A new approach to develop the <scp>R</scp>aman carbonaceous material geothermometer for low‐grade metamorphism using peak width

Abstract

Abstract The R aman spectra of carbonaceous material ( CM ) from 19 metasediment samples collected from six widely separated areas of S outhwest J apan and metamorphosed at temperatures from 165 to 655°C show systematic changes with metamorphic temperature that can be classified into four types: low‐grade CM ( c. 150–280° C ), medium‐grade CM ( c. 280–400°C), high‐grade CM ( c. 400–650°C), and well‐crystallized graphite (&gt; c. 650°C). The Raman spectra of low‐grade CM exhibit features typical of amorphous carbon, in which several disordered bands (D‐band) appear in the first‐order region. In the R aman spectra of medium‐grade CM , the graphite band ( G ‐band) can be recognized and several abrupt changes occur in the trends for several band parameters. The observed changes indicate that CM starts to transform from amorphous carbon to crystallized graphite at around 280°C, and this transformation continues until 400°C. The G ‐band becomes the most prominent peak at high‐grade CM suggesting that the CM structure is close to that of well‐crystallized graphite. In the highest temperature sample of 655°C, the R aman spectra of CM show a strong G ‐band with almost no recognizable D ‐band, implying the CM grain is well‐crystallized graphite. In the R aman spectra of low‐ to medium‐grade CM , comparisons of several band parameters with the known metamorphic temperature show inverse correlations between metamorphic temperature and the full width at half maximum ( FWHM ) of the D 1‐ and D 2‐bands. These correlations are calibrated as new R aman CM geothermometers, applicable in the range of c. 150–400°C. Details of the methodology for peak decomposition of R aman spectra from the low to medium temperature range are also discussed with the aim of establishing a robust and user‐friendly geothermometer.