RAMAN SPECTROSCOPY OF CARBONACEOUS MATERIAL AS A GEOTHERMOMETER IN LOW-GRADE METASEDIMENTS

Raman spectroscopy of carbonaceous material (CM) is used to assess peak temperatures in low to mid-grade metasediments. Sedimentary rocks generally contain CM derived from organic material originally present within the sediment. Previous work has demonstrated that with increasing metamorphic temperature, the poorly ordered CM is progressively transformed into graphite. This process is largely independent of pressure and therefore the degree of graphitization holds potential as a geothermometer (Beyssac et al., 2002, J. Metamorphic Geol., v. 20, p. 859-871). The degree of graphitization in a sample may be characterized by Raman spectroscopy. Beyssac et al. (2002) showed that the relative area beneath the disordered CM peak is sensitive to temperature and they used natural samples to correlate this parameter to peak metamorphic temperature over the interval of 330 to 600°C. However, since all sediments generally contain some well-ordered CM, estimates from low-grade samples will be at least 300°C even if the actual peak temperature was significantly lower. To extend the applicability of the graphite thermometer into lower-grade conditions, a series of sub-greenschist facies samples from the Olympic Mountains in Washington State have been analyzed. Independent temperature estimates for these samples are derived from published fission-track and vitrinite reflectance data. Other aspects of the Raman spectra are explored for temperatures below 300°C to identify potential tools for geothermometry. For example, it is shown that the width of the disordered peak is sensitive to temperature variation at low-grade conditions.

The graphite thermometer is applied to a suite of samples from the Miocene Phyllite-Quartzite Unit exposed in Crete, Greece. Peak metamorphic temperatures are estimated to range between 300 and 400°C. These data are consistent with previous estimates for peak metamorphic temperatures in the area, confirming the utility of Raman analysis of CM for geothermometry.