HR: 17:45h
AN: V54C-08    [Abstracts]
TI: Isotopic Constraints on the Origin and Evolution of Geothermal Fluids, Long Valley, CA
AU: * Brown, S T
EM: stbrown@lbl.gov
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd MS70A-4418, Berkeley, CA 94720, United States
AU: Kennedy, B M
EM: bmkennedy@lbl.gov
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd MS70A-4418, Berkeley, CA 94720, United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd MS70A-4418, Berkeley, CA 94720, United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: University of California, Berkeley, Dept of Earth and Planetary Science, Berkeley, CA 94720, United States
AU: Evans, W C
EM: wcevans@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. M/S 434, Menlo Park, CA 94025, United States
AB: Successful management of geothermal resources requires hydrologic models that define and predict fluid flow in fracture-dominated systems. Such models are necessary to assess the resource potential, the impact of fluid production on the reservoir and the impact that new wells will have on existing production. We present new data on variations in O, D, and Sr isotopes in thermal waters of the Long Valley (California, USA) geothermal system and use them to assess fluid flow and water-rock interaction in the system. Geothermal well water samples collected June 2005 to June 2007 have temperatures of 41-190°C, δ¹⁸O from -16.5 to -13.5‰, and δD from -123 to -111‰. The low values suggest recharge from the west and north rims of the caldera, consistent with a general west-to-east decrease in temperature. Both δ¹⁸O and δD are displaced from the local meteoric water line and are positively correlated with Cl- concentrations for all thermal water samples hotter than 50°C. The Sr isotope ratios in the currently producing part of the field are clustered near 0.708, with a small west-to-east gradient of decreasing values from 0.7080 to 0.7078 over a distance of a few kilometers. These values are higher than those of the primary reservoir rocks (0.7060-0.7065), so the Sr isotopic ratio of the hot fluids must be set in another rock type. Granites from the northern and western rims of the caldera have ⁸⁷Sr/⁸⁶Sr = 0.7078-0.7100 and are possible sources of hot (200°C+) water entering the Long Valley geothermal system. The O isotope values of the fluids are also far out of isotopic equilibrium with host rocks. The small gradient in Sr isotope ratios, associated with minimal shift in O isotopes in this part of the system, probably reflects water-rock reaction, and indicates that flowing waters are contained in widely spaced (10m or more) fractures. Observed variations in O and D isotopes outside of the producing part of the field are inconsistent with a water rock reaction model, rather it appears another process such as mixing, as suggested in previous studies, or boiling exerts primary control on the water isotopes. The inferred large fracture spacing suggests that the thermal response time and O isotope response time of the system are of order 10-100 years, and therefore that changes to the system hydrology could be reflected in fluid O isotope values. Based on comparison of our 2006 isotope data and earlier data (Goff et al., 1991), there is little evidence of any change to the system hydrology over 15 years.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting