In search of the Earth-forming reservoir: Mineralogical, chemical, and isotopic characterizations of the ungrouped achondrite NWA 5363/NWA 5400 and selected chondrites
In search of the Earth-forming reservoir: Mineralogical, chemical, and isotopic characterizations of the ungrouped achondrite NWA 5363/NWA 5400 and selected chondrites
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DOI：
10.1111/maps.12834
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发表时间：
2017-05-01
影响因子：
2.2
通讯作者：
Papanastassiou, Dimitri A.
中科院分区：
文献类型：
--
作者：
Burkhardt, Christoph;Dauphas, Nicolas;Papanastassiou, Dimitri A.
陨石的高精度同位素数据表明，长期存在的球粒均匀储层的概念并不总是适用于描述大块地球和其他行星体的同位素组成。为了减轻这种同位素危机的影响，并更好地了解陨石与地球形成储层的成因关系，我们对未分类的NWA 5363和NWA 5400进行了全面的岩石学、元素和多同位素（O、Ca、Ti、Cr、Ni、Mo、Ru和W）研究，这两种陨石的陆地O同位素特征之前都有报道。此外，我们还获得了Pillistfer （EL6）、allean （H6）和Allende （CV3）球粒陨石的同位素数据，并汇编了未分化和分化陨石的可用异常数据。NWA 5363和NWA 5400的化学成分惊人地相似，除了可追踪沙漠风化的流体流动元素。结果表明，NWA 5363和NWA 5400是原始无球粒岩母体的配对样品，经过硅酸盐熔体萃取和亲铁元素的加入，认为这两种岩石是抗性岩组合。铪-钨年表在CAI之后得出的模型年龄为2.2 +/- 0.8Myr，这可能是不确定的。确认了NWA 5363/NWA 5400的陆地O同位素特征；然而，Ca, Ti, Cr， Mo和Ru的核合成异常的发现表明，NWA5363/NWA 5400母体并不是通过已知陨石的混合来解释地球组成的缺失环节。在这个缺失的环节或陆地储层的直接样本被确定之前，将提供如何使用球粒陨石估计地球整体同位素组成的指导方针。
High-precision isotope data of meteorites show that the long-standing notion of a chondritic uniform reservoir is not always applicable for describing the isotopic composition of the bulk Earth and other planetary bodies. To mitigate the effects of this isotopic crisis and to better understand the genetic relations of meteorites and the Earth-forming reservoir, we performed a comprehensive petrographic, elemental, and multi-isotopic (O, Ca, Ti, Cr, Ni, Mo, Ru, and W) study of the ungrouped achondrites NWA 5363 and NWA 5400, for both of which terrestrial O isotope signatures were previously reported. Also, we obtained isotope data for the chondrites Pillistfer (EL6), Allegan (H6), and Allende (CV3), and compiled available anomaly data for undifferentiated and differentiated meteorites. The chemical compositions of NWA 5363 and NWA 5400 are strikingly similar, except for fluid mobile elements tracing desert weathering. We show that NWA 5363 and NWA 5400 are paired samples from a primitive achondrite parent-body and interpret these rocks as restite assemblages after silicate melt extraction and siderophile element addition. Hafnium-tungsten chronology yields a model age of 2.2 +/- 0.8Myr after CAI, which probably dates both of these events within uncertainty. We confirm the terrestrial O isotope signature of NWA 5363/NWA 5400; however, the discovery of nucleosynthetic anomalies in Ca, Ti, Cr, Mo, and Ru reveals that the NWA5363/NWA 5400 parent-body is not the missing link that could explain the composition of the Earth by the mixing of known meteorites. Until this missing link or a direct sample of the terrestrial reservoir is identified, guidelines are provided of how to use chondrites for estimating the isotopic composition of the bulk Earth.