Structural disorder of graphite and implications for graphite thermometry
- 1Department of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand
- 2Department of Chemistry, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand
- 3Dipartimento di Scienze della Terra, Università degli Studi La Sapienza, Rome, Italy
- 4Faculty of Science, Earth and Planetary Sciences, Hokkaido University, Sapporo, Japan
Abstract. Graphitization, or the progressive maturation of carbonaceous material, is considered an irreversible process. Thus, the degree of graphite crystallinity, or its structural order, 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 megapascal (MPa) and aseismic velocities of 1, 10 and 100 µm s−1. The degree of structural order both in the starting and resulting materials was analyzed by Raman microspectroscopy. Our results demonstrate structural disorder of graphite, manifested as changes in the Raman spectra. Microstructural observations show that brittle processes caused the documented mechanical modifications of the aggregate graphite crystallinity. We conclude that the calibrated graphite
thermometer is ambiguous in active tectonic settings.