Abiogenic methanogenesis in crystalline rocks

Journal Article · Wed Dec 01 00:00:00 EST 1993 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(93)90610-9· OSTI ID:5207917

Lollar, B S; Frape, S K ^[1]; Weise, S M ^[2]; Fritz, P ^[3]; Macko, S A ^[4]; Welhan, J A ^[5]

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1. Univ. of Waterloo, Ontario (Canada)
2. Institut fuer Hydrologie (G.S.F), Neuherberg (Germany)
3. UFZ, Umweltforschungszentrum, Leipzig-Halle (Germany)
4. Univ. of Virginia, Charlottesville, VA (United States)
5. Idaho State Univ., Pacatello, ID (United States)

Isotopically anomalous CH[sub 4]-rich gas deposits are found in mining sites on both the Canadian and Fennoscandian shields. With [delta][sup 13]C[sub CH4] values from -22.4 to -48.5% and [delta]D[sub CH4] values from -133 to -372%, these methane deposits cannot be accounted for by conventional processes for bacterial or thermogenic methanogenesis. Compositionally the gases are similar to other CH[sub 4]-rich gas occurrences found in Canadian and Fennoscandian shield rocks. However, the isotopically anomalous gases of this study are characterized by unexpectedly high concentrations of H[sub 2] gas, ranging from several volume percent up to 30 vol%. The H[sub 2] gases are consistently depleted in the heavy isotope, with [delta]D[sub H[sub 2]] values of -619 to -659%. [sup 3]He/[sup 4]He ratios in the range of 0.4 X 10[sup [minus]8] to 4.4 X 10[sup [minus]8] indicate that there is no resolvable component of mantle-derived He in these deposits. Based on these results a mantle-derived source for the C-bearing gases is unlikely. Several lines of evidence support an alternative abiogenic origin for the gases from Sudbury, Canada, and Juuka and Pori, Finland. The D-depleted H[sub 2] gas and calculated [Delta]D[sub (CH[sub 4]-H[sub 2]]) equilibration temperatures of 110-170[degrees]C at all three sites are in good agreement with results obtained for abiogenically produced CH[sub 4] and H[sub 2] in ophiolite sequences in Oman and the Philippines. Serpentinization and the hydration of ultramafic rocks are the proposed mechanisms for CH[sub 4] and H[sub 2] production in these ophiolite sequences. The widespread occurrence of serpentinized and altered ultramafic rocks at Juuka and at a number of other sites in both Canada and Finland implies that similar mechanisms may be involved in gas production at at least three sites on the shields. Abiogenic processes of CH[sub 4] production may be considerably more widespread than previously anticipated.

• OSTI ID: 5207917
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 57:23-24; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
HYDROGEN
ORIGIN
METHANE
NATURAL GAS DEPOSITS
CARBON 13
DEUTERIUM
HELIUM 3
HELIUM 4
IGNEOUS ROCKS
ISOTOPE RATIO
METAMORPHIC ROCKS
ALKANES
CARBON ISOTOPES
ELEMENTS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC DEPOSITS
HELIUM ISOTOPES
HYDROCARBONS
HYDROGEN ISOTOPES
ISOTOPES
LIGHT NUCLEI
MINERAL RESOURCES
NONMETALS
NUCLEI
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
RESOURCES
ROCKS
STABLE ISOTOPES
030200* - Natural Gas- Reserves
Geology
& Exploration