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 66m thick Great Whin Sill in County Durham, England, shows that the time-scale of heating was on the order of similar to 40-270years with a temperature range of similar to 300-680 degrees C within similar to 110m from the sill contact. Raman spectroscopy reveals a significant increase in the crystallinity of the CM (from a less crystallized D1-band width of similar to 110cm(-1) up to a well crystallized R2 peak area ratio of similar to 0.43) within similar to 50m from the sill contactcorresponding to an apparent increase in temperature of up to similar to 200 degrees C when estimated using conventional Raman CM geothermometry. Over this distance, the temperatures derived from thermal modelling exceed similar to 400 degrees C and heating occurred over a time-scale of c. 100years. 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.