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</em>_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</em>_peak). To test this hypothesis, we have compiled fluid inclusion T</em>_peak, mean random vitrinite reflectance (R</em>_m), and present-day T</em>_peak from 46 diverse geologic systems that have been at T</em>_peak from 10⁴ to 10⁶ yr. Present T</em>_peak ranged from 65 to 345 °C, T</em>_h modes and means ranged from 59 to 350 °C, and R</em>_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</em>_h and T</em>_peak data for systems thought to be currently at maximum temperature demonstrate close agreement between T</em>_h and present T</em>_peak in sedimentary basins. Although caution should be applied, the relation suggests that T</em>_h of aqueous fluid inclusions in calcite may be a useful measure of maximum temperature. This study also compares T</em>_h to mean random vitrinite reflectance (R</em>_m) to offer further support for the use of T</em>_h as a measure of T</em>_peak, and to provide a better understanding of R</em>_m. T</em>_h correlates well with R</em>_m and results in a curve similar to R</em>_m vs. T</em>_peak calibrations determined by other workers. The strong correlation (correlation coefficient r = 0.93) between T</em>_peak and R</em>_m in these systems suggests that maximum temperature is the major control on thermal maturation.