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Book Chapter

Geochemistry of Light Hydrocarbons in Subduction-Related Volcanic and Hydrothermal Fluids

By
Yuri A. Taran
Yuri A. Taran
Institute of Geophysies, Universidad Nacional Autónoma de México 04510, Mexico
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Werner F. Giggenbach
Werner F. Giggenbach
Institute of Geological and Nuclear Sciences, P.O. Box 31312, Lower Hutt, New Zealand
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Published:
January 01, 2005

Abstract

During the last 20 years of his outstanding career, Werner Giggenbach collected and analyzed hundreds of samples of volcanic and hydrothermal gases from White Island volcano and geothermal systems in New Zealand, as well as many volcanoes and geothermal systems over the world. Hundreds of samples were analyzed for C1-C6 hydrocarbons, including benzene. On the basis of the data set obtained for the White Island volcano, together with other available data, several general trends in the behavior of CH4, C2-C4 hydrocarbons, and benzene are apparent, based on application of techniques developed by W. Giggenbach for the interpretation of crustal fluid composition in a high-temperature environment. The trends can be divided into two main types involving temperature-dependent equilibrium and mixing of carbon from magmatic and sedimentary sources.

The common statement that the CH4 concentration in volcanic gases decreases with increasing temperature is not true, as there are no temperature-dependent trends in the CH4/CO2 behavior until magmatic temperatures are reached. The concentrations of methane in hydrothermal fluids are controlled mainly by the source output, comprising organic matter buried with sedimentary rocks. Thermal decomposition of this organic matter at upper crust levels produces CH4 and light hydrocarbons as well as nitrogen accompanied by a very high N2/Ar ratio. Therefore, the CH4-rich end member of hydrothermal fluids tends to have a high N2/Ar ratio. By contrast, subduction-related magmatic fluids have almost no methane despite having a high N2/Ar ratio due to degradation of subducted organic-rich oceanic sediments. Hence, volcanic gases and hydrothermal fluids are characterized by two different relationships between CH4 concentration and N2/Ar ratio.

Two systems show a good correlation with sampling temperature in volcanic gases: alkane-alkene pairs with the same number of carbon atoms and ethene-benzene. Their concentration ratios (C2H6/C2H4, C3H8/C3H6, ΣC4H10/ΣC4H8, C2H4/C6H6) in volcanic gases are strongly dependent on the temperature of the fumarole, and these ratios change with temperature along metastable equilibrium paths. This means that the chemical redox reactions alkene + H2 = alkane and C6H6 + 3H2 = 3C2H4 are fast but kinetically controlled, probably through catalysis by oxides and sulfur species. Variations in alkane-alkane ratios in terms of the 2Cn = Cn 1 + Cn + 1 equilibrium, either for volcanic gases or for hydrothermal fluids, show no systematic trends, and even at magmatic temperatures (>800°C), the observed Cn/Cn 1 ratios often correspond to negative equilibrium temperatures.

The reaction CO2 + 4H2 = CH4 + 2H2O, mistakenly called the Fischer-Tropsch reaction by some geochemists, is unlikely to apply under redox and temperature conditions in the hydrothermal and magmatic environment. The only natural inorganic process in which reduction of CO2 (and carbon) could be possible is the serpentinization of Mg-rich mafic rocks. Under conditions prevailing in the crust, the only process that results in equilibration within the CH4-CO2 system is the oxidation of methane. This can be facilitated by natural catalysts, which are usually oxides, but not native metals as in the case of the reduction of CO2.

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Special Publications of the Society of Economic Geologists

Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth

Stuart F. Simmons
Stuart F. Simmons
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Ian Graham
Ian Graham
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Society of Economic Geologists
Volume
10
ISBN electronic:
9781629490342
Publication date:
January 01, 2005

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