Origin of volcanic sulfur in Iceland : a sulfur isotope study

Sammanfattning: The chemistry and sulfur isotope composition have been determined in fresh and altered rocks and geothermal fluids in Iceland. Sulfur isotope ratios were analysed in 27 selected volcanic rocks, eight lava incrustations and three volcanic gas samples.The 63% of the basalts ranges from -2.0 o/oo to + 0.4 o/oo and shows little difference between the tholeiitic and alkaline rock suites. The few intermediate and acid rocks analysed exhibit heavier 634s values up to +4.2 °/oo. It issuggested that this distribution is due to either the processes that form siliceous magmas or to the degassing process. More than 90 % of the rocks in Iceland are basalts. The bedrock could therefore be estimated to show 63% of -0.8 o/oo, which equals the average 63% of the basalts. Isotopic equilibrium occurs in the magma prior to degassing but disequilibrium persists in the degassed rocks. Degassing is a disequilibrium process, as indicated by the large range in 63% of the volcanic gas samples.The small range of the 63% in Icelandic basalts and the similar 63% values of Icelandic and other oceanic basalts suggest that the mantle is homogeneous in its sulfur isotopic composition. The 63% of the mantle is suggested to be in the range of the oceanic basalts from -0.5 to +1.0 o/oo.The lava incrustations of sulfate composition are from 5 recent eruptions and occur as thenardite or as aphtithalite-thenardite mixtures. The incrustations show small sulfur isotope fractionation as compared with corresponding lavas and volcanic gas samples. The sulfate incrustations are formed through oxidation ofS02 from the emitted volcanic gas and subsequent reaction with metal halides. It is indicated that the sulfate incrustations serve as a late stage volcanic gas sample with respect to sulfur isotopes.The Icelandic geothermal fluids sampled are from eight geothermal systems, and cover a temperature range of 70-350 °C, a Cl- concentration range of IQ- 28000 ppm and a ô3% range of sulfide from -10.9 to +3.0 °/oo and of sulfate from -1.0 to +21.2 °/oo.As most of the samples are hot springs, shallow drillholes and mixed waters, the chemistry of the fluids is slightly modified through reactions in the upflow zones. The chemical composition suggests that several fluid-mineral equilibria are attained, although some components seem to be limited by slow leaching rates.The sulfur isotope fractionation between sulfide and sulfate suggests chemical and isotopic disequilibrium between sulfide and sulfate in all geothermal systems investigated. Sulfate reduction occurs in the high temperature fluids; however, sulfur isotope exchange is lacking in the low temperature fluids.With a few exceptions, leaching of basaltic bedrock constitutes the dominant process and source of sulfur in the geothermal systems, although the ô3% 0f the fluid could vary between -1 and +11 °/oo. Both pulsative and continuous influxes of magmatic gas occurred at Krafla volcanic centre in N. Iceland during the rifting episode in 1975-1984. Influxes of marine sulfate occur in the high temperature systems at the Reykjanes peninsula and in the low temperature systems located close to the coast at low altitude, as indicated by correlations of increasing Cl- with increasing 63%. The 63% values in the systems are further modified during mixing and recirculation of oxidized high temperature sulfur allowing the large deviations observed in the measured 63%.The 63% of the high temperature fluids changes rapidly in response to reactions in the systems; but the 63% would not change much through time in the low temperature waters. The ô3% could easily be used to trace the origin of sulfur in the low temperature systems.