The importance of heating duration for Raman CM thermometry: Evidence from contact metamorphism around the Great Whin Sill intrusion, UK

The degree of crystallinity of amorphous carbonaceous material (CM) increases with heating. Previous studies have shown good correlations between maximum temperature and the structural state of CM as measured by Raman spectroscopy. Changes in CM are also expected to depend on the time-scale of heating, but there are very few data that can be used to define this relationship. Thermal modelling of contact metamorphism developed around the 66 m thick Great Whin Sill in County Durham, England, shows that the time-scale of heating was on the order of ~40-270 years with a temperature range of ~300-680 °C within ~110 m from the sill contact. Raman spectroscopy reveals a significant increase in the crystallinity of the CM (from a less crystallized D1-band width of ~110 cm^-1 up to a well crystallized R2 peak area ratio of ~0.43) within ~50 m from the sill contact-corresponding to an apparent increase in temperature of up to ~200 °C when estimated using conventional Raman CM geothermometry. Over this distance, the temperatures derived from thermal modelling exceed ~400 °C and heating occurred over a time-scale of c. 100 years. Combining the results of this study with the results of previous work shows both the maximum attained temperature and the duration of heating have a significant effect on the resulting Raman spectra and demonstrates the utility of this method to derive quantitative descriptions of the kinetics of CM crystallization in rocks.