Assessing Clumped Isotope Thermometry for the Formation Temperature versus Burial Recrystallisation of Dolomite

Clumped isotope thermometry has been shown to record the temperature of precipitation for carbonate minerals in surface and near-surface environments. However, the ability of a mineral to hole its clumped isotope signature at deeper burial conditions is less clear. We present data from dolostones buried between 2-6 km and we investigate whether the clumped isotope thermometer records the temperature of dolomitisation or subsequent burial recrystallization. A first suite of reservoir samples from an oil field in Western Africa recovered from 2848-3705 metres below sea floor yielded temperatures of 93 °C to 107 °C using the calibration of Passey and Henkes (2012). δ18OSMOW values of +3 to +4 ‰ were calculated (Land, 1980) for the dolomitizing fluid. The relatively narrow range of temperatures and parent fluid oxygen isotope compositions recorded over ~100 metres of stratigraphy suggests that in this case the clumped isotope thermometer is recording the temperature of dolomitisation. By contrast, preliminary analysis from samples of Upper Jurassic dolomite in the Aquitaine basin (France) occurring at present-day depths of 2960-5351 metres yielded temperatures of 35 °C to 124 °C. An initial observation is that the clumped isotope temperatures seem to increase with depth, suggesting that the clumped isotope paleothermometer may be recording a burial recrystallization process. By combining results of the two case studies, we aim at characterizing the temperature at which dolomite reopens for clumped isotope paleo-thermometry.