Abstract
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.