Petrographic criteria for fluid mobility of graphitic carbon in terrestrial and extraterrestrial samples

Graphitic carbon is a widespread precipitate in terrestrial and extraterrestrial samples. However it has a range of possible origins, which can be difficult to distinguish, including the in situ alteration of organic matter, thermal alteration of hydrocarbons, and precipitation from C–O–H fluids. Petrographic characteristics help to understand the origin of the graphite, including relationships with rock fabric, paragenetic sequences and evidence for fluid mobility. Characterization of a range of terrestrial samples will allow better interpretation of the petrography of carbon in extraterrestrial samples. In particular, improved petrographic data from carbonaceous chondrites and ureilite meteorites should help to distinguish the origin of carbon in their parent bodies.