Transmission electron microscopy of impact‐generated platinum group element alloys from Barberton spherule layers: New clues to their formation
Transmission electron microscopy of impact‐generated platinum group element alloys from Barberton spherule layers: New clues to their formation
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巴伯顿小球层撞击生成的铂族元素合金的透射电子显微镜：其形成的新线索
DOI：
10.1111/maps.13109
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发表时间：
2018
影响因子：
2.2
通讯作者：
Reimold
中科院分区：
文献类型：
--
作者：
Mohr-Westheide;Greshake;Reimold
已知最古老的大型火流星撞击地球的代表是南非巴伯顿绿岩带（BGB）和西澳大利亚皮尔巴拉克拉通的大约3.47-3.2 Ga的太古代球粒层。这些地层被认为是撞击沉积物，因为它们的铂族元素含量过高，表明有外星成分。在某些情况下，这之后是对地外铬同位素比率的测量。最近，来自BGB的太古代球粒层中的地外PGE特征被局部化，并且与与Ni，Cr-富尖晶石相关的亚微米PGE合金微量元素的存在正相关。然而，这些铂族矿物（PGM）相的实际形成尚未解决。撞击体的原生陨石颗粒、撞击熔化的产物或撞击蒸气羽流的冷凝都被认为是可能的遗传过程。解决这个问题需要详细的微观分析调查的内部微观化学和微观结构组成，纹理特征，和不同阶段之间的晶体学关系。在这里，我们报告了对六种此类PGE微粒的首次透射电子显微镜（TEM）研究结果，这些微粒包裹在Ni-Cr尖晶石中或出现在来自BARB 5 ICDP钻孔岩芯和CT 3勘探岩芯的巴伯顿球粒层的基质中。结果包括各种化学和结构的PGM组合物，很难解释一个单一的过程，导致的结论是，几个过程可能已经参与了形成的PGM在太古代球粒层从BGB。有证据支持这些铂族金属的形成，从尖晶石主相的出溶，从熔融相沉淀，并从气相（的影响蒸汽羽流）冷凝。
The oldest known large bolide impacts onto Earth are represented by approximately 3.47–3.2 Ga old Archean spherule layers of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara craton in West Australia. These layers were recognized as impact deposits by their excessively high platinum group element (PGE) contents that are indicative of an extraterrestrial component. This was followed by measurements of extraterrestrial Cr isotopic ratios, in some cases. Recently, the extraterrestrial PGE signature in Archean spherule layers from the BGB was localized and positively associated with the presence of submicrometer PGE alloy micronuggets associated with Ni,Cr‐rich spinel. The actual formation of these platinum group mineral (PGM) phases has, however, not yet been resolved. Primary meteoritic particles from the impacting body, the products of impact melting, or condensation from impact vapor plumes have all been proposed as possible genetic process. Resolving this requires detailed microanalytical investigation of the internal microchemical and microstructural compositions, textural characteristics, and crystallographic relationships between the different phases. Here, we report the results of a first transmission electron microscopy (TEM) study of six such PGE microparticles enclosed in Ni‐Cr spinel or occurring in groundmass of Barberton spherule layers from the BARB5 ICDP drill core and from the CT3 exploration core. Results include a variety of chemical and structural PGM compositions that are difficult to explain by a single process, leading to the conclusion that several processes may have been involved in the formation of PGMs in Archean spherule layers from the BGB. There is evidence supporting formation of these PGMs by exsolution from the spinel host phase, precipitation from a melt phase, and condensation from a gas phase (of the impact vapor plume).
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