Noble gas evidence for two fluids in the Baca (Valles Caldera) geothermal reservoir

Abstract

Noble gas elemental and isotopic abundances were measured in steam from four wells in the Baca geothermal reservoir located in the Valles Caldera, New Mexico. The 40Ar 36Ar ratio and noble gas elemental abundances relative to 36Ar are all strongly correlated with 1/ 36Ar, the inverse of the argon content. Ratios of ( α, n)-produced 21Ne∗ and radiogenic 40Ar∗ to total 4He (dominantly radiogenic) are nearly constant at 2.1 × 10−8 and 0.20, respectively. The 3He 4He ratio covers a restricted range of 3.9 to 4.8 times atmospheric. The high 3He content of the gas indicates the presence of a helium component ultimately derived from the mantle. Kr and Xe isotopic compositions are close to atmospheric; excess 129Xe∗ is <0.25% of the total 129Xe. The high degree of linear correlation among the various noble gas results strongly suggests that the Baca reservoir contains two distinct fluids that are produced in varying proportions from individual wells. The noble gases in fluid A (~2900 mg/1 C1) are air-like, but with lighter gases and isotopes preferentially enriched. The fluid A 36Ar content is low, only 13% that of 10°C air-saturated water (ASW). The second fluid, B (~ 1700 mg/1 C1), is the dominant carrier of the radiogenic and mantle-derived gases. The heavier non-radiogenic gases are preferentially enriched in fluid B, and its 36Ar content is very low, only 5–7% ASW. The source of the noble gases in fluid A is tentatively ascribed to leaching of the relatively young (<1.4 m.y.) volcanic Bandelier Tuff. The radiogenic gases and mantle-derived helium in fluid B suggest a deeper source, possibly including gases escaping from a magma.