Composition and origin of refractory-metal-rich assemblages in a Ca,Al-rich Allende inclusion
Composition and origin of refractory-metal-rich assemblages in a Ca,Al-rich Allende inclusion
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富钙、铝阿连德包裹体中富难熔金属组合的组成和起源
DOI：
10.1016/0016-7037(94)90479-0
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发表时间：
1994
影响因子：
5
通讯作者：
B. Spettel
中科院分区：
文献类型：
--
作者：
H. Palme;I. Hutcheon;B. Spettel
金属-氧化物-硫化物组合，此后称为不透明组合(OA)或熔融组合，具有高含量的难熔金属(Ir、Os、Ru、Pt等)。是从Allende Egg 6富钙富铝包裹体(B1型)中分离出来的。仪器中子活化分析(INAA)被应用于包括Zelda在内的七个OAS，Armstronget al的一篇早期论文中描述了这一点。(1987)。在INAA之后，四个OAS的抛光切片被准备用于岩石学和矿物学研究。此外，还对散装的鸡蛋6包裹体和矿物分离物进行了几次中子活化分析。块状包裹体表现出对难熔亲石元素(Ca、Al、Se、Rees等)的富集性。和难熔金属，典型的来自阿连德的粗晶CaI。OAS和母体Egg 6夹杂物的联合研究表明，该夹杂物以及其他许多夹杂物中难熔金属的主要宿主相不是这里分析的大型孤立OAS，而是许多细小的亚微观难熔金属合金，它们细小地分散在夹杂物的主要矿物中。Egg 6夹杂物中的难熔金属含量与冷凝成因定性上是一致的。然而，详细的计算表明，美洲国家组织中难熔和非难熔金属的丰度不能用简单的凝聚模型来匹配。此外，美洲国家组织在难熔金属绝对丰度和相对丰度方面的差异要求在略有不同的条件下形成不同的美洲国家组织。观测到的难熔金属在OAS中的分布反映了伴随着主要的难熔富金属组合的硫化和氧化而发生的重新分布。铂和Rh主要集中在NiFe、Os、Ru和Re中，它们是由更均匀的合金出溶形成的微小、微米级的OsRu金块。Ir在FeNi和OsRu金块之间分配，而Mo仅集中在MoS_2中。两个主要的硫化物相--磁黄铁矿和镍黄铁矿--除了约0.15%的钼外，其它难熔金属的含量都不到0.04%。钨在大块OAS中含量很低，尽管凝聚模型预测W很高。实验表明，OAS周围硅酸盐中的W-含量异常高，这表明在实验室模拟实验中，在包裹体中加入OAS后，W重新分布。Fremdlinger成因的模型必须包括几个阶段：(1)通过冷凝形成难熔金属合金，(2)在高温下并入现有的CaI中，(3)OAS的氧化和硫化以及元素的广泛再分布，包括挥发性W化合物和Mo化合物的损失。
Metal-oxide-sulfide assemblages, henceforth opaque assemblages (OA) or Fremdlinge, with high contents of refractory metals (Ir, Os, Ru, Pt, etc.) were separated from the Allende Egg 6 Ca, Al-rich inclusion (Type B1). Instrumental neutron activation analysis (INAA) was applied to seven OAs including Zelda, described in an earlier paper byArmstronget al. (1987). After INAA, polished sections were prepared of four OAs for petrographic and mineralogical studies. In addition, several INAA analyses of the bulk Egg 6 inclusion and mineral separates were performed. The bulk inclusion shows enrichments in refractory lithophile elements (Ca, Al, Se, REEs, etc.) and refractory metals, typical of coarse-grained CAIs from Allende. The combined study of OAs and the parent Egg 6 inclusion indicates that the major host phases of refractory metals in this inclusion, and probably in many others, are not the large isolated OAs analyzed here but numerous smaller submicroscopic refractory metal-rich alloys finely dispersed in the major minerals of the inclusion.The Egg 6 OAs have refractory metal contents qualitatively compatible with an origin by condensation. Detailed calculations, however, demonstrate that the abundances of both refractory and non-refractory metals in the OAs cannot be matched by simple condensation models. In addition, differences among OAs in absolute and relative abundances of refractory metals require formation of individual OAs under slightly different conditions. The observed distribution of refractory metals within OAs reflects redistribution accompanying sulfurization and oxidation of the primary refractory metal-rich assemblages. Platinum and Rh are primarily concentrated in NiFe, Os, Ru, and Re in tiny, micron-sized OsRu-nuggets formed by exsolution from a more homogeneous alloy. Iridium partitions between FeNi and OsRu-nuggets, while Mo is exclusively concentrated in MoS2. The two major sulfide phases, pyrrhotite and pentlandite, have less than 0.04% of any of the refractory metals, except for about 0.15% Mo. Tungsten is very low in the bulk OAs, although condensation models predict high W. It is demonstrated that W-contents in silicates surrounding OAs are unusually high, indicating redistribution of W after incorporation of OAs in the inclusion in aggreement with laboratory simulation experiments. A model for the origin of the Fremdlinge must involve several stages: (1) Formation of refractory metal alloys by condensation, (2) incorporation into existing CAIs at elevated temperatures, and (3) oxidation and sulfurization of OAs and extensive redistribution of elements including loss of volatile W-compounds and Mo-compounds.