Transport of Elements By Geothermal And Volcanic Vapor
Abstract
Volatile species provide information on physical and chemical origin and processes in volcanic geothermal systems. Volatile metals and metalloids may be transported by both liquid and vapor phases that are often present in these systems, and some elements (e.g. As, Hg, Pb) may even be enriched in the vapor phases. The speciation and transport mechanism, as well as the origin of the elements in vapor is often poorly understood. Our aim is to characterize elemental concentrations and transport mechanism in the vapor phase in active geothermal systems. Samples of geothermal and volcanic vapor were collected from two-phase well discharges from Iceland and high temperature fumaroles in Vulcano Island and Solfatara field in Italy. Vapor and liquid phases were analyzed for major components and trace elements. Single phase volcanic vapor discharges had temperatures between 144 and 407°C. Their composition was dominated by H2O, but contained significant amounts of CO2 and acid gases (SO2, HCl, HF) whereas also H2S, H2 and N2 were present. In geothermal steam, with discharge pressures up to 42 bars, acid gases were absent and the steam consisted of 99% H2O, the rest being CO2, H2S, N2, and H2. Volcanic vapor condensates were analyzed for over 50 trace elements. Precipitation of condensate samples upon sampling and storage is commonly encountered problem. Different sampling protocols and sample treatments were tested to avoid and study their effects on the measured concentrations. The measured concentrations of some elements (e.g. Mg, Al, Sb) were found to show significant differences depending on the treatment, where as others (e.g. B, Ni, Zn) were not affected. In volcanic vapor, metals and metalloids were found in concentrations from lower ppb level up to tens of ppm. B shows highest concentrations followed by e.g. As, Pb, Zn, Mn, Ni, and Cu. In two phase well discharges, however, metals and metalloids were observed to be depleted in vapor compared to the liquid phase. Nonetheless, volatiles including B were observed in elevated concentrations (5.5-345 ppb) in the steam phase, and others including As, Hg, Cu, were found in significant measurable concentrations (0.01-2 ppb) and sometimes enriched in the steam phase. Compared to B, which behaves like a true volatile in volcanic vapor, other elements are depleted compared to the host rocks. Together with literature data, these data indicates that temperature variations of a given element within a given area are limited. Thermochemical modeling suggests the trace elements to be transported in the volcanic vapor mainly as chloride species, but fluorides, oxychlorides, sulfur species and elemental gas may also be of importance for example for Al, Ti, As, and Sn transport.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V51G..04K
- Keywords:
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- 1034 GEOCHEMISTRY / Hydrothermal systems;
- 8430 VOLCANOLOGY / Volcanic gases