Thermal Metamorphism of Cm Carbonaceous Chondrites: Effects on Phyllosilicate Mineralogy and Presolar Grain Abundances

Introduction: CM carbonaceous chondrites have been constituent materials of hydrated planetesimals in the early solar system. They consist mainly of hydrous minerals as a result of extensive aqueous alteration. At a certain period after the aqueous alteration, many CM chondrites have been heated to higher temperature [e.g., 1], which caused evaporation loss of highly volatile elements [2] and dehydration of hydrous minerals [e.g., 3-5]. However, it remains unknown when the heating took place and what induced the heating. To address this issue, a combined study on mineralogy and noble gas isotopes was carried out on CM chondrites, which were heated to various extents based on the results of previous mineralogical and bulk chemical studies [2-5]. Our noble gas study includes analysis of acid-residues, diamond separates, and SiC+spinel separates prepared by chemical processing of two thermally metamorphosed CM chondrites.