Abstract EN:

Metasediments generally contain carbonaceous material (CM) deriving from the evolution of organic matter originally present in the host rock. During metamorphism, organic matter is progressively transformed into graphite. The study of graphitization along the Schistes Lustrés unit (western Alps) by transmission electron microscopy and Raman spectroscopy shows that (1) the mechanism of graphitization depends on the type of precursor, (2) pressure promotes graphitization by inducing microtextural and subsequent structural evolution of CM and (3) the degree of organization of CM is very sensitive to the metamorphic grade. More generally, by characterizing the degree of organization of CM from several metamorphic belts with well constrained P-T conditions, we calibrate a new geothermometer which is an indicator of the peak metamorphic conditions. A first application of this geothermometer to the lesser himalayan series (Nepal) demonstrates its applicability and shows the presence of strong thermal gradients through these units. To decipher the role of each parameter during graphitization, we have performed a series of experiments using synthetic cokes and a coal as starting material. In these experiments, the respective roles of temperature (up to 1473 K), hydrostatic pressure (up to 8 GPa), time, fluid and type of precursor are discussed. The experimental products are strongly structurally and microtexturally heterogeneous, but the mechanisms of graphitization in these experiments are very similar to those observed in natural samples. Non graphitizing CM under a pure thermal effect are transformed into graphite under pressure, because of the microtextural modifications induced by pressure. The comparison between natural and experimental graphitization shows that during the graphitization under pressure and if the duration is long enough, CM is progressively transformed into a metastable intermediate stage, the evolution of the latter being controlled by temperature.