Lithium Isotope Geochemistry of the Yellowstone Hydrothermal System
Neil C. Sturchio, Lui-Heung Chan, 2005. "Lithium Isotope Geochemistry of the Yellowstone Hydrothermal System", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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An exploration of the Li isotope variations of thermal waters and various rock types from drill cores and outcrops in the Yellowstone National Park, Wyoming-Montana, hydrothermal system was undertaken to investigate simple models for the Li cycle and to consider their implications for the geochemical dynamics of the system. Thermal waters have Li concentrations ranging from 0.27 to 6.5 mg/kg, and δ7Li values from 1.0 to 6.5 per mil. Rocks have Li concentrations ranging from 2.0 to 32.7 mg/kg (in sedimentary rock), and 15.2 to 282 mg/kg (in rhyolite); Li concentrations in rhyolite increase with the extent of hydrothermal alteration because Li from thermal water is incorporated in hydrothermal minerals. The range in δ7Li values for the sedimentary rock is 1.6 to 22.4, and δ7Li is generally higher in limestone than in shale. The range of δ7Li values for the rhyolite is −3.6 to +7.5 per mil; δ7Li values generally increase with an increasing extent of hydrothermal alteration. Relationships of Li and Cl concentrations in thermal waters indicate a loss of Li by incorporation into hydrothermal minerals in altered rhyolite. Simple isotopic fractionation models for Li isotope exchange between thermal water and altered rhyolite, with mass transfer constraints from Li/Cl ratios of thermal waters, can account for the observed variations in δ7Li. Massbalance considerations indicate that the long-term discharge of Li from the Yellowstone hydrothermal system may be sustained in large part by deep input of high-salinity magmatic brine.
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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.