Articles | Volume 9, issue 1
Research article
27 Feb 2018
Research article |  | 27 Feb 2018

Structural disorder of graphite and implications for graphite thermometry

Martina Kirilova, Virginia Toy, Jeremy S. Rooney, Carolina Giorgetti, Keith C. Gordon, Cristiano Collettini, and Toru Takeshita

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.

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.