VOLATILE FLUXES INTEGRATED OVER 4 DECADES AT GRIMSVOTN VOLCANO, ICELAND

Samples of the subglacial lake in the crater of the tholeiitic basalti c caldera Grimsvotn in Iceland were obtained by using a hot-water dril l to sink two boreholes through the 250-m- thick ice shelf covering th e 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 temperatu re ranges from O-degrees-C in the upper part to temperatures of 1-degr ees-C to 4-degrees-C near the bottom of the lake. The lake water pH ra nges between 7.0 and 5.7. The crater lake is assumed to be closed, wit h respect to volatile components released from subsurface magma, excep t 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 a nd 1991 and corrected these rates for dissolution of bedrock into the lake water and seepage of solutes to groundwater. The corrected mean r elease rates are 5.3 x 10(7) kg C yr-1, 5.3 X 10(6) kg S yr-1, 6.6 x 1 0(5) kg Cl yr-1, and 1.5 x 10(5) kg F yr-1. The emission rate estimate s for Grimsvotn, one of the most active volcanoes in Iceland, are the longest integrated estimates obtained for an active volcano and are eq ual to or lower than those of other major active volcanoes worldwide. This difference may imply that published release rates for other volca noes are overestimated, because they are usually not integrated over t ime. The values of the S/Cl and F/Cl ratios for noneruptive periods ar e 0.53 +/- 0.20 and 0.013 +/- 0.003, and for the two eruptive events a re 0.69 and 2.14, and 0.034 and 0.041, respectively. The response of t he elemental ratios to eruptive events, followed by the return to lowe r ratios, supports the assumption of steady state, because no long-ter m accumulation of volatiles occurs. The energy output from the volcano , estimated from the amount of ice melted by hydrothermal heat, is 425 0 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 Grim svotn, while a significant portion of available Cl and C are released. These calculations also reveal that coverage of a volcano by a glacie r, and subsequent raising of the water table, may cause significant sc rubbing of magmatic gases so that these gases do not reach the atmosph ere.