  The CM carbonaceous chondrite meteorites provide a record of low temperature
aqueous reactions in the early solar system. A number of CM chondrites also
experienced short-lived, post-hydration thermal metamorphism at temperatures of
200C to over 750C. The exact conditions of thermal metamorphism and the
relationship between the unheated and heated CM chondrites are not well
constrained but are crucial to understanding the formation and evolution of
hydrous asteroids. Here we have used position-sensitive-detector X-ray
diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission
infrared (IR) spectroscopy to characterise the mineralogy and water contents of
14 heated CM and ungrouped carbonaceous chondrites. We show that heated CM
chondrites underwent the same degree of aqueous alteration as the unheated CMs,
however upon thermal metamorphism their mineralogy initially (300 to 500C)
changed from hydrated phyllosilicates to a dehydrated amorphous phyllosilicate
phase. At higher temperatures (over 500C) we observe recrystallisation of
olivine and Fe-sulphides and the formation of metal. Thermal metamorphism also
caused the water contents of heated CM chondrites to decrease from 13 wt
percent to 3 wt percent and a subsequent reduction in the intensity of the 3
micron feature in IR spectra. We estimate that the heated CM chondrites have
lost 15 to 65 percent of the water they contained at the end of aqueous
alteration. If impacts were the main cause of metamorphism, this is consistent
with shock pressures of 20 to 50 GPa. However, not all heated CM chondrites
retain shock features suggesting that some were instead heated by solar
radiation. Evidence from the Hayabusa2 and ORSIRS-REx missions suggest that
dehydrated materials may be common on the surfaces of primitive asteroids and
our results will support upcoming analysis of samples returned from asteroids
Ryugu and Bennu.
