Comparative mineralogy of magmatic inclusions in olivine from the Chassigny and Nakhla martian meteorites

摘要

Chassigny and Nakhla frequently contain magmatic inclusions in olivine. Some Al-, Ti-rich augites in Chassigny magmatic inclusions show reverse zoning from the Mg-poor core to the Mg-rich rim. This unique chemical zoning in Chassigny magmatic inclusion gives important implications for the crystallization of the magmatic inclusions. The crystallization stage of the Mg-poor core is different from that of the Mg-rich rim because the zoned augite has a sharp boundary between the Mg-rich and Mg-poor regions. The presence of such Al-, Ti-augite with reverse zoning implies that the bulk composition of the magmatic inclusion changed into Mg-rich. Such a compositional change of the magmatic inclusion can be produced by the melting of the surrounding olivine. There is a possibility that the melting of surrounding olivine has occurred during a shock event. In order to examine the possibility of the melting by a shock event, this study compared texture and constituent minerals of Chassigny magmatic inclusions with those of Nakhla magmatic inclusions. Although Chassigny magmatic inclusions have remarkable radial cracks around them, Nakhla magmatic inclusions only have ambiguous radial cracks. The presence of radial cracks in the Chassigny magmatic inclusion shows that the inclusions have been affected by a heavy impact event because cracks around magmatic inclusions formed by the fracturing of a phenocryst induced by rapid compression and decompression.