Geochemistry of Light Hydrocarbons in Subduction-Related Volcanic and Hydrothermal Fluids
Yuri A. Taran, Werner F. Giggenbach, 2005. "Geochemistry of Light Hydrocarbons in Subduction-Related Volcanic and Hydrothermal Fluids", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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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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Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth
To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.