Amphibole-bearing Multiphase Solid Inclusions in Olivine and Plagioclase from a Layered Gabbro : Origin of the Trapped Melts
Amphibole-bearing Multiphase Solid Inclusions in Olivine and Plagioclase from a Layered Gabbro : Origin of the Trapped Melts
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DOI：
10.1093/petrology/egr067
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发表时间：
2012-02
影响因子：
3.9
通讯作者：
T. Hoshide;M. Obata
中科院分区：
文献类型：
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作者：
T. Hoshide;M. Obata
本文研究了日本层状辉长岩室富崎辉长岩中包裹在橄榄石和斜长石中的含角闪石多相固体包裹体（MSI）的岩石学和矿物学特征。利用波长色散（WDS）和能量色散（EDS）技术结合模态分析，得到了MSI的体组成。橄榄石中的微晶硅多呈球形或多角形，主要由寄生角闪石、正辉石和云母（黑云母和云母）组成。侵入体冷缘橄榄石斑晶的微晶硅由斜长石和角闪石组成，而非寄生石。其中许多被发现在其整体成分中富含橄榄石成分，因此它们不能被视为简单的捕获玄武岩熔体或其衍生物。斜长石中的微晶硅形状较为不规则，主要出现在含钙斜长石岩心的被吸收部分；它们主要由寄生石和钠斜长石组成。通过对与主橄榄石的Fe^Mg交换反应进行校正，试图获得MSI熔体的原始组成。校正后发现，冷缘的MSI最接近公布的初始玄武岩熔体成分，而橄榄辉长岩中橄榄石为主的MSI在校正后仍具有相当高的橄榄石成分。由于斜长石基本上不含Mg和Fe，因此斜长石承载的MSI被认为代表了原始被捕获熔体的成分，因此这种捕获后的改性对斜长石承载的MSI可能是最小的。它们的特征是富含斜长石成分。我们认为（修正后）被困熔体的异常成分是由岩浆室结晶边界层下部的水的引入引起的橄榄石和斜长石的局部溶解造成的。因此，MSI为玄武岩岩浆凝固过程中岩浆储层下部结晶边界层含水溶蚀硅酸盐提供了新的证据。
This study presents new data regarding the petrographic and mineralogical characteristics of amphibole-bearing multiphase solid inclusions (MSI) enclosed in olivine and plagioclase from a well-characterized Japanese layered gabbro, the Murotomisaki Gabbroic Intrusion. Bulk compositions of the MSI were obtained using wavelength-dispersive (WDS) and energy-dispersive (EDS) techniques combined with modal analysis. Many MSI in olivines are spherical or polygonal in shape and are mainly composed of pargasitic amphibole, orthopyroxene, and mica (biotite and aspidolite). The MSI in olivine phenocrysts in the chilled margins of the intrusion are composed of plagioclase and hornblende rather than pargasite. Many of them are found to be enriched in olivine components in their bulk compositions, and therefore they cannot be regarded as simple trapped basaltic melts or their derivatives. The MSI in plagioclase are more irregular in shape and occur in the resorbed part of calcic plagioclase cores; they are mainly composed of pargasite and sodic plagioclase. Attempts were made to obtain the original composition of the MSI melts by applying a correction for Fe^Mg exchange reactions with the host olivine. It was found that after the correction, the MSI in the chilled margin were closest to the published initial basaltic melt composition, whereas the olivine-hosted MSI from the olivine gabbros were still fairly high in olivine components despite the correction. Such post-entrapment modification was probably minimal for the plagioclase-hosted MSI because plagioclase is essentially free of Mg and Fe, and therefore plagioclase-hosted MSI are believed to represent the compositions of the original trapped melts. These are characteristically enriched in plagioclase components. We believe that the anomalous compositions of the trapped melts (after the corrections) were generated by local dissolution of olivine and plagioclase caused by the introduction of water from lower horizons of the crystallization boundary layer in a magma chamber. The MSI are therefore considered to provide new evidence for the dissolution of silicates by hydrous fluxing in a lower crystallizing boundary layer of a magma reservoir during solidification of basaltic magma.