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Solid Earth An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/se-2017-74
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
20 Jul 2017
Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Solid Earth (SE).
Structural Disorder of Graphite and Implications for Graphite Thermometry
Martina Kirilova1, Virginia Toy1, Jeremy S. Rooney2, Carolina Giorgetti3, Keith C. Gordon2, and Cristiano Collettini3 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
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.

Citation: Kirilova, M., Toy, V., Rooney, J. S., Giorgetti, C., Gordon, K. C., and Collettini, C.: Structural Disorder of Graphite and Implications for Graphite Thermometry, Solid Earth Discuss., https://doi.org/10.5194/se-2017-74, in review, 2017.
Martina Kirilova et al.
Martina Kirilova et al.
Martina Kirilova et al.

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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 with increasing shear strain, which we attribute to mechanical reworking in the brittle field.
Graphite crystallinity irreversibly increases with temperature and it has been calibrated as a...
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