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Solid Earth An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 1
Solid Earth, 9, 223–231, 2018
https://doi.org/10.5194/se-9-223-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Solid Earth, 9, 223–231, 2018
https://doi.org/10.5194/se-9-223-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 27 Feb 2018

Research article | 27 Feb 2018

Structural disorder of graphite and implications for graphite thermometry

Martina Kirilova1, Virginia Toy1, Jeremy S. Rooney2, Carolina Giorgetti3, Keith C. Gordon2, Cristiano Collettini3, and Toru Takeshita4 Martina Kirilova et al.
  • 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.

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Short summary
Graphite crystallinity “irreversibly” increases with temperature and it has been calibrated as a thermometer recording peak temperatures experienced by a rock. To examine the possibility of mechanical modifications of graphite structure and the impacts on graphite thermometry we performed deformation experiments. Raman spectroscopy demonstrates a reduction in crystallinity due to mechanical reworking in the brittle field. This finding clearly compromises the validity of the graphite thermometry.
Graphite crystallinity “irreversibly” increases with temperature and it has been calibrated...
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