Noble and reactive gases of Palinpinon geothermal field (Philippines): Origin, reservoir processes and geodynamic implications

Abstract

Palinpinon is a high-temperature, liquid-dominated volcano-geothermal system located on southern Negros Island, Philippines, associated with subduction of Negros-Sulu arc (Early Pliocene to Recent). In 2001, eleven (11) producing wells of the Palinpinon geothermal field were analyzed for major gas components and for noble gases isotopic composition. Geothermal gases are dominated by H2O, with CO2 and H2S being the most abundant species of the dry fraction. Chemical and isotope data indicate that two main components feed the geothermal system: (i) a deep component, enriched in CO2, H2S, H2 and He, related to volcano-hydrothermal interactions occurring in the roots of the geothermal system, and (ii) a surficial component, enriched in N2, Ar, Ne, related to natural meteoric recharge of the reservoir. The noble gas fraction is dominated by argon of atmospheric origin, as denoted by 40Ar/36Ar ratios between 295 and 310. Helium, in excess above the reference concentrations in air and air-saturated water (ASW), has an isotopic signature (3He/4He ratios between 6.96 to 7.94 RA) in the range of values normally observed for subduction-related volcanism. 3He/4He and CO2/3He (between 12.1×109 to 28.7×109) ratios support the hypothesis that most of the deep gases are directly derived from a magmatic source and/or from the scavenging of an organic-depleted, basalt-rich crust. Water–rock interactions cause some geothermal overprinting of the deep magmatic component, allowing redox conditions in the reservoir to be controlled by the Fe(II)–Fe(III) buffer. Based on CO2/CH4 and H2/Ar ratios, maximum equilibrium temperatures between 300 and 350°C have been estimated in the geothermal reservoir. Chemical data indicate that the geothermal reservoir is largely flushed by steam derived from the boiling of waters of meteoric recharge and reinjected brines.