Magnetic insights into the aqueous alteration of CM chondrites

Abstract

CM chondrites are the archetypal group of aqueously altered meteorites, preserving records of water-rock interactions on planetesimals. Here, we couple rock magnetic properties with modal mineralogy measured from 48 CM chondrites to explore aqueous alteration through its influence on the magnetic mineralogy of these meteorites. These magnetic properties reflect a complex mixture of Fe-Ni metal, pyrrhotite, and magnetite in varying proportions and grain sizes. As the degree of alteration increases, the relative abundance of metal and pyrrhotite decreases with respect to magnetite, which forms predominantly as very fine grains (tens to hundreds of nanometers)—smaller, on average, than in CI1 chondrites. However, there is no trend between degree of alteration and magnetic properties, arguing that aqueous alteration may not be the only process that influenced the magnetic mineralogy of CM chondrites. Our measurements also indicate that transient heating events may have induced detectable changes in this mineralogy. We find hints of these changes throughout our datasets, possibly explaining the absence of simple trends with extent of aqueous alteration and arguing that weak/short-duration heating may be widespread among CM chondrites. If so, the magnetic mineralogy of many CM chondrites reflects a subtle interplay between aqueous alteration and weak transient heating.