HR: 0830h
AN: V21C-0534 [PDF]
TI: Volatile Light Hydrocarbon Compositions of the Central American Arc and Yellowstone National
Park
AU: * Burnett, B J
EM: bettina3@unm.edu
AF: Dept. of Earth \& Planet. Sci., University of New Mexico, Albuquerque, NM 87131 United States
AU: Fischer, T P
EM: fischer@unm.edu
AF: Dept. of Earth \& Planet. Sci., University of New Mexico, Albuquerque, NM 87131 United States
AU: Sharp, Z D
EM: zsharp@unm.edu
AF: Dept. of Earth \& Planet. Sci., University of New Mexico, Albuquerque, NM 87131 United States
AU: Hilton, D R
EM: drhilton@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0244 United States
AU: de Leeuw, D
EM: gdeleeuw@insci14.ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0244 United States
AB:
The presence of organic compounds in volcanic and geothermal emissions has generally been attributed to the degradation of
previously existing crustal organic material. Samples collected from areas of documented high sedimentary content are
therefore expected to contain a higher quantity and greater diversity of hydrocarbons than samples collected in areas with
more pronounced mantle input. We sampled 17 fumaroles and hot springs in Central America and 26 in Yellowstone National Park
in order to examine the relationship between the physical and chemical characteristics of the sampling location and
hydrocarbon distribution.
El Salvador and Honduras gas samples were analyzed for inorganic compounds, light hydrocarbons and C isotopes of methane.
Collection temperatures of the El Salvador samples ranged between 78 and $875\deg$C. Values of $\delta$$^{13}$C-CH$_{4}$ for
the El Salvador samples vary between -32 and -30 $\permil$, while the Honduras samples range between -39 and -24 $\permil$.
These values fall within the accepted range for thermogenic methane (marine source rock -40 to -30 $\permil$ and humic
source rock -30 to -25 $\permil$). Of the hydrocarbons, methane was present in the greatest concentration, although
hydrocarbons up to pentane were detected. At the highest temperatures, pentane was the only higher hydrocarbon and it was
present in one sample (C1/C5 of 34), whereas in the lower temperature samples hydrocarbon variability and quantity was
greater (C1/C3 of 41 to 448; C1/C4 of 132 to 2463; C1/C5 of 277 to 6305). Hydrocarbon amounts and diversity were much higher
in samples with lower $^{3}$He/$^{4}$He ratios (3.99 to 6.38 R$_{c}$/R$_{A}$) than in samples with high ratios (6.67 to 7.56
R$_{c}$/R$_{A}$).
In Yellowstone National Park, gas samples were collected from sites within the caldera ($^{3}$He/$^{4}$He of 4.73 to 15.47
R$_{c}$/R$_{A}$), along the caldera rim, and outside of the caldera. Temperatures range from 12 to $94\deg$C. These samples
have been analyzed for inorganic compounds, light hydrocarbons, and C isotopes of CO$_{2}$. The $\delta$$^{13}$C-CO$_{2}$
values of the samples range from -4 to -1 $\permil$, with the lightest values seen along the caldera rim and outside the
caldera, while the heaviest values are seen primarily at sites inside the caldera. The lighter C isotope values fall within
the range of magmatic CO$_{2}$ (-8 to -4 $\permil$). To produce the heavier values, magmatic carbon may have mixed with
limestone-derived CO$_{2}$ ($\delta$$^{13}$C = 0). Relationships between light hydrocarbon compositions, major element gas
chemistry and isotopic variations within these geothermal systems will be discussed.
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting