[SMP37-P07] A comparative study on graphitization process in limestone and pelites at low pressure contact metamorphism
Keywords:Graphitization, Carbonaceous material, Micro-Raman spectroscopy, XRD, HRTEM
We estimated the effective activation energy of about 280 kJ / mol for Tono samples and about 300 kJ / mol for Tanohata samples, based on Nakamura et al. (2017) model. These values are considerably lower than the activation energy of graphitization reported in previous studies. It can be deduced that actual activation energies for natural graphitization in contact metamorphism could be lower than that was estimated here due to the influence of H2O activity and/or catalysis. Samples in the temperature range from 350 to 420 °C have bimodal exothermal peaks and reflections in both DTA-TG and XRD, respectively. This suggests that CMs with different crystallinity are mixed in the sample. Therefore, graphitization at low pressure contact metamorphism is an extremely heterogeneous process. In particular, CM in limestone tends to have higher heterogeneity, and each parameter obtained by micro-Raman spectroscopy also has larger error. CM in limestone has large value of d002 spacing, FWHM, and lattice distortion (εC) in the low temperature side, and the value decreased sharply as temperature increased. When observing such a heterogeneous sample with HRTEM, the CM in the limestone had a unique microstructure in which the amorphous core was covered with a thick graphene sheet. It is thought that this unique microstructure was generated by the reaction between carbonate and CM.
These results suggest that the graphitization of CM in limestone progress rapidly when accelerated by the catalytic effect of coexisting carbonate minerals. Since this process increases error of Raman spectra measurements, it is necessary to take into account the host rock effect in order to accurately estimate metamorphic temperature using RSCM thermometry.
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