Infrared diffuse reflectance spectra of several thermallymetamorphosed carbonaceous chondrites

Infrared diffuse reflectance spectra (2.53-25μm) were measured for several Antarctic carbonaceous chondrites. Thermally metamorphosed carbonaceous chondrites with CI-CM affinities (Belgica (B)-7904,Yamato (Y)-793321,Y-82162,and Y-86720), which were recently found in the Antarctic collection, show weaker absorption bands near 3μm due to hydrous minerals, compared with the Murchison (CM) carbonaceous chondrite. These results show that hydrous minerals are dehydrated as a result of thermal metamorphism, consistent with the results of chemical, mineralogical, and petrologic studies. B-7904,Y-793321,and Y-86720 show weaker absorption bands near 6.9μm due to carbonates than those of Murchison. For Y-82162,the 1450cm^ (6.9μm) band is relatively strong unlike the other thermally metamorphosed carbonaceous chondrites despite the weak 3μm band of all the thermally metamorphosed ones measured. The shape of the spectral curves near 3μm of the thermally metamorphosed carbonaceous chondrites resembles those of Antarctic ordinary chondrites affected by terrestrial weathering, implying that major absorption features near 3μm of the thermally metamorphosed carbonaceous chondrites may be due to secondary hydrous minerals produced by terrestrial weathering