Structural Disorder of Graphite and Implications for Graphite Thermometry

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Abstract

Abstract. Graphitization, or the progressive maturation of carbonaceous material, is considered an irreversible process. Thus, the degree of graphite crystallinity has been calibrated as an indicator of the peak metamorphic temperatures experienced by the host rocks. However, discrepancies between temperatures indicated by graphite crystallinity versus other thermometers have been documented in deformed rocks. To examine the possibility of mechanical modifications of graphite structure and the potential impacts on graphite thermometry we performed laboratory deformation experiments. We sheared highly crystalline graphite powder at normal stresses of 5 and 25 MPa and aseismic slow sliding velocities of 1 µm/s, 10 µm/s and 100 µm/s. The degree of graphite crystallinity both in the starting and resulting materials was analyzed by Raman microspectroscopy. Our results demonstrate consistent decrease of graphite crystallinity with increasing shear strain. We conclude that the calibrated graphite thermometer is ambiguous in active tectonic settings and we suggest that a calibration that accounts for shear strain is needed.

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