骨状相对丰度。 Pd的贫化可以与CAI的初始形成(通过缩合)或随后通过蒸发过程进行处理有关。软骨的绝对HSE模式类似于CAI,尽管它们的绝对丰度较低。软骨中的钯耗竭可能反映了形成时的固体金属-液体金属分馏,或者是作用于前体材料的过程的结果。基质样品的所有HSE几乎具有软骨样的绝对丰度。因此,基质是这里检查的唯一主要软骨成分,在Pd中没有相对损耗。质量平衡表明存在未确定的富Pd载体,尽管此处显示的数据集可能太有限而无法代表某些软骨成分(例如软骨)的典型HSE丰度。在4568 Ma原始参考等时线的不确定性范围内,分析的二十四个阿连德软骨成分中只有六个中的〜(187)Re-〜(187)Os同位素系统。与预期等时线的偏差很可能反映了过去20亿年内的后期开放系统行为,在某些情况下,甚至可能是由于陆地变化而引起的。开放系统的行为最容易在阿连德的毫米大小子样本中观察到,与该尺度上的Re和/或Os迁移率一致。

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