Thermal alteration of CM carbonaceous chondrites: Mineralogical changes and metamorphic temperatures
Thermal alteration of CM carbonaceous chondrites: Mineralogical changes and metamorphic temperatures
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DOI：
10.1016/j.gca.2021.02.011
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发表时间：
2021-02-26
影响因子：
5
通讯作者：
Russell, S. S.
中科院分区：
文献类型：
--
作者：
King, A. J.;Schofield, P. F.;Russell, S. S.
CM碳质球粒陨石提供了低温（750摄氏度）的记录。热变质作用的确切条件以及未加热和加热CM陨石之间的关系并没有很好的约束，但对于理解含水小行星的形成和演化至关重要。本文利用位置灵敏探测器X射线衍射（PSD-XRD）、热重分析（TGA）和透射红外（IR）光谱分析了14个加热的CM和未分组的碳质方解石的矿物学和水含量。我们发现，加热CM方解石进行了相同程度的水蚀变作为未加热的CM，然而，在热变质作用，其矿物学最初（300-500摄氏度）从水合层状硅酸盐变成脱水的无定形层状硅酸盐相。在更高的温度下（>500摄氏度），我们观察到橄榄石和铁硫化物的再结晶以及金属的形成。热变质作用还导致加热CM方解石的水含量从类似于13重量%减少到类似于3重量%，并随后减少在红外光谱中的3 μ m特征的强度。我们估计，加热的CM方解石在水蚀变结束时损失了约15 - >65%的水。如果撞击是变质作用的主要原因，这与类似于20-50 GPa的冲击压力是一致的。然而，并不是所有加热的CM陨石都保留了激波特征，这表明有些陨石是被太阳辐射加热的。来自Hayabusa 2和ORSIRS-REx任务的证据表明，脱水物质在原始小行星的表面可能很常见，我们的研究结果将支持即将对小行星Ryugu和Bennu返回的样本进行分析。(C)2021爱思唯尔有限公司保留所有权利。
The CM carbonaceous chondrite meteorites provide a record of low temperature (750 degrees C. The exact conditions of thermal metamorphism and the relationship between the unheated and heated CM chondrites are not well constrained but are crucial to understanding the formation and evolution of hydrous asteroids. Here we have used position-sensitive-detector X-ray diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission infrared (IR) spectroscopy to characterise the mineralogy and water contents of 14 heated CM and ungrouped carbonaceous chondrites. We show that heated CM chondrites underwent the same degree of aqueous alteration as the unheated CMs, however upon thermal metamorphism their mineralogy initially (300-500 degrees C) changed from hydrated phyllosilicates to a dehydrated amorphous phyllosilicate phase. At higher temperatures (>500 degrees C) we observe recrystallisation of olivine and Fe-sulphides and the formation of metal. Thermal metamorphism also caused the water contents of heated CM chondrites to decrease from similar to 13 wt% to similar to 3 wt% and a subsequent reduction in the intensity of the 3 mu m feature in IR spectra. We estimate that the heated CM chondrites have lost similar to 15 - >65% of the water they contained at the end of aqueous alteration. If impacts were the main cause of metamorphism, this is consistent with shock pressures of similar to 20-50 GPa. However, not all heated CM chondrites retain shock features suggesting that some were instead heated by solar radiation. Evidence from the Hayabusa2 and ORSIRS-REx missions suggest that dehydrated materials may be common on the surfaces of primitive asteroids and our results will support upcoming analysis of samples returned from asteroids Ryugu and Bennu. (C) 2021 Elsevier Ltd. All rights reserved.