Other
Scientific paper
May 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51.1245b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 51, Issue 5, pp.1245-1256
Other
6
Scientific paper
Two types of white micas are found in drillhole samples within the geothermal system at Coso Hot Springs. Low-permeability zones of the crystalline basement contain coarse-grained relict muscovite, whereas rock alteration near fracture zones at temperatures > 150°C is characterized by abundant finegrained sericite in association with secondary calcite and quartz and unaltered relict microcline. In this hydrothermal sericite there is an increase in interlayer K, octahedral Mg + Fe total and tetrahedral Al with increasing temperature between ~215° to >250°C. Thermodynamic activity of the Al 2 Si 4 O 10 (OH) 2 component of serielles, calculated using site-mixing approximations of and (1985), decreases with increasing temperature between ~215° and >250°C. As a consequence of the observed variations in the interlayer, octahedral and tetrahedral site occupancies, the activity of KAl 2 (AlSi 3 O 10 (OH) 2 is essentially constant in the Coso seriates over this temperature range. The calculated equilibrium distribution of aqueous species in the hydrothermal solutions produced from well 16-8, together with cation-activity phase diagrams that account for variations in sericite composition, requires a pH of ~6.7-6.8 at temperatures between 236° and 250°C. Comparison of predicted and observed phase relations with fluid compositions indicate that seriates are in local equilibrium with the geothermal reservoir fluid, whereas relict metamorphic muscovites are metastable. Although the compositions of hydrothermal sericites are a complex function of temperature, pressure and geothermal fluid composition, compositional relations observed at Coso together with published compositions of hydrothermal dioctahedral layer-silicates from the Salton Sea geothermal system demonstrates that elemental compositions of interlayer K and tetrahedral Al increase systematically with increasing temperature despite the dramatic differences in fluid compositions between these two geothermal systems. This suggests that the observed variations in interlayer and tetrahedral site occupancy is largely dependent on the enthalpy of hydrolysis reactions representing equilibrium between sericite solid solutions and the geothermal reservoir fluids.
Bird Dennis K.
Bishop Barbara P.
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