Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?

Isotopically anomalous CH 4-rich gas escapes at low flow rate and ambient temperature from seeps in serpentinized ultramafic rock in the Zambales Ophiolite, Philippines. The major components of the gas are CH 4 (55 mole%) and H 2 (42 mole%); the CH 4 CO 2 ratio is > 1800 and the CH 4 He ratio is 9.2·10 4. The δ 13C-value of the CH 4 is −7.0±0.4‰ (PDB), ∼8‰ higher than the highest published values for CH 4 in other natural gases and hot springs, but similar to values commonly attributed to mantle carbon. The 3 He 4 He ratio is 5.70·10 −6, 4.1 times the atmospheric ratio, indicative of a substantial mantle He component. The δD-values of CH 4 and H 2 are −136 and −590‰, respectively, consistent with equilibration temperatures of 110–125°C. Carbon and He isotopic data could be consistent with derivation of the Zambales gas directly from a reduced mantle. However, phase equilibria and H isotope data indicate that the gas also could have been produced by reduction of water and carbon during low-temperature serpentinization of the ophiolite.