2005 Volume 100 Issue 6 Pages 260-272
The presence of abundant phyllosilicates in many carbonaceous chondrites indicates a prevailing activity of low-temperature aqueous alteration in primitive asteroids. However, among the hydrous carbonaceous chondrites known, more than 20 samples show evidence of having been heated at elevated temperatures with corresponding phyllosilicate dehydration. The mineralogical features of dehydration suggest that the heating occurred in situ in meteorites, which demonstrates that there are some hydrated asteroids that have been heated at a certain period after aqueous alteration. Recent studies have uncovered details of heating and dehydration processes in hydrous carbonaceous chondrites: step-by-step changes in mineralogy, trace element chemistry, carbonaceous materials, and reflectance spectra have been clarified. Based on data from synchrotron X-ray diffraction analysis of the matrix, heated hydrous carbonaceous chondrites have been classified as Stages I-IV, with the temperature of heating increasing from I to IV. In spite of recent progress, heat sources are poorly defined, mainly due to a lack of chronological information on the timing of the heating, and therefore more data are needed to fully clarify the thermal metamorphism of hydrous carbonaceous chondrites.