Chemical Geology, 71 (1988) 211-222 211 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands METHANE-HYDROGEN GAS SEEPS, ZAMBALES OPHIOLITE, PHILIPPINES: DEEP OR SHALLOW ORIGIN? T.A. ABRAJANO', N.C. STURCHIO', J.K. BOHLKE 1, G.L. LYON 2, R.J. POREDA 3'* and C.M. STEVENS 1 ~Argonne National Laboratory, Argonne, IL 60439 (U.S.A.) 2Institute of Nuclear Science, DSIR, Lower Hutt (New Zealand) 3Isotope Laboratory, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093 (U.S.A.) (Revised and accepted July 7, 1988) Abstract Abrajano, T.A., Sturchio, N.C., Bohlke, J.K., Lyon, G.L., Poreda, R.J. and Stevens, C.M., 1988. Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin? In: M. Schoell (Guest-Editor), Origins of Meth- ane in the Earth. Chem. Geol., 71: 211-222. Isotopically anomalous CH4-rich gas escapes at low flow rate and ambient temperature from seeps in serpentinized ultramafic rock in the Zambales 0phiolite, Philippines. The major components of the gas are CH4 (55 mole% ) and H2 (42 mole% ); the CH4/CO 2 ratio is > 1800and the CH4/He ratio is 9.2.10 4. The • 13C-value of the CH4 is - 7.0 + 0.4%o (PDB), ~ 8%ohigher 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 JD-values of CH4 and H2 are - 136 and - 590%o,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. 1. Introduction The importance of deep Earth carbon as a source of significant hydrocarbon deposits is a subject of much interest and debate. According to conventional theory, hydrocarbon deposits (oil and gas) are produced by diagenetic alter- *Present address: Department of Geological Sciences, Uni- versity of Rochester, Rochester, NY 14672, U.S.A. ation of sedimentary organic matter as a con- sequence of bacterial activity or deep burial and heating in sedimentary basins (Hunt, 1979; Schoell, 1983). In contrast, it has been pro- posed that vast amounts of primordial CH4 and other hydrocarbons are stored in the Earth's mantle and could be a source of significant de- posits in the crust (Gold and Soter, 1980; Giar- dini et al., 1982). It is also possible that subducted sedimentary organic carbon is pres- ent at mantle depths and migrates to shallower 0009-2541/88/$03.50 © 1988 Elsevier Science Publishers B.V.