Compositional, Order/Disorder, and Stable Isotope Characteristics of Al‐Fe Epidote, State 2–14 Drill Hole, Salton Sea Geothermal System

Abstract

Epidote (Ca2Fe3Si3O12(OH)‐Ca2Al3Si3O12(OH)) is a common hydrothermal mineral in metasediments and veins at depths >900 m in the State 2–14 drill hole of the Salton Sea geothermal system. The mole fraction of Ca2Fe3Si3O12(OH) in epidotes (Xps) from this drill hole ranges from 0.11 to 0.42, and complex compositional zoning of octahedral Fe3+ and Al is typical within single grains. With increasing depth there is an overall, but irregular, decrease in the Fe3+ content of epidotes in metasandstones. In most samples, vein epidotes are more Fe3+ ‐rich and exhibit a wider compositional range than metasandstone epidotes from the same depth. Octahedral Fe3+ in the M(1) sites in four epidotes, evaluated by 57Fe Mössbauer spectroscopy, ranges from 7.5 ± 1.7% to 11.4 ± 1.5% of the total iron. The most ordered epidote formed in veins at 2618 m in the biotite zone. This epidote has an ordering parameter (σ = 1–2XFe3+,M(1)) of 0.85 ± 0.03 which corresponds to a calculated state of equilibrium order/disorder at ∼390° ± 60°C, in close agreement with the probable downhole temperature of ∼340°C, Two epidote samples from the chlorite + calcite zone (1420 m, ∼265°C; 1867 m, ∼300°C) are more disordered, corresponding to calculated states of equilibrium order/disorder of >450°C, These data suggest that the analyzed epidotes from the chlorite + calcite zone are in a metastable state of substitutional order/disorder. The measured value of δDepidote from 1420 m is −96‰, which is 5–6‰ lighter than epidotes from 1867 m (δDepidote = −90‰), 2227 m (δDepidote = −91‰), and 2618 m (δDepidote = −90 ‰). When compared to the reservoir fluid from the December 1985 flow test, the derived fractionation of hydrogen isotopes between epidote and the geothermal brine is −19‰ at 1867 m and ∼340°C. This value is in accord with published experimental hydrogen isotope fractionations between iron‐rich epidote and aqueous electrolyte solutions. Therefore, considering the complexity of the Salton Sea brine, the isotopic temperature is similar to the inferred downhole temperature.