The speciation of aqueous dissolved sulfur was determined in hydrothermal waters in Iceland. The waters sampled included hot springs, acid-sulfate pools and mud pots, sub-boiling well discharges and two-phase wells. The water temperatures ranged from 4 to 210°C, the pHT was between 2.20 and 9.30 at the discharge temperature and the SO4 and Cl concentrations were 0.020-52.7 and <0.01-10.0mmolkg-1, respectively. The analyses were carried out on-site within ~10min of sampling using ion chromatography (IC) for sulfate (SO42-), thiosulfate (S2O32-) and polythionates (SxO62-) and titration and/or colorimetry for total dissolved sulfide (S2-). Sulfite (SO32-) could also be determined in a few cases using IC. Alternatively, for few samples in remote locations the sulfur oxyanions were stabilized on a resin on site following elution and analysis by IC in the laboratory. Dissolved sulfate and with few exceptions also S2- were detected in all samples with concentrations of 0.02-52.7mmolkg-1 and <1-4100μmolkg-1, respectively. Thiosulfate was detected in 49 samples of the 73 analyzed with concentrations in the range of <1-394μmolkg-1 (S-equivalents). Sulfite was detected in few samples with concentrations in the range of <1-3μmolkg-1. Thiosulfate and SO32- were not detected in <100°C well waters and S2O32- was observed only at low concentrations (<1-8μmolkg-1) in ~200°C well waters. In alkaline and neutral pH hot springs, S2O32- was present in significant concentrations sometimes corresponding to up to 23% of total dissolved sulfur (STOT). In steam-heated acid-sulfate waters, S2O32- was not a significant sulfur species. The results demonstrate that S2O32- and SO32- do not occur in the deeper parts of <150°C hydrothermal systems and only in trace concentrations in ~200-300°C systems. Upon ascent to the surface and mixing with oxygenated ground and surface waters and/or dissolution of atmospheric O2, S2- is degassed and oxidized to SO32- and S2O32- and eventually to SO42- at pH >8. In near-neutral hydrothermal waters the oxidation of S2- and the interaction of S2- and S0 resulting in the formation of Sx2- are considered important. At lower pH values the reactions seemed to proceed relatively rapidly to SO42- and the sulfur chemistry of acid-sulfate pools was dominated by SO42-, which corresponded to >99% of STOT. The results suggest that the aqueous speciation of sulfur in natural hydrothermal waters is dynamic and both kinetically and source-controlled and cannot be estimated from thermodynamic speciation calculations.
|Number of pages||15|
|Journal||Geochimica et Cosmochimica Acta|
|Publication status||Published - 15 May 2011|
Bibliographical noteFunding Information:
The present study was supported by the Research Fund of the University of Iceland and the Science Fund of the National Research Council, Iceland, with grants to A. Stefánsson, and a grant from the Nordic Volcanological Center awarded to H. Kaasalainen. We are greatful to I. Gunnarsson for help in the field and laboratory and to S. Arnórsson for some initial discussion on the subject. We also wish to thank A. Migdisov, two anonymous reviewers and associate editor L. Benning for their suggestions and comments, which significantly improved this paper.