Isotopically anomalous CH4-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 CH4 (55 mole%) and H2 (42 mole%); the CH4 CO2 ratio is > 1800 and the CH4 He ratio is 9.2·104. The δ13C-value of the CH4 is -7.0±0.4‰ (PDB), ∼8‰ higher than the highest published values for CH4 in other natural gases and hot springs, but similar to values commonly attributed to mantle carbon. The 3He 4He ratio is 5.70·10-6, 4.1 times the atmospheric ratio, indicative of a substantial mantle He component. The δD-values of CH4 and H2 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.
ASJC Scopus subject areas
- Geochemistry and Petrology