Formation of spinel-, hibonite-rich inclusions found in CM2 carbonaceous chondrites
Formation of spinel-, hibonite-rich inclusions found in CM2 carbonaceous chondrites
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在 CM2 碳质球粒陨石中发现富含尖晶石、黑铝石的包裹体的形成
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发表时间：
2005
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通讯作者：
S. Fallon
中科院分区：
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作者：
S. Simon;L. Grossman;I. Hutcheon;D. Phinney;P. Weber;S. Fallon
摘要本文报道了Murchison （CM2）碳质球粒陨石中40个富尖晶石包裹体的岩石学、矿物化学、体化学和体同位素组成。根据矿物组合鉴定出7种类型的包裹体：尖晶石-hibonite-钙钛矿；spinelperovskite -辉石;spinel-perovskite-melilite;spinel-hibonite-perovskite-melilite;spinel-hibonite;spinel-pyroxene;和spinel-melilite-anorthite。与不含hibonite的包裹体相比，含hibonite的包裹体具有贫钛尖晶石，且尖晶石-hibonite-钙钛矿包裹体的TiO2平均体积含量最高（7.8%）。包裹体的CaO/Al2O3体积比在0.005 ~ 0.21之间，远低于太阳值0.79。富含希波尼特、尖晶石的包裹体由凝聚计算无法预测共存的相组成；在平衡层序中，hibonite之后是melilite，然后是尖晶石。因此，硅辉石-千英石或千英石-尖晶石包裹体应占主导地位。一种解释是。失踪的黄长石。是它如预期的那样凝结了，但由于在加热和熔炼球粒前体时Mg和Ca的蒸发而丢失了。如果这一理论是正确的，假设瑞利分馏伴随着蒸发作用，那么缺镁铝石的球粒将具有同位素重的Mg和Ca。除1例FMg = 4.3±2.6外。/amu和同位素轻Ca （FCa = -3.4±2.0）。/amu)，但我们分析的所有包裹体的同位素组成都在其2σ不确定度范围内。因此，我们没有发现明显的质量依赖分馏的证据。非瑞利蒸发的必要条件是不可能的，如果不是不现实的，我们首选的解释是，在含hibonite和尖晶石的包裹体中普遍缺乏千英石，这是相对于尖晶石，千英石凝结的动力学抑制。由于hibonite和尖晶石的晶体结构相似，尖晶石应该比千晶石更容易由hibonite形成。
Abstract We report petrography, mineral chemistry, bulk chemistry, and bulk isotopic compositions of a suite of 40 spinel-rich inclusions from the Murchison (CM2) carbonaceous chondrite. Seven types of inclusions have been identified based on mineral assemblage: spinel-hibonite-perovskite; spinelperovskite- pyroxene; spinel-perovskite-melilite; spinel-hibonite-perovskite-melilite; spinel-hibonite; spinel-pyroxene; and spinel-melilite-anorthite. Hibonite-bearing inclusions have Ti-poor spinel compared to the hibonite-free ones, and spinel-hibonite-perovskite inclusions have the highest average bulk TiO2 contents (7.8 wt%). The bulk CaO/Al2O3 ratios of the inclusions range from 0.005 to 0.21, well below the solar value of 0.79. Hibonite-, spinel-rich inclusions consist of phases that are not predicted by condensation calculations to coexist; in the equilibrium sequence, hibonite is followed by melilite, which is followed by spinel. Therefore, hibonite-melilite or melilite-spinel inclusions should be dominant instead. One explanation for the .missing melilite. is that it condensed as expected, but was lost due to evaporation of Mg and Ca during heating and melting of spherule precursors. If this theory were correct, melilite-poor spherules would have isotopically heavy Mg and Ca, assuming Rayleigh fractionation accompanied evaporation. Except for one inclusion with FMg = 4.3 ± 2.6./amu and another with isotopically light Ca (FCa = -3.4 ± 2.0./amu), however, all the inclusions we analyzed have normal isotopic compositions within their 2σ uncertainties. Thus, we found no evidence for significant mass-dependent fractionation. Conditions necessary for non-Rayleigh evaporation are unlikely if not unrealistic, and our preferred explanation for the general lack of melilite among hibonite-, spinel-bearing inclusions is kinetic inhibition of melilite condensation relative to spinel. Because of similarities between the crystal structures of hibonite and spinel, it should be easier for spinel than for melilite to form from hibonite.