In situ observation of D-rich carbonaceous globules embedded in NWA 801 CR2 chondrite.
In situ observation of D-rich carbonaceous globules embedded in NWA 801 CR2 chondrite.
复制标题
对嵌入 NWA 801 CR2 球粒陨石中的富含 D 的碳质球进行原位观察。
DOI：
10.1016/j.gca.2013.08.007
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发表时间：
2013
影响因子：
5
通讯作者：
H.
中科院分区：
文献类型：
--
作者：
Hashiguchi;M.;Kobayashi;S. and Yurimoto;H.
使用同位素显微镜在 NWA 801 CR2 球粒陨石的基质中鉴定出 85 个富含 D 的碳质颗粒。使用二次电子显微镜结合 X 射线元素图谱对 67 富含 D 的碳质颗粒的出现进行了表征。 H 和 C 以及 D 富集的密切关联表明，富含 D 的碳质颗粒对应于有机物。富含 D 的有机颗粒以大约 660 ppm 的浓度分散在整个基质中。富含D的碳质颗粒的形态为直径最大约1μm的球状，分为四种类型：环状球状体、圆形球状体、不规则形状球状体和球状聚集体。环球是含有硅酸盐和/或氧化物的环状有机物，中心有或没有空隙。这是首次报道硅酸盐和氧化物颗粒被富含 D 的有机物包围。球粒聚集体由几种富含 D 的有机球粒与硅酸盐混合组成。环小球的形态与天文学推断的分子云或太阳外星云中产生的核幔颗粒非常相似，表明有机小球是由冰幔中的紫外光解作用形成的。附着在富含 D 的有机球粒上的硅酸盐或氧化物是迄今为止在球粒陨石中首次观察到的，并且可能是 CR2 球粒陨石的独特性质。环形球体、圆形球体、不规则形状球体和球体聚集体的氢同位素组成分别为δD=3000-4800、2900-8100、2700-11,000和2500-11,000‰。这些有机球的D/H比的变化似乎归因于有机自由基的D/H比的变化或富含D的有机自由基的含量的差异。四种富D碳质物质的氢同位素组成没有显着差异。 D-富集表明这些有机球体是在早期太阳系的冷分子云和/或外原行星盘中形成的。附着在环形球体和球体聚集体上的硅酸盐和氧化物的氧同位素组成范围为 δ17O = -49 至 50‰ 和 δ18O = -46 至 64‰。氧同位素组成与太阳系材料的氧同位素组成没有区别，这表明有机球体是在太阳系外层而不是在太阳系前环境中形成的。因此，NWA 801中的环形球体和球体聚集体可能是在早期太阳系的外原行星盘中形成的。在这项研究中没有发现表现出明显的前太阳起源的有机球体。缺乏清晰的太阳前特征可能表明，早期太阳星云中发生了太阳前有机物质的同位素组成或形态的变化。
Eighty-five D-rich carbonaceous particles were identified in the matrix of the NWA 801 CR2 chondrite using isotope microscopy. The occurrence of 67 D-rich carbonaceous particles was characterized using secondary electron microscopy combined with X-ray elemental mapping. The close association of H and C, and D-enrichment suggests that the D-rich carbonaceous particles correspond to organic matter. The D-rich organic particles were scattered ubiquitously throughout the matrix at a concentration of approximately 660 ppm. The morphology of the D-rich carbonaceous particles is globular up to about 1 μm in diameter and is classified into four types: ring globules, round globules, irregular-shaped globules, and globule aggregates. The ring globules are ring-shaped organic matter containing silicate and/or oxide, with or without a void in the center. This is the first report of silicate and oxide grains surrounded by D-rich organic matter. The globule aggregates are composed of several D-rich organic globules mixed with silicates. Morphology of ring globules is very similar to core-mantle grain produced in the molecular cloud or in the outer solar nebula inferring by astronomy, suggesting that the organic globules have formed by UV photolysis in the ice mantle. Silicates or oxides attached to D-rich organic globules are the first observation among chondrites so far and may be unique nature of CR2 chondrites. The hydrogen isotopic compositions of the ring globules, round globules, irregular-shaped globules, and globule aggregates are δD = 3000–4800, 2900–8100, 2700–11,000, and 2500–11,000‰, respectively. Variations of D/H ratio of these organic globules seemed to be attributed to variations of D/H ratio of the organic radicals or differences of content of the D-rich organic radicals. There are no significant differences in the hydrogen isotopic compositions among the four types of D-rich carbonaceous matter. The D-enrichments suggest that these organic globules have formed in a cold molecular cloud and/or the outer protoplanetary disk of the early solar system. The oxygen isotopic compositions of the silicates and oxides attached to the ring globules and globule aggregates range from δ17O = −49 to 50‰ and δ18O = −46 to 64‰. The oxygen isotopic compositions are not distinct from those of solar system materials, which suggests that the organic globules were formed in the outer solar system rather than in the presolar environment. Therefore, it is possible that the ring globules and globule aggregates in NWA 801 may have formed in the outer protoplanetary disk of the early solar system. Organic globules that exhibit clear presolar origin were not identified in this study. The lack of clear presolar signatures might suggest that modifications of isotopic compositions or morphologies of the presolar organic matter occurred in the early solar nebula.