Origin of Iodine and 129I in Volcanic and Geothermal Fluids from the North Island of New Zealand: Implications for Subduction Zone Processes
Udo Fehn, Glen T. Snyder, 2005. "Origin of Iodine and 129I in Volcanic and Geothermal Fluids from the North Island of New Zealand: Implications for Subduction Zone Processes", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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The mobilization of volatile elements during the subduction of marine crust and sediments is an important question for the understanding of the marine element budget. Geochemical behavior and halflife (15.7 Ma) of 129I make this isotopic system a useful tracer for processes associated with the subduction and recycling of marine sediments. The North Island of New Zealand is particularly suited for testing this system because samples of volcanic and geothermal waters are accessible from a variety of settings across the volcanic arc. We report here on waters recently collected from locations sampled earlier by Giggenbach et al. (1993), specifically from the East Coast, the Taupo Volcanic Zone, and the Northland, representing the fore arc, main arc, and the zone behind the volcanic arc, respectively. Although a significant number of the samples showed the presence of anthropogenic 129I, preanthropogenic ratios could be determined for waters from the East Coast and the Taupo Volcanic Zone. Waters from the Taupo Volcanic Zone had the lowest iodine concentrations and 129I/I ratios close to 300 × 10−15. In contrast, waters sampled in the East Coast had the highest iodine concentrations coupled with 129I/I ratios as low as 55 × 10−15. Whereas samples from the Northland had intermediate values in iodine concentrations, the initial 129I/I ratios for this set of samples could not be determined due to the presence of anthropogenic 129I. The ratios for the Taupo Volcanic Zone are compatible with iodine derived from marine sediments, mobilized from the entire sediment column undergoing subduction in this area. The ratios in the East Coast suggest the presence of a substantially older component in these fluids, also of marine origin, but mobilized from formations in the accretionary wedge. The ages derived for these samples (~70 Ma) are in good agreement with ages estimated for hydrocarbons found in this area, suggesting a common source; the positive correlation observed between iodine and methane concentrations supports this interpretation. Results from this study demonstrate that processes leading to release of geothermal fluids are substantially different in fore- and main-arc settings. A comparison of the data from New Zealand to results from volcanic areas in Japan, Central America, and South America shows that the 129I system gives site-specific results and can be used successfully to determine origin and history of fluids in subduction zones.
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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.