HR: 0800h
AN: V51B-1485    [Abstracts]
TI: Low Molecular Weight Hydrocarbon Production at the Lost City Hydrothermal Field
AU: * Proskurowski, G
EM: giora@ocean.washington.edu
AF: WHOI, WHOI MS#4, Woods Hole, MA 02543 United States
AU: Lilley, M
EM: lilley@ocean.washington.edu
AF: UW School of Oceanography, University of Washington, Seattle, WA 98195 United States
AU: Olson, E
EM: olson@ocean.washington.edu
AF: UW School of Oceanography, University of Washington, Seattle, WA 98195 United States
AU: Kelley, D
EM: kelley@ocean.washington.edu
AF: UW School of Oceanography, University of Washington, Seattle, WA 98195 United States
AU: Frueh-Green, G
EM: gretli@erdw.ethz.ch
AF: ETH-Zentrum, Department of Earth Sciences, ETH-Zentrum, Zurich, CH-8092 Switzerland
AB: Here we present concentration, and stable and radiocarbon isotope data from hydrocarbons dissolved in hydrogen-rich fluids venting at the ultramafic-hosted Lost City Hydrothermal Field. The cool (<90°C) fluids venting from the spectacular carbonate chimneys at Lost City contain abundant low molecular weight hydrocarbons. A log-linear relationship (Schulz-Flory distriubution) between methane (mmol/kg), ethane (?mol/kg), propane (?mol/kg), and butane (nmol/kg) concentrations indicates synchronous production that is compatible with abiogenic formation such as Fischer-Tropsch type (FTT) genesis. However, a Schulz-Flory distribution of low-molecular weight hydrocarbons is commonly observed in the formation of petroleum via the pyrolysis of organic matter. Radiocarbon isotopic evidence suggests that the carbon source to Lost City hydrocarbons is carbon found in the host rocks, with no contribution from seawater bicarbonate. The underlying ultramafic host rocks contain <600 ppm non-carbonate carbon, most likely as a graphite found along grain boundaries, however a macro-molecular organic carbon source has not been eliminated (Delacour et al., 2004). While the absence of a large organic matter source argues for an abiogenic formation mechanism, a thermogenic source cannot yet be completely ruled out. The forthcoming carbon and hydrogen stable isotope analysis of Lost City hydrocarbons is expected to shed light on the formation mechanism. Both abiogenic FTT reactions and thermogenic formation imprint distinctive trends in the carbon isotopes of C1-C4 alkanes (Sherwood Lollar et al., 2002). Our initial findings, pending isotopic confirmation, illustrate the simple minimum requirements for extensive hydrocarbon production in nature: ultramafic rocks and water. Delacour, A. et al., 2004. Fluid-Rock Interaction in the Basement of the Lost City Vent Field: Insights from Stable and Radiogenic Isotopes. Eos, Transactions, American Geophysical Union, 85(47): Abstract B13A-0198. Sherwood Lollar, B., Westgate, T.D., Ward, J.A., Slater, G.F. and Lacrampe, C.G., 2002. Abiogenic formation of alkanes in the Earth's crust as a minor source for global hydrocarbon reservoirs. Nature (London), 416(6880): 522-524.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1038 Mantle processes (3621)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1055 Organic and biogenic geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005