Paleotemperatures from fluid inclusions and coal rank of carbonaceous material of the Tertiary formations in Northwest Kyushu, Japan

Abstract

Homogenization temperatures of fluid inclusions in quartz and/or calcite that coexist with carbonaceous material in sedimentary rocks from selected Tertiary coalfields have been measured. A correlation between these fluid inclusion temperatures and vitrinite reflectance as a rank parameter of coal is presented. All mineral samples analyzed here were collected outside of high coal rank regions adjacent to intrusive rocks in each coalfield. The minimum values of homogenization temperatures obtained in this study are at least regarded as representing temperatures attained by the sedimentary rocks during burial metamorphism.
This study indicates a direct relationship between minimum value of homogenization temperature (T in °C) and vitrinite reflectance (R_max, in %) with a regression equation of the form; R_max=0.352 exp 0.00976•T. Previous works on the temperature of R_mex are based mainly on present temperatures such as present geothermal gradients or pyrolysis experiments. As the reaction rate of thermal maturation of carbonaceous material differs in diagenesis, geothermal systems, and contact metamorphism, the equation obtained here may be employed as a useful tool to estimate paleotemperatures in Tertiary sedimentary basins.