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arXiv 1503.03241astro-ph.EP
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普通球粒陨石微量元素地球化学:I型/II型球粒二分法

Trace element geochemistry of ordinary chondrite chondrules: the type I/type II chondrule dichotomy

Emmanuel Jacquet, Olivier Alard, Matthieu Gounelle

摘要:

我们报告微量元素浓度的硅酸盐相的球粒从LL 3普通陨石Bishunpur和Semarkona。结果表明,碳质辉长岩中稀土元素的富集顺序为橄榄石<辉石<中稳岩,重稀土元素富集1-2个数量级(CI标准化)相对于轻稀土元素(LREE)在铁镁硅酸盐,虽然没有发现单一的橄榄石具有非常大的LREE/HREE分馏。II型球粒中橄榄石的难熔亲石不相容元素(如稀土元素)比I型球粒中的橄榄石少约2倍。这表明I型和II型球粒中的橄榄石分别通过分批结晶和分步结晶形成,这意味着II型球粒在比I型球粒更快的冷却速率(> ~ 10 K/h)下形成。在这两种球粒的橄榄石中测得了可观的钠浓度(3-221 ppm); II型球粒似乎表现为封闭系统,这可能需要在原行星或微行星附近形成球粒。无论如何,在II型球粒形成区域中较高的固体浓度可以解释与I型球粒相比,它们记录的较高的氧逸度。I型和II型球粒形成于不同的环境中,高固体浓度和/或氧逸度与快速冷却速率之间的相关性是球粒形成模型必须考虑的关键约束。

英文摘要:

We report trace element concentrations of silicate phases in chondrules from LL3 ordinary chondrites Bishunpur and Semarkona. Results are similar to previously reported data for carbonaceous chondrites, with rare earth element (REE) concentrations increasing in the sequence olivine < pyroxene < mesostasis, and heavy REE (HREE) being enriched by 1-2 orders of magnitude (CI-normalized) relative to light REE (LREE) in ferromagnesian silicates, although no single olivine with very large LREE/HREE fractionation has been found. On average, olivine in type II chondrules is poorer in refractory lithophile incompatible elements (such as REE) than its type I counterpart by a factor of ~2. This suggests that olivine in type I and II chondrules formed by batch and fractional crystallization, respectively, implying that type II chondrules formed under faster cooling rates (> ~ 10 K/h) than type I chondrules. Appreciable Na concentrations (3-221 ppm) are measured in olivine from both chondrule types; type II chondrules seem to have behaved as closed systems, which may require chondrule formation in the vicinity of protoplanets or planetesimals. At any rate, higher solid concentrations in type II chondrule forming regions may explain the higher oxygen fugacities they record compared to type I chondrules. Type I and type II chondrules formed in different environments and the correlation between high solid concentrations and/or oxygen fugacities with rapid cooling rates is a key constraint that chondrule formation models must account for.

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