The Geochemistry of Rare Earth Elements and Yttrium in Geothermal Waters
Scott A. Wood, 2005. "The Geochemistry of Rare Earth Elements and Yttrium in Geothermal Waters", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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The rare earth elements (REE) have potential as tracers in geothermal systems, but the scarcity of data on REE systematics in thermal waters, and on REE mineral solubility and complex stability at elevated temperatures and pressures, has impeded progress. In this paper, information relevant to the use of REE in geothermal systems is reviewed. The REE form their strongest complexes with ligands such as hydroxide, fluoride, carbonate, sulfate, and phosphate. Chloride complexes are very weak at standard conditions, but much experimental data at higher temperatures suggest that their stability increases considerably with increasing temperature. Although chloride complexation by itself is unlikely to result in inter-REE fractionation, experiments show that chloride brines exsolved from silicate melts are enriched in the light REE compared to the melt. Chloride complexes are likely to be the dominant REE species in sea-floor hydrothermal vent fluids and many chloride-rich continental thermal waters. Experimental data at elevated temperatures for chloride complexes, combined with those for acetate and very limited data for hydroxide complexes, suggest that published theoretical estimates of REE complex stability constants are in need of revision. No reliable data exist for fluoride, carbonate, sulfate, or phosphate complexes at elevated temperature. Only very limited experimental data exist for the solubility of REE minerals (e.g., monazite, allanite, xenotime, and bastnäsite), the partitioning of REE between aqueous fluids and minerals (e.g., apatite, fluorite, scheelite, zircon), the sorption of REE onto mineral surfaces, and the behavior of REE during water-rock interaction. The scarcity of these types of data is currently the biggest impediment to quantitative modeling of the behavior of the REE in geothermal systems.
In the last couple of decades, there has been a large increase in the amount of information on REE systematics of geothermal waters, owing in large part to analytical advances. Considerable data on thermal waters from mid-ocean ridge sea-floor hydrothermal vents demonstrate that such fluids generally contain 10 to 10,000 times as much REE as seawater, and have light REE-enriched, chondrite-normalized patterns with strong positive Eu anomalies. However, such fluids are not net sources of REE to seawater owing to scavenging of REE by Fe oxyhydroxides formed on mixing of hydrothermal fluids with oxidized seawater. On the other hand, off-axis hydrothermal vent fluids may suffer removal of REE before emerging onto the sea floor. The REE geochemistry of continental geothermal waters shows a greater variety of patterns. However, pH is a major control on both absolute REE contents and chondrite-normalized patterns. Low-pH, acid-sulfate waters typically have the highest REE concentrations and most of the REE appear to be present in true solution. Many, but not all, such waters have a distinctive “gull's wing” chondrite-normalized pattern, in which La, Ce, and Pr are depleted with respect to host rocks. Near-neutral chloride- and bicarbonate-type waters have much lower overall REE concentrations, and filtered aliquots typically contain lower REE concentrations than unfiltered aliquots of the same sample, suggesting that the REE are present dominantly in or on suspended particles. Unfiltered and, to a lesser extent, filtered aliquots of these near-neutral waters commonly have chondrite-normalized patterns that closely parallel those of their reservoir rocks, indicative of minimal REE fractionation during water-rock interaction. However, some fluids exhibit strong positive Eu anomalies not necessarily seen in their reservoir rocks, possibly a result of water-rock interaction at higher temperatures (≥250°C) or under unusually reducing conditions. Finally, there is some evidence to suggest that REE are lost to solid phases upon vapor-liquid separation.
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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.