Samples of the subglacial lake in the crater of the tholeiitic basaltic caldera Grimsvotn in Iceland were obtained by using a hot-water drill to sink two boreholes through the 250-m- thick ice shelf covering the lake. The lake generally shows an increase in solutes with increased depth, as solutes are added from the lake's bottom and dilute glacial meltwater is added continuously from above. The crater lake temperature ranges from O-degrees-C in the upper part to temperatures of 1-degrees-C to 4-degrees-C near the bottom of the lake. The lake water pH ranges between 7.0 and 5.7. The crater lake is assumed to be closed, with respect to volatile components released from subsurface magma, except for periodic draining by jokulhlaups. From the periodicity and water chemistry of the jokulhlaups, we have estimated the volcano's average release rates of carbon, sulfur, chlorine and fluorine between 1954 and 1991 and corrected these rates for dissolution of bedrock into the lake water and seepage of solutes to groundwater. The corrected mean release rates are 5.3 x 10(7) kg C yr-1, 5.3 X 10(6) kg S yr-1, 6.6 x 10(5) kg Cl yr-1, and 1.5 x 10(5) kg F yr-1. The emission rate estimates for Grimsvotn, one of the most active volcanoes in Iceland, are the longest integrated estimates obtained for an active volcano and are equal to or lower than those of other major active volcanoes worldwide. This difference may imply that published release rates for other volcanoes are overestimated, because they are usually not integrated over time. The values of the S/Cl and F/Cl ratios for noneruptive periods are 0.53 +/- 0.20 and 0.013 +/- 0.003, and for the two eruptive events are 0.69 and 2.14, and 0.034 and 0.041, respectively. The response of the elemental ratios to eruptive events, followed by the return to lower ratios, supports the assumption of steady state, because no long-term accumulation of volatiles occurs. The energy output from the volcano, estimated from the amount of ice melted by hydrothermal heat, is 4250 MW over the last four decades. Using the energy output to calculate magma solidification rates and maximum possible volatile release rates, we observe that emissions of F and S are strongly suppressed at Grimsvotn, while a significant portion of available Cl and C are released. These calculations also reveal that coverage of a volcano by a glacier, and subsequent raising of the water table, may cause significant scrubbing of magmatic gases so that these gases do not reach the atmosphere.
