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

Abstract

Relative 3He, 4He, Ne, Ar, N 2 and CO 2 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 R A, CO 2/ 3He ratios of 10 to 50 × 10 9 and N 2/Ar ratios of 200 to 500. The data suggest that He is essentially of upper mantle origin and that CO 2 and N 2 are predominantly derived from subducted sediments. Highest CO 2/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 CO 2-rich magmatic environment created by the supply of volatiles from two subduction systems. The low 3He/ 4He ratios of 0.4 and 0.6 R A 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.