Thermal metamorphism of CI and CM carbonaceous chondrites: An internal heating model

Abstract

Abstract— Infrared diffuse reflectance spectra were measured for several thermally metamorphosed carbonaceous chondrites with CI‐CM affinities which were recently found from Antarctica. Compared with other CI or CM carbonaceous chondrites, these Antarctic carbonaceous chondrites show weaker absorption bands near 3 μm due to hydrous minerals, and weaker absorption bands near 6.9 μm due to carbonates, interpreted as thermal metamorphic features. These absorption bands also disappear in the spectra of samples of the Murchison (CM) carbonaceous chondrite heated above 500 °C, implying that the metamorphic temperatures of the Antarctic carbonaceous chondrites considered here were higher than about 500 °C. Model calculations were performed to study thermal metamorphism of carbonaceous chondrites in a parent body internally heated by the decay of the extinct nuclide 26Al. The maximum temperature of the interior of a body more than 20 km in radius is 500–700 °C for the bulk Al contents of CI and CM carbonaceous chondrites, assuming a ratio of 26Al/27Al = 5 × 10−6 which has been previously proposed for an ordinary‐chondrite parent body. The metamorphic temperatures experienced by the Antarctic carbonaceous chondrites considered here may be attainable by an internally heated body with an 26Al/27Al ratio similar to that inferred for an ordinary‐chondrite parent body.