Metamorphic grade of organic matter in six unequilibrated ordinary chondrites
Metamorphic grade of organic matter in six unequilibrated ordinary chondrites
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DOI：
10.1111/j.1945-5100.2003.tb00043.x
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发表时间：
2003-05
影响因子：
2.2
通讯作者：
E. Quirico;P. Raynal;M. Bourot-Denise
中科院分区：
文献类型：
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作者：
E. Quirico;P. Raynal;M. Bourot-Denise
摘要-在一级碳带区域用拉曼光谱研究了6种非平衡普通碳酸盐（UOC）（Semarkona [LL 3.0]、Bishunpur [L/LL 3.1]、Krymka [LL 3.1]、Chainpur [LL 3.4]、Inman [L/LL 3.4]和Tieschitz [H/L 3.6]）中捕获的有机质（OM）的热变质等级。碳质球粒陨石Renazzo（CR2）也被研究并用作比较的参考对象，因为以前的研究指出该陨石中的OM是所有球粒陨石中最原始的。结果表明：OM热变质等级遵循Renazzo << Semarkona <<其他UOC的等级; 2）与研究对象的岩石学类型有很好的相关性; 3）与同位素富集度δ 15 N也有很好的相关性。这些结果与早期的宇宙化学研究，特别是亚历山大等人（1998年）提出的设想惊人地一致。母岩中的热变质作用是UOC中OM的主要演化过程，说明星云加热作用非常微弱，OM埋藏导致了高δD和δ 15 N的初始同位素组成的不稳定。此外，Renazzo，Semarkona和其他UOC之间的明确区别表明：1）Semarkona是一个非常独特的UOC-到目前为止最原始的; 2）拉曼光谱是一个有效的和有价值的工具，用于推导岩相子类型（特别是低的），应该在未来使用，以补充目前的技术。我们将我们的结果与其他现有技术进行比较，即诱导热释光和不透明岩相学。还取得了其他成果。首先，腐殖煤不是陨石有机质的严格有效的标准物质，但有助于研究热变质作用的演化趋势。其次，陆地风化作用对OM结构有巨大影响，特别是在Inman，这是一个发现。最后，早期关于Semarkona的细粒球粒边缘和基质可能是富含蒙皂石的IDP的来源的说法是无效的，因为它们的OM的结构有序程度不同。
Abstract— The thermal metamorphism grade of organic matter (OM) trapped in 6 unequilibrated ordinary chondrites (UOCs) (Semarkona [LL 3.0], Bishunpur [L/LL 3.1], Krymka [LL 3.1], Chainpur [LL 3.4], Inman [L/LL 3.4], and Tieschitz [H/L 3.6]) has been investigated with Raman spectroscopy in the region of the first‐order carbon bands. The carbonaceous chondrite Renazzo (CR2) was also investigated and used as a reference object for comparison, owing to the fact that previous studies pointed to the OM in this meteorite as being the most pristine among all chondrites. The results show that the OM thermal metamorphic grade: 1) follows the hierarchy Renazzo << Semarkona << other UOCs; 2) is well correlated to the petrographic type of the studied objects; and 3) is also well correlated with the isotopic enrichment δ15N. These results are strikingly consistent with earlier cosmochemical studies, in particular, the scenario proposed by Alexander et al. (1998). Thermal metamorphism in the parent body appears as the main evolution process of OM in UOCs, demonstrating that nebular heating was extremely weak and that OM burial results in the destabilization of an initial isotopic composition with high δD and δ15N. Furthermore, the clear discrimination between Renazzo, Semarkona, and other UOCs shows: 1) Semarkona is a very peculiar UOC—by far the most pristine; and 2) Raman spectroscopy is a valid and valuable tool for deriving petrographic sub‐types (especially the low ones) that should be used in the future to complement current techniques. We compare our results with other current techniques, namely, induced thermo‐luminescence and opaques petrography. Other results have been obtained. First, humic coals are not strictly valid standard materials for meteoritic OM but are helpful in the study of evolutionary trends due to thermal metamorphism. Second, terrestrial weathering has a huge effect on OM structure, particularly in Inman, which is a find. Finally, the earlier statement that fine‐grained chondrule rims and matrix in Semarkona could be the source of smectite‐rich IDPs is not valid, given the different degree of structural order of their OM.