Carbon is an essential element on the Earth’s surface. It is involved in the formation of certain minerals (carbonates) as well as biomolecules. Metamorphic rocks also contain carbonaceous materials (CM) with various possible origins. Solid CM (partially or completely graphitized CM) play a major role in the petrological and geochemical evolution of a subducted rock. If our knowledge of metamorphic CM increased over the last years based on naturalist, experimental or theoretical studies, many issues remain as to the source of CM in metamorphic rocks for example.The role of mineralization as a factor of preservation of CM of biological origin (biogenic) is still poorly understood. We studied natural samples (Marybank formation, New Zealand) metamorphozed in the blueschist facies and which contain carbonaceous plant fossils that display a remarkable morphological preservation at the microscopic scale. We investigate mineralogical and chemical processes that contributed to preserve, or obliterate, information carried by the original biomaterial, and, more generally, by the fossil. To do so, we have characterized the fossils and the minerals using analytical technics with high special resolution. We show that the remarkable morphological preservation is accompanied by the advanced recristallization of the mineralogy compositing the fossils. We also show the presence of spectacular TiO2 mineral nanocristals in the CM composing the fossils and we discuss about possible mechanisms leading to the formation of these exceptional mineralizations.Complex abiotic processes, intimately linked to fluid-rock interactions, allow the formation of graphitic CM in metamorphic rocks. These processes hinder the study and interpretation of CM in rocks. Nevertheless, they also reveal the major role of fluids and mineral assemblages in the metamorphic dynamics of carbon. We carried a detailed study of a contact between serpentinites and metasediments (Malaspina, Alpine Corsica) that display complete carbonate destabilization. Because of the reduced conditions imposed by the underlying serpentinite, the inorganic carbon released has precipitated and formed graphite. We use geochemical, mineralogical and petrological complementary tools that allow to distinguish different categories of CM in these rocks, and we propose a well constrained scenario for the formation for this abiotic graphite. This study allows discussing the role of redox gradients on the dynamics of carbon in a metamorphic rock.These are all examples stressing the important, and yet poorly explored, petrological role of CM in metamorphic rocks.
