Poorly crystalline carbonaceous matter in high grade metasediments: implications for graphitisation and metamorphic fluid compositions

Abstract

Poorly crystalline carbonaceous matter was observed in chlorite to sillimanite grade metasediments from the Trois Seigneurs Massif, in contrast to other studies of carbon crystallinity which observed well crystallised graphite under upper greenschist facies conditions. Using transmission electron microscopy four types of carbon particle were identified; globular carbon, composite flakes, homogeneous flakes and crystalline graphite. Globular carbon and composite flakes are poorly crystalline microporous carbon. Homogeneous flakes decompose in the electron beam and are probably composed of heavy volatile hydrocarbons. Graphite is confined to samples from retrograde shear zones and often occurs with globular carbon. The lack of graphitisation in metasediments is probably a consequence of the microporous structure of the carbonaceous matter combined with low fO2. The preservation of carbonaceous matter in the Trois Seigneurs metasediments is not compatible with the metasediments having been externally buffered by a high XH2O fluid syn-metamorphism. An alternative hypothesis of internal buffering is preferred to explain the carbonaceous matter in the Trois Seigneurs metasediments.