Accretionary rims on inclusions in the Allende meteorite
Accretionary rims on inclusions in the Allende meteorite
复制标题
阿连德陨石内含物的增生边缘
DOI：
10.1016/0016-7037(85)90227-3
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发表时间：
1985
影响因子：
5
通讯作者：
L. Grossman
中科院分区：
文献类型：
--
作者：
G. MacPherson;A. Hashimoto;L. Grossman
阿连德的许多包裹体，特别是那些形状不规则的包裹体，被一系列在质地、矿物学和矿物化学上彼此不同的薄层所包围。所有其它层下面的层含有：IA，辉石针+橄榄石+ hedenbergite和andradite块状；IB，橄榄石甜甜圈；或IC，矩形橄榄石晶体。向外的下一层（II）包含微小的（<5 μm）橄榄石板和第三层（5 - 10 μm）橄榄石板条。最后一层，IV，是由富镁辉石针包围的赤铁矿+钙镁辉石团块。它将第三层从阿连德基质中分离出来，阿连德基质比第三层分选更差，硫化物含量更高。霞石和硫化铁是大多数层的共同成分，后者颗粒特别细，在第二层中含量丰富。虽然不是每一层都存在于每一个内含物上，但层的顺序是恒定的。这些边缘是增生聚集体的证据包括高度不平衡矿物组合的存在，以及它们是由许多自面体晶体组成的高度多孔体，很少有共生体。此外，在包裹体表面的地形空洞处，层最厚，而内层则不存在或不连续，这表明除了在口袋中，晶体的吸积概率最初很低，后来在形成了一个由凝聚晶体组成的软垫后，晶体的吸积概率变得更大。星云模型似乎最好地解释了不同矿物、矿物化学和质地的组合在不同边缘层的分离，在星云模型中，长而缓慢的冷却历史允许在颗粒/气体分离过程中凝结时进行区分。
Many inclusions in Allende, particularly those with irregular shapes, are surrounded by a sequence of thin layers which differ from one another in texture, mineralogy and mineral-chemistry. The layer underlying all others contains either: IA, pyroxene needles + olivine + clumps of hedenbergite and andradite; IB, olivine doughnuts; or IC, rectangular olivine crystals. The next layer outward, II, contains tiny (<5 μm) olivine plates and Layer III large (5–10 μm) olivine laths. The final layer, IV, occurs as clumps of andradite + hedenbergite surrounded by magnesium-rich pyroxene needles. It separates Layer III from the Allende matrix which is more poorly sorted and more sulfide-rich than Layer III. Nepheline and iron sulfide are common constituents of most layers, the latter being particularly fine-grained and abundant in Layer II. Although not every layer is present on every inclusion, the sequence of layers is constant. Evidence that the rims are accretionary aggregates includes the presence of highly disequilibrium mineral assemblages and the fact that they are highly porous masses consisting of many euhedral crystals with few intergrowths. In addition, the layers are thickest in topographic hollows on the surfaces of inclusions and the inner layers are absent or discontinuous beyond such irregularities, suggesting that the probability of accretion of crystals was low initially, except in pockets, and became greater later, after a soft cushion of accreted condensate crystals had already formed. Separation of assemblages of different mineralogy, mineral-chemistry and texture into different rim layers seems best explained by nebular models in which long, slow cooling histories allow differentiation during condensation by grain/gas separation processes.