Fluid inclusion evidence for recent temperature increases at Fenton Hill hot dry rock test site west of the Valles Caldera, New Mexico, U.S.A.

Abstract

The fluid inclusions in calcite veins and those in quartz of the host Precambrian rocks from the GT-2 drill hole have been studied microthermometrically to determine the recent thermal history of the Fenton Hill Hot Dry Rock test site west of the Valles caldera, New Mexico. The calcite veins were collected from 1876 m and 2624 m depth. They contain primary liquid-rich inclusions and secondary liquid-rich and monophase liquid inclusions. The homogenization temperature ( T h ) and final melting point of ice ( T m ) of these inclusions and the secondary inclusions in quartz from the host Precambrian rocks were measured using α USGS-type gas flow heating/freezing stage. The CO 2 content was also determined semiquantitatively using a microscope crushing stage. The trapping temperature was determined on the isochore under the assumption of lithostatic pressure. NaCl eq. salinity was also determined from T m after correction for CO 2 . Microthermometry of primary inclusions in calcite and secondary inclusions in quartz indicates that the calcite veins precipitated from low-salinity geothermal fluids at temperatures at least 10–15°C lower than the thermal maximum recorded in the secondary inclusions in quartz of the Precambrian rocks. The lowest temperature determined from the minimum trapping temperature of secondary inclusions in calcite is 26°C lower than the present borehole temperature of 178°C at 2624 m. After this cooling the temperature increased again up to the present geothermal profile.