Constraints on the thermal history of Taylorsville Basin, Virginia, U.S.A., from fluid-inclusion and fission-track analyses: implications for subsurface geomicrobiology experiments

Abstract

Microbial populations have been found at the depth of 2621–2804 m in a borehole near the center of Triassic Taylorsville Basin, Virginia. To constrain possible scenarios for long-term survival in or introduction of these microbial populations to the deep subsurface, we attempted to refine models of thermal and burial history of the basin by analyzing aqueous and gaseous fluid inclusions in calcite/quartz veins or cements in cuttings from the same borehole. These results are complemented by fission-track data from the adjacent boreholes. Homogenization temperatures of secondary aqueous fluid inclusions range from 120° to 210°C between 2027- and 3069-m depth, with highest temperatures in the deepest samples. The salinities of these aqueous inclusions range from 0 to ∼ 4.3 eq wt% NaCl. Four samples from the depth between 2413 and 2931 m contain both two-phase aqueous and one-phase methane-rich inclusions in healed microcracks. The relative CH 4 and CO 2 contents of these gaseous inclusions was estimated by microthermometry and laser Raman spectroscopy. If both types of inclusions in sample 2931 m were trapped simultaneously, the density of the methane-rich inclusions calculated from the Peng-Robinson equation of state implies an entrapment pressure of 360 ± 20 bar at the homogenization temperature (162.5 ± 12.5°C) of the aqueous inclusions. This pressure falls between the hydrostatic and lithostatic pressures at the present depth 2931 m of burial. If we assume that the pressure regime was hydrostatic at the time of trapping, then the inclusions were trapped at 3.6 km in a thermal gradient of ∼ 40°C/km. The high temperatures recorded by the secondary aqueous inclusions are consistent with the pervasive resetting of zircon and apatite fission-track dates. In order to fit the fission-track length distributions of the apatite data, however, a cooling rate of 1–2°C/Ma following the thermal maximum is required. To match the integrated dates, the thermal maximum would have occurred at ∼ 200 Ma. The timing of the maximum temperature is consistent with rapid burial of the Taylorsville Basin to twice its present-day depth and thermal re-equilibration with a 40°C/km geothermal gradient, followed by slow exhumation. The results may imply that the microorganisms did not survive in situ, but were transported from the cooler portions of the basin sometime after maximum burial and heating.