FLUID-VOLCANO INTERACTION IN AN ACTIVE STRATOVOLCANO - THE CRATER LAKE SYSTEM OF POAS VOLCANO, COSTA-RICA

Seismic and geochemical data collected at Poas volcano, Costa Rica, since 1978 suggest that temperature and chemical variations recorded in subaerial fumaroles and the crater lake are related to episodic release of heat and volatiles associated with hydrofracturing of the upper margin of the shallow magma body. Power outputs associated with these events approach 600 MW and are superimposed on a baseline energy flux of approximately 200 MW. The baseline heat flux suggests a magma solidification rate of 0.012 km3/yr and background volatile release rates of 1000 t/d H2O, 66 t/d, S, 13 t/d Cl, and 0.5 t/d F. These fluxes are comparable to fluxes of F, Cl and S exiting the magmatic/hydrothermal system through acidic flank springs and are about a factor of five less than estimated volatile fluxes through summit fumaroles during the high-temperature event of 1981-1983. Mass balance considerations suggest that heat released at the lake surface is primarily supplied by the ascent of heated brines supplemented by condensation of fumarolic steam. Calculated average seepage rates out of the lake (almost-equal-to 450 kg/s) indicate rapid convection of acidic lake brine through underlying lake sediments and pyroclastic deposits. Circulation of the extremely corrosive lake brine through the volcanoclastic material beneath the lake may enhance subsurface permeabilities. Fluxes of rock-forming elements observed in the Rio Agrio basin on the northwest flank of Poas suggest that dissolution and removal of volcano-clastic material occurs at a rate of approximately 1650 m3/yr. However, estimated sulfur fluxes from the cooling magma body suggest that porosity created by this dissolution could quickly be filled by the deposition of native sulfur. Finally, analysis of heat and water budgets for the crater lake over the period 1978-1989 indicates that the recent decline and disapperance of the lake was caused by variations in summit rainfall combined with an increase in subsurface heat flow to the lake in 1987-1988. The increase in heat flow is related to continued degassing of high-temperature volatiles following a hydrofracturing/magma ascent episode that occurred in 1986.