Contrasting degrees of recrystallization of carbonaceous material in the Nelson aureole, British Columbia and Ballachulish aureole, Scotland, with implications for thermometry based on Raman spectroscopy of carbonaceous material

Abstract

The degree of recrystallization of carbonaceous material (CM), as monitored by Raman microspectroscopy, was examined as a function of metamorphic grade in two well‐studied contact aureoles containing carbonaceous pelites: the Nelson aureole, British Columbia and the Ballachulish aureole, Scotland. Here, we use (a) the R2 ratio extracted from the Raman spectrum of CM as a proxy for the degree of graphitization (0.0 in perfect graphite then increasing with structural defects) and (b) the second‐order S1 band (~2,700 cm−1) as a marker for the tridimensional ordering of CM. The Nelson aureole (garnet–staurolite–andalusite–sillimanite–K‐feldspar sequence, ~550–650°C, 3.5–4.0 kbar) was developed in rocks that were unmetamorphosed prior to contact metamorphism, whereas the Ballachulish aureole (cordierite–andalusite–K‐feldspar–sillimanite sequence, ~550–700°C, ~3.0 kbar) was developed in rocks that had been metamorphosed to garnet grade conditions (~7 kbar, ~500°C) c. 45 Ma before contact metamorphism. Thirty‐one samples were examined from Nelson and 29 samples from Ballachulish. At Nelson, the R2 ratio steadily decreases from ~0.25 to 0.0 as the igneous contact is approached, whereas at Ballachulish, the R2 ratio remains largely unchanged from regional values (~0.20–0.25) until less than 100 m from the igneous contact. The second‐order S1 band reveals that carbonaceous material (CM) was transformed to highly “ordered” locally tridimensional graphitic carbon at Ballachulish by regional metamorphism prior to contact metamorphism, whereas CM was still a disordered turbostratic (bidimensional) material before contact metamorphism in the case of Nelson. Pretexturation of CM likely induced sluggish recrystallization of CM and delayed graphitization in the Ballachulish aureole. Temperatures of recrystallization of the CM in the two aureoles were estimated using different published calibrations of the thermometry based on Raman Spectroscopy of Carbonaceous Material (RSCM), with differences among the calibrations being minor. In the Nelson aureole, temperatures are in reasonable agreement with those indicated by the metapelitic phase equilibria (all within 50°C, most within 25°C). In the Ballachulish aureole, the retarded crystallization noted above results in increasing underestimates of temperatures compared to the metapelitic phase equilibria (up to ~75°C too low within 200 m of the igneous contact). Our study calls for careful attention when using RSCM thermometry in complexly polymetamorphosed rocks to assess properly the meaning of the calculated temperature.

Pressure dependence of graphitization: implications for rapid recrystallization of carbonaceous material in a subduction zone

“…The difference between turbostratic and amorphous structures prior to pressure-induced graphitization might play an important role in natural graphitization under low-pressure conditions. Such nanostructural differences in the precursor material may lead to variability in natural graphitization in various types of metamorphic terrain (Wada et al 1994;Beyssac et al 2019). However, the effect of reorganization in the nanostructure becomes small with increasing pressure, and activation volumes of two starting materials demonstrate the similar ΔV ≠ values of − 14 ~ − 10 cm 3 mol −1 .…”

Thermal History of the Northern Taiwanese Slate Belt and Implications for Wedge Growth During the Neogene Arc‐Continent Collision

“…Organic matter present in rocks is transformed into graphitic carbonaceous material during metamorphism in a systematic process that can be used as a geothermometer (e.g., Beyssac et al, , ). The graphitization process is irreversible, such that retrograde metamorphic reactions do not affect temperature estimates.…”

“…Organic matter present in rocks is transformed into graphitic carbonaceous material during metamorphism in a systematic process that can be used as a geothermometer (e.g., Beyssac et al, 2002Beyssac et al, , 2019. The…”

Raman spectra of a graphite–nontronite association in marbles from Oltrek Island (Lake Baikal, Russia)

“…An evaluation of the graphitization processes in the selected Oltrek samples can be carried out using the prominent D and G bands of carbon and defining specific Raman parameters that systematically change with the increasing metamorphism. [30][31][32][33][34][35][36][37][38][39] The first-and second-order Raman spectra have been correlated with changes in the structure of graphite; in their first-order Raman spectra, carbonaceous materials (CM) exhibit an intense band around 1,580 cm −1 (G), an additional band around 1,350 cm −1 (D1), and a weak band around 1,620 cm −1 (D2). In the second-order Raman spectra of graphite, the main observed bands are at 2,450 cm −1 , a doublet at 2,695 and 2,735 cm −1 (G′), and bands at 2,945 cm −1 (D1 + G), 3,176 cm −1 (2G), and 3,244 cm −1 (2D2) observed in samples excited with 514.5 nm wavelength.…”

“…Raman spectroscopy of the carbonaceous material has been used to evaluate the metamorphic temperature. [32][33][34][35][36][37][38]43,44] With an increase in regional metamorphism, the G-band in the Raman spectra becomes more intense while the width of this band becomes narrower.…”