Fluid-inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer

Theory, laboratory experiments, and empirical observation suggest that many aqueous fluid inclusions in calcite reequilibrate during overheating, and therefore some homogenization temperatures (T_h) record a temperature close to the maximum reached by the rock. This characteristic suggests that aqueous fluid inclusions in calcite can be used to establish maximum temperature (T_peak). To test this hypothesis, we have compiled fluid inclusion T_peak, mean random vitrinite reflectance (R_m), and present-day T_peak from 46 diverse geologic systems that have been at T_peak from 10⁴ to 10⁶ yr. Present T_peak ranged from 65 to 345 °C, T_h modes and means ranged from 59 to 350 °C, and R_m data ranged from 0.4% to 4.6%, spanning the temperature and thermal maturity range associated with burial diagenesis, hydrothermal alteration, and low-grade metamorphism.

Plots of T_h and T_peak data for systems thought to be currently at maximum temperature demonstrate close agreement between T_h and present T_peak in sedimentary basins. Although caution should be applied, the relation suggests that T_h of aqueous fluid inclusions in calcite may be a useful measure of maximum temperature. This study also compares T_h to mean random vitrinite reflectance (R_m) to offer further support for the use of T_h as a measure of T_peak, and to provide a better understanding of R_m. T_h correlates well with R_m and results in a curve similar to R_m vs. T_peak calibrations determined by other workers. The strong correlation (correlation coefficient r = 0.93) between T_peak and R_m in these systems suggests that maximum temperature is the major control on thermal maturation.