129-6 Cross Comparison Between Raman Spectroscopy of Carbonaceous Material and Quartz Opening Angle Thermometry on the Phyllite-Quartzite Unit in Crete Greece

Abstract:

Quantifying the temperature history of rocks is essential for testing models for various geologic processes and understanding the tectonic history of an area. Two commonly employed methods for determining temperature are Raman Spectroscopy of Carbonaceous Material (RSCM) thermometry and quartz opening-angle thermometry. The RSCM thermometer is based on the Raman spectra of the organic material commonly found in metasedimentary rocks, which is progressively transformed from poorly organized organic material and to highly ordered graphite with increasing temperature. The quartz opening angle thermometer is based on the crystallographic preferred orientations of quartz, which commonly develop a cross-girdled appearance. The angle between the limbs on a pole figure is controlled by the relative activity of slip systems in quartz, particularly the competition between basal <a> and prism [c], and which are dependent on deformation temperature. Although both methods are widely used, there has been little systematic comparison of these two approaches, in part due to the challenge of finding samples suitable for both techniques. Here, we describe new RSCM and quartz opening angle data from samples of the Phyllite-Quartzite unit, a package of metasediments exposed in Crete, Greece. We find RSCM temperatures ranging from ~350-400°C and quartz opening angle temperatures from ~390-520°C. Direct comparisons from individual samples show that opening angles are consistently ~90°C hotter than estimates based on RSCM. Exploratory analyses into samples from other geologic terrains suggest that the temperature offset observed may be typical of the Crete rocks rather than a systematic bias in the two methodologies. One potential explanation for the observed mismatch is the geologic history of the Cretan rocks – which involved rapid subduction, burial, and exhumation –  provided insufficient time for the graphitization process to proceed to equilibrium, producing lower-than-expected temperature estimates. Our ongoing work seeks to better document and interpret these observations.

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