Relationships between type I and type II chondrules: Implications on chondrule formation processes
Relationships between type I and type II chondrules: Implications on chondrule formation processes
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I 型和 II 型球粒之间的关系：对球粒形成过程的影响
DOI：
10.1016/j.gca.2015.03.033
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发表时间：
2015
影响因子：
5
通讯作者：
C. Soulié
中科院分区：
文献类型：
--
作者：
J. Villeneuve;G. Libourel;C. Soulié
在不平衡的球粒陨石中，球粒中的铁镁硅酸盐表现出很宽的mg# = Mg/（Mg + Fe）范围，允许将斑状球粒细分为I型（mg# > 0.9）或II型（mg# < 0.9）。虽然这两种类型的球粒都是在相对于典型太阳星云的氧化条件下形成的，但一般推断II型球粒形成的氧化条件比I型更强。为了检查这种氧化还原差异是否是在球粒形成期间建立的，或者反映了它们的前体的差异，我们进行了一组实验，旨在在氧化条件下加热富含橄榄石（A）的I型球粒代用品，即镁橄榄石+ Fe金属+ Ca-Mg-Si-Al玻璃混合物。我们表明，高温（等温）氧化IA型组合是一个非常有效和快速的过程（例如，几十分钟），形成类似于IIA型球粒的纹理。由于Fe金属气泡的快速溶解，熔体中FeO的增加以及镁橄榄石的溶解使得熔体达到铁橄榄石饱和。铁橄榄石的结晶发生在中稳态中的新晶体或剩余的未吸收的镁橄榄石颗粒（残体）上的过度生长。中断这一过程在任何时候完成之前，通过快速冷却允许复制的纹理和化学多样性的整个范围内观察到的A型球粒，即从I型到II型。球粒形成过程中的几个影响可以推断从所提出的实验。I型球粒或I型球粒的碎片很可能是参与大多数II型球粒形成的主要前体物质。斑状橄榄石II型球粒的形成很可能是高温下化学不平衡产生晶体生长的过程的结果，而不是仅通过冷却速率产生结晶的过程。这质疑球粒热历史的可靠性（如冷却速率值）迄今推断生产斑状纹理仅从动态冷却速率实验。A型球粒的形成是一个非常快速的过程。在1500-1800 °C下，经过短至几十分钟、不超过几百分钟的亚等温加热或缓慢冷却（<50 K/h）后，A型球粒通过快速冷却（>103- 104 K/h）终止其形成，以保持其玻璃态中稳态。这种推断的热历史与星云激波模型不一致，因此我们主张对星子的影响导致快速熔化和蒸发，可能提供形成球粒所需的高密度和高度挥发性富集的气体环境。在这种情况下，陨石球粒及其多样性应该是由喷射出的碎片与撞击蒸气羽流的不同程度的相互作用造成的; IIA型陨石球粒中记录的最具氧化性的条件很可能最接近撞击蒸气羽流造成的条件。
In unequilibrated chondrites, the ferromagnesian silicates in chondrules exhibit wide ranges of mg# = Mg/(Mg + Fe), allowing to sub-divide porphyritic chondrules into either type I (mg# > 0.9) or type II (mg# < 0.9). Although both chondrule types formed under oxidizing conditions relative to the canonical solar nebula, it is generally inferred that type II chondrules formed in more oxidizing conditions than type I. In order to check whether this redox difference was established during chondrule formation, or reflects differences in their precursors, we have undertaken a set of experiments aimed at heating type I olivine-rich (A) chondrule proxy, i.e. forsterite + Fe metal + Ca–Mg–Si–Al glass mixtures, under oxidizing conditions. We show that high temperature (isothermal) oxidation of type IA-like assemblages is a very efficient and rapid process (e.g. few tens of minutes) to form textures similar to type IIA chondrules. Due to the rapid dissolution of Fe metal blebs, a FeO increase in the melt and in combination with the dissolution of magnesian olivine allows the melt to reach ferroan olivine saturation. Crystallization of ferroan olivine occurs either as new crystal in the mesostasis or as overgrowths on the remaining unresorbed forsterite grains (relicts). Interruption of this process at any time before its completion by rapid cooling allows to reproduce the whole range of textures and chemical diversity observed in type A chondrules, i.e. from type I to type II.Several implications on chondrule formation processes can be inferred from the presented experiments. Type I chondrules or fragments of type I chondrules are very likely the main precursor material involved in the formation of most type II chondrules. Formation of porphyritic olivine type II chondrules is very likely the result of processes generating crystal growth by chemical disequilibrium at high temperature rather than processes generating crystallization only by cooling rates. This questions the reliability of chondrule thermal history (e.g. cooling rate values) hitherto inferred for producing porphyritic textures from dynamical cooling rate experiments only. Type A chondrule formation can be a very fast process. After periods of sub-isothermal heating or slow cooling (<50 K/h) as short as several tens of minutes and no longer than few hundreds of minutes at 1500–1800 °C, type A chondrules terminates their formation by a fast cooling (>103–104K/h) in order to preserve their glassy mesostasis. Such inferred thermal history being at odds with nebular shock models, we thus advocate that impacts on planetesimals causing rapid melting and vaporization may provide the high density and highly volatile-enriched gaseous environments required to form chondrules. In this scenario, chondrules and their diversity should result from various degrees of interaction of the ejected fragments with the impact vapor plume; the most oxidizing conditions recorded in type IIA chondrules being very likely the closest to those imposed by the impact vapor plume.