Pyrrhotite and the remanent magnetization of SNC meteorites: a changing perspective on Martian magnetism

A combined magnetic and mineralogical study of 10 Shergotty¯Nakhla¯Chassigny type (SNC) meteorites, including six recent desert finds, provides strong evidence that, like in ALH84001, the natural remanence (NRM) of basaltic shergottites (apart from Los Angeles, LA) is carried by pyrrhotite. Low Curie point titanomagnetite is the major magnetic carrier only in nakhlites, Chassigny and LA. LA appears to be by far the most magnetic SNC. The dominant role of pyrrhotite has major implications on the interpretation of Martian magnetic anomalies and SNC paleomagnetism: in particular pyrrhotite undergoes magnetic and/or phase transition at room temperature in a pressure range of 1.6¯3.6 GPa, much lower than the peak shock pressure experienced by shergottites. Any impact-related pressure above this value should thus totally reset remanent magnetism. It is also suggested that the very low thermal stability of remanence in some SNCs may be due to hexagonal pyrrhotite in metastable ferrimagnetic state or shock-induced substructures in monoclinic pyrrhotite.