Helium isotopic and chemical composition of gases from volcanic-hydrothermal systems in the Philippines

Relative 3He, 4He, Ne, Ar, N2 and CO2 contents of gases collected from high temperature geothermal systems associated with active subduction zones in the Philippines, (Cagua, Alto Peak, Mahagnao, Mambucal and Mount Apo) correspond to 3He/4He ratios of 6.9 to 7.6 RA, CO2/3He ratios of 10 to 50 × 109 and N2/Ar ratios of 200 to 500. The data suggest that He is essentially of upper mantle origin and that CO2 and N2 are predominantly derived from subducted sediments. Highest CO2/He ratios are observed for systems along the centerline between major zones of active subduction to the E and W of the Philippine islands. The high values may tentatively be explained in terms of derivation from an especially CO2-rich magmatic environment created by the supply of volatiles from two subduction systems. The low 3He/4He ratios of 0.4 and 0.6 RA for two springs on the island of Palawan indicate that the thermal waters there are the result of only residual geothermal activity associated with an, at present, inactive subduction zone.