RAMAN SPECTROSCOPY OF GRAPHITE IN MARBLE: EVIDENCE FOR ACCELERATED GRAPHITIZATION IN MARBLE RELATIVE TO PELITIC ROCKS

Raman spectra of carbonaceous materials including graphite (CM) have been shown by many workers to vary systematically with metamorphic grade. As temperature increases, “disorder” bands at ~1350 cm^-1 (D1-peak) and ~1620 cm^-1 (D2-peak) decrease in intensity as the principal graphite band at ~1580 cm^-1 (G-peak) increases. The ratio R of the areas of these peaks (D1/(D1+D2+G)) has been quantitatively calibrated by Beyssac et al. (2002, JMG 20, 859-871) for pelitic rocks in the range 330-650°C. They focused on CM inclusions within transparent host minerals to eliminate problems associated with polishing and heating of CM grains, and used polished sections cut perpendicular to foliation to minimize variability that results from the orientation of graphite crystals. We observe the disorder/order band ratio R to be lowest when the laser beam is directed perpendicular to (0001).

We applied these methods to 37 marble samples from the Grenville Province, analyzing mostly inclusions in calcite and generally 4-8 analyses per sample. Independent T estimates from calcite-dolomite and calcite-graphite thermometry range from 450 to 735°C. Instrumental settings are 532-nm laser excitation and 11 mW of laser power at the sample. Linear regression to all samples yields T(°C) = -725R + 640 (r² = 0.55). However, within-sample standard deviations tend to be large, and the T spread at low values of R is large (~200°). A more selective subset of 26 samples from the Elsevir terrane yields T(°C) = -535R + 595 (r² = 0.65). Our spectra show that graphite in marble is more structurally mature than graphite in pelites metamorphosed at the same temperature. For example, using Beyssac et al.'s Raman calibration for pelites yields temperatures that are >50° too high for the marble samples. Graphitization appears to be accelerated in marble relative to pelitic rocks, perhaps due to higher oxygen fugacity or advanced graphite crystallization with coarsening. Greater variability in graphite orientation could also contribute to observed differences. Raman CM thermometry in marble appears to be useful only at temperatures below ~550°C and with a ±50° uncertainty. With better understanding of the variability due to orientation, Raman spectroscopy may prove useful in recognizing within-sample heterogeneity and discriminating among organic precursors.