Isotopic Analysis- Gas At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)

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Exploration Activity: Isotopic Analysis- Gas At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)

Exploration Activity Details
Location Long Valley Caldera Geothermal Area
Exploration Technique Isotopic Analysis- Fluid
Activity Date 2000 - 2001

Usefulness useful
DOE-funding Unknown

Exploration Basis
The study integrates detailed results from pump tests, fluid level monitoring, temperature logging, and fluid sampling/analysis of the LVEW with information obtained from other wells drilled on or near the resurgent dome to provide a comprehensive conceptual model of the different stages of hydrothermal activity, flow, and recharge in the Long Valley caldera groundwater system. Fluids were sampled from LVEW during flow testing in May 2000, July 2000, and September 2001 in order to characterize the composition of aqueous fluids encountered below the resurgent dome, provide data required to apply geothermometers for estimating reservoir temperatures, determine potential sources of water in the wellbore, and identify magmatic gases present in the well fluids.
Water samples were collected from the pump discharge line at the surface during each flow test. Dissolved gases were sampled at the surface using 1-1 glass flow-through tubes and cold-welded copper tubes. Downhole water samples were also collected from 2600 m depth after the flow test in July 2000. The temperature, pH, specific conductance, and alkalinity of each water sample were measured in the field within minutes following collection. Field processing of water samples was conducted in accordance with standard USGS protocols outlined by Wilde et al. (1998), and the chemical constituents of each sample were analyzed at the USGS Central Laboratory in Denver, CO. Hydrogen and oxygen isotopic analyses of the water samples, as well as analyses of the chemical compositions and carbon isotope ratios of gas samples collected in the glass tubes, were conducted at the USGS laboratories in Menlo Park, CA. Gas samples collected in the copper tubes were analyzed for their helium isotope contents by B.M. Kennedy at the University of California at Berkeley, CA. Isotopic ratios of helium and CO2 determined for gases sampled at LVEW were 3.66 Ra (3He/4He ratio) and -6.4 %o (delta C13 in dissolved inorganic carbon). These values resemble those of thermal water equilibrated with a magmatic gas input (Sorey et al., 1998; Evans et al., 2002), and the relatively heavy delta C13 value is similar to those measured in other thermal waters in the caldera, although the dissolved carbon could also be sourced from metamorphic basement rocks beneath the resurgent dome. Together, the chemical (see separate activity entries) and isotopic characteristics of fluids sampled from LVEW suggest that the well is not directly connected with the shallow hydrothermal system that flows laterally from the west around the southern edge of the resurgent dome.


Additional References