Stable-Isotope Studies Of Rocks And Secondary Minerals In A Vapor-Dominated Hydrothermal System At The Geysers, Sonoma County, California

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Journal Article: Stable-Isotope Studies Of Rocks And Secondary Minerals In A Vapor-Dominated Hydrothermal System At The Geysers, Sonoma County, California

Abstract
The Geysers, a vapor-dominated hydrothermal system, is developed in host rock of the Franciscan Formation, which contains veins of quartz and calcite whose Δ18O values record the temperatures and isotopic compositions of fluids prevailing during at least two different episodes of rock-fluid interaction. The first episode took place at about 200°C, during which marine silica and carbonate apparently interacted with ocean water entrapped in the sediments to form veins of quartz and calcite whose Δ18O values were around +19 and +16%, respectively. The calculated water/mineral ratios were less than unity. The water may have profoundly influenced the Δ18O values of spilitic basalts during their metamorphism to greenstones. Serpentinization and structural emplacement of ophiolite slabs were isotopically unrelated to this episode, which was essentially a low-grade (post-Cretaceous?) burial metamorphism. D/H ratios of actinolite, chlorite, and micas in host rocks were more profoundly altered during this episode than were 18O/16O ratios. A paleogeothermal gradient of about 53°C/km has been inferred for this episode, from Δ18O-depth distributions of vein minerals. The second episode, in part recorded by cogenetic vein quartz and calcite Δ18O values of +4 to +6% and +1 to +3%, respectively, began with large quantities of meteoric water circulating in fractures in the rock at temperatures of 160-180°C in response to the initiation of the Pliocene-Pleistocene Clear Lake magmatism. The temperature rose, and with the restricted circulation of fluids the ancestral hot-water system evolved into the presently active vapor-dominated system, which according to the cogenetic vein quartz and calcite Δ18O values involved temperatures as high as 320°C and fluid/mineral ratios near unity. The change in the oxygen-isotopic composition of the serpentinite within the host rock during this later activity was negligible. The Δ13C values of vein calcite at The Geysers reflect both a marine carbonate and organic component of carbon, but carbon-isotope exchange has been facilitated by the vapor-dominated hydrothermal fluid to a greater degree than in any other episode or in other hot-water systems.

Authors 
Steven J. Lambert and Samuel Epstein








Published Journal 
Journal of Volcanology and Geothermal Research, 1992





DOI 
Not Provided
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Citation

Steven J. Lambert,Samuel Epstein. 1992. Stable-Isotope Studies Of Rocks And Secondary Minerals In A Vapor-Dominated Hydrothermal System At The Geysers, Sonoma County, California. Journal of Volcanology and Geothermal Research. (!) .