HR: 0800h
AN: B31B-0986    [Abstracts]
TI: Multidimensional Field Mapping of Gaseous C-H-O-S Species in Hydrothermal Systems: Distinguishing Potential Sites for Hydrocarbon Generation
AU: * Schwandner, F M
EM: fschwandner@asu.edu
AF: Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Dunn, E E
EM: eileen.dunn@asu.edu
AF: Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Dunn, E E
EM: eileen.dunn@asu.edu
AF: Department of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287-1604 United States
AU: Shock, E L
EM: eshock@asu.edu
AF: Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Shock, E L
EM: eshock@asu.edu
AF: Department of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287-1604 United States
AB: Organic compounds in hydrothermal gas emissions have been documented since the mid-1800's, yet their origin is still a matter of some debate. Thermal alteration such as maturation and cracking can produce thermogenic hydrocarbons from pre-existing organic matter in hydrothermal systems. Gas-phase radical reactions and catalytic hydrogenation reactions of CO₂ and CO to methane and higher hydrocarbons have also been suggested as being responsible for observations of organic compounds in hydrothermal emissions.
Recently published data indicated that some organic signatures in volcanic-hydrothermal systems cannot be explained by pre-existing organic matter alone, and more representative analyses are now required to shed light on this question. Choosing a representative site within a hydrothermal field for sampling is in itself a complicated task, and heterogeneities can be easily missed. Spatial analysis of the distribution of C-O-H-S species in the gas phase can potentially indicate possible sites of increased hydrocarbon generation potentials via the catalytic hydrogenation pathway. This approach offers the advantage of providing information in the field that can be used to judge appropriate sampling locations prior to the more complex and costly standard organic analyses of gaseous emissions.
A portable multi-sensor system with electrochemical and infrared sensors can in a short time provide large spatial data sets that yield potential target areas for selectively sampling organic compounds. Statistical methods, including probability tests and spatial correlation of concentrations and fluxes of selected species, can be applied later to yield information on the number of populations as well as genetic relationships between different populations.
This approach was tested at three acid-sulfate sites in Yellowstone National Park, USA. The chosen sites were the Greater Obsidian Pool area (GOPA, Mud Volcanoes hot spring group), the Sylvan Springs area, and the Washburn Springs area. The test sites represent different structural regimes of the Yellowstone Caldera complex: (a) inner caldera radial faults related to the Sour Creek resurgent dome at GOPA, (b) an extra-caldera regional fault systems in a region where local seismicity appears to be focused to in recent decades (Sylvan), and (c) the caldera rim ring fracture system (Washburn). Flux data on CO₂, H₂, CO, and H₂S were acquired, as well as temperature/depth profiles which yielded soil temperatures, geothermal gradient and heat flux data.
The results indicate that at least two populations are present in all four species at all sites, and that the dominant populations of H₂ and CO₂ appear to be structurally controlled. In contrast, CO and H₂S appear to form high-flux clusters around hot pools. The former are explained by a strong influence of deeper processes such as magmatic degassing, while the latter may be explained by more shallow chemical or biological processes. A magmatic signature (high CO₂/H₂S ratios) is not evident along lineaments but appears localized. High reduced gas fluxes are observed at ground wetted by adjacent thermal pools, and similarly, the ground's thermal budget appears to be strongly controlled by localized conductive heating by thermal waters rather than advective heat transport. These findings provide the context for the organic compounds found in these and other hydrothermal and volcanic gas emissions.
UR: http://geopig.asu.edu
DE: 1055 Organic and biogenic geochemistry
DE: 8045 Role of fluids
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
DE: 8430 Volcanic gases
DE: 8494 Instruments and techniques
SC: Biogeosciences [B]
MN: Fall Meeting 2005