The age of the geothermal system and the granitic host rock at Coso geothermal system in California is poorly known. This is mainly due to a paucity of vein-type minerals (e.g. adularia, sericite) that can be directly dated. A downhole 40Ar/39Ar thermochronology study of granitic host-rock K-feldspar is presently being undertaken at the New Mexico Geochronology Research Laboratory at New Mexico Tech. The technique couples the measurement of argon loss from K-feldspar and knowledge of the diffusion parameters of transport in K-feldspar to estimate the longevity of the system at present day temperature and also to obtain an estimate of the host rock age. The study centers around a vertical distribution of samples obtained from Coso well 73-19 that reaches temperatures of 325oC at a depth of 1550m and thus represents one of the hottest producing wells in the Coso system. Four samples from Coso well 73-19, from depths 550m (downhole temperature 150oC), 700m (200oC), 1085m (275oC), and 1850m (325oC), were isolated from the granitic host rock chip samples. These samples were analyzed by the 40Ar/39Ar age spectrum method where the data recorded significant and variable degrees of argon loss due to the range and magnitude of the present day temperatures sampled. The amount of argon loss from the samples analyzed increases as expected as depth and temperature increase downhole, yielding integrated ages that decrease with depth, but initial steps do not reach 0 apparent age. Lovera et al's (1989) Multi Diffusion Domain method was used to compare thermal history models with measured data. The modeling supports a duration for the Coso geothermal system of definitely less than 100 ka, and possibly as short as 10 ka. This is much younger (< 100 ka vs. < 300 ka) than previous age estimates for the Coso geothermal system. Additional thermochronological sampling at Coso is planned to test the validity of these preliminary results.

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