Structural Disorder of Graphite and Implications for Graphite Thermometry

Graphitization, or the transformation of organic matter into crystalline graphite, is induced by compositional and structural changes during diagenesis and metamorphism. The irreversible nature of this process has allowed the degree of graphite crystallinity to be calibrated as an indicator of the peak temperatures reached during progressive metamorphism. However, discrepancies between temperatures indicated by the graphite crystallinity vs. other thermometers have been documented in numerous fault zones, such as the Alpine Fault rocks, New Zealand (Kirilova et al, in review) and the Hidaka metamorphic belt, Japan (Nakamura et al., 2015). We hypothesise this is because the calibrated graphite thermometers disregard the effects of mechanical modifications of the graphite structure. To examine this possibility, we have carried out laboratory deformation experiments, combined with Raman microspectroscopy to investigate the impacts of structural disordering of graphite on the graphite ’thermometry’. Our experiments were performed in the Brittle Rock Deformation Versatile Apparatus (BRAVA) at INGV, Rome. We systematically sheared highly crystalline graphite powder at room temperature, normal stresses of 5 MPa and 25 MP and sliding velocities of 1 m/s, 10 m/s and 100 m/s to total displacements of 20 mm, 10 mm and 5 mm. We then analysed the degree of graphite crystallinity in the resulting material by Raman microspectroscopy. Our results show consistent decrease of graphite crystallinity with increasing shear strain; spectra area ratios (R2) drop from ~0.1 in the initial material to ~0.5 in the deformed powder. We infer this is due to mechanical modification and thus conclude that graphite thermometers are unreliable in brittely deformed rocks. Temperatures derived from the thermometer in deformed rocks should be treated as minimum estimates of temperatures experienced by them.