PORTABLE RAMAN SPECTROSCOPIC ANALYSIS OF GRAPHITE IN NEOPROTEROZOIC METASEDIMENTARY ROCKS FROM CENTRAL VIRGINIA: TESTING A FIELD-BASED APPROACH TO GENERATE GEOTHERMOMETRY DATA

Geothermometry data provide critical information to constrain the geologic history in metamorphic, hydrothermal, and sedimentary rock packages. Traditional geothermometry studies in metamorphic rocks, such as thermodynamic modeling and fluid inclusion microthermometry, require advanced analytical data generated by specialized instruments (e.g., from an electron microprobe, mass spectrometer, or a microscope equipped with a heating/cooling stage) and an in-depth understanding of thermodynamics and phase equilibria. While these workflows are standard for constraining peak metamorphism, they can require time intensive sample preparation, the presence of specific mineral assemblages or mineral textures, and access to specialized laboratory equipment. Alternatively, Raman spectroscopy-based geothermometers, coupled with the current generation of highly portable Raman spectrometers, provide an opportunity to explore a rapid-analysis approach for generating geothermometric data in field settings from graphite-bearing rocks.

Here we present efforts to test such an approach on graphite-bearing Neoproterozoic rocks from the Blue Ridge and Piedmont regions of central Virginia. Samples comprise low-grade metasedimentary rocks containing graphite, either as porphyroblasts or finely disseminated in the matrix. The empirical geothermometer used was newly developed for a 785 nm laser using a scaled total area method. Field data were benchmarked by laboratory measurements and initial results show peak metamorphic temperatures ranged from lower to upper greenschist (~300-450 °C) to lower amphibolite (~500 °C) facies consistent with the regional geologic history of the sample sites. X-ray diffraction data (XRD) were used to quantify the amount of graphite in the rocks. XRD data were also used to assess the minimum amount of graphite required to produce a measurable Raman signal and to check for the presence of minerals that could have interfering peaks in the graphite diagnostic region. Our preliminary results suggest that portable Raman spectroscopy may represent a rapid, robust, and relatively affordable field-based approach for mapping peak metamorphic temperatures in graphite-bearing rocks; work is on-going to further develop this method.