Fluid Inclusion Evidence for Recent Temperature Rising 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 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 volcanic activity in the Jemez mountains began in Miocene time with basaltic and rhyolitic eruptions, culminating in the ignimbrite eruptions forming the Valles caldera 1.45 Ma and 1.12 Ma ago (Smith and Bailey, 1966; Doell et al., 1968; Gardner et al., 1986; Self et al., 1986). The resurgent doming and ring fracture volcanism followed the caldera collapse (Smith and Bailey, 1968). The youngest volcanism is the Banco Bonito obsidian at 0.13 Ma on the southern margin of the caldera (Marvin and Dobson, 1979). Deep holes were drilled at Fenton Hill to test a method for extracting geothermal energy from hot and impermeable rocks (Smith, 1975). The active heat source for them is believed to be present beneath the Valles caldera (Laughlin, 1981). An early thermal modeling for the HDR test site was constructed based on the assumption of simple cooling of the magma reservoir related to the caldera-forming ignimbrite eruptions (Kolstad and MacGetchin, 1978). The reexamination of the geotherm of the deep holes and an 40Ar/39Ar isotopic study, however, indicate recent temperature increasing at the HDR test site (Harrison et al., 1986).