Testing for robustness on estimation of graphitization degree by Raman spectroscopy

Abstract

A Raman geothermometer, which utilizes the degree of graphitization of carbonaceous material, has been widely applied to estimate the recrystallization temperature of metapelite. This study evaluates the degree of graphitization of carbonaceous material, which is affected by several factors, and tests the robustness of the Raman carbonaceous material geothermometer defined by the R2 [= D1/(G + D1 + D2) area ratio] value. The main results are as follows. (1) Laser radiation over 6 mW at the sample surface caused a significant decrease in the R2 value, owing to the local increase in surface temperature of the carbonaceous material, and thus gave an overestimation of the recrystallization temperature. On the contrary, laser irradiation of 2 mW showed no distinct alteration of the spectrum during continuous analyses up to 120 s. (2) Carbonaceous materials occurring as matrix and inclusion phases in silicate minerals in a thin section showed no significant difference in R2 value. (3) The average R2 value of 10 samples collected from an outcrop at a scale of 2–3 m was 0.483 ± 0.012, corresponding to a temperature of 416 ± 5 °C. This result implies that an arbitrary sample can likely represent the R2 value of the entire outcrop from which the sample was collected. (4) No distinct alteration of the R2 value around a shear zone width of 1–1.5 m was measured in the metapelites. The degree of graphitization of carbonaceous material was not noticeably altered by deformation during exhumation and local fracturing. Moreover, the Raman analysis of the carbonaceous material under the appropriate laser power condition can estimate the peak metamorphic temperature of rocks regardless of scale from thin section to outcrop.