INVESTIGACIONES GEOFISICAS Y CARACTERISTICAS GEOQUIMICAS DE LAS AGUAS HIDROTERMALES: VOLCAN POAS, COSTA RICA

A geophysical and geochemical preliminar research has been carried out in the active caldera of Poas volcano during 1980-82. The water of the intra-caldera lake has a very strong acidity (pH <1) due to the high content of Cl-and SO 4 -2 . The concentration of metal cations is considerably high as well. Temperature calculated (T = 229°C), by means of chemical geothermometer for the mixing of phreatic waters with hot fluids, are in the range of those detected in geothermal systems associated to andesitic rocks. The temperature increase from 92°C (December, 1980) to 960°C (March-November, 1981) in the N-E section of the lava dome seems to be related to the fracturing (probably connected to the seismic crisis of July, 1980) of the upper plug of the cooling magma body at depth. This mechanism induced the exolution of relatively high temperature gaseous phases from below. Near surface oxidation phenomena might have been effective, thus contributing to increase the temperature of gases. Magnetic measurements confirm that the temperature increase was superficial and confined within a small area surrounding the central dome. In the light of the data hereby presented we suggest that the local temperature increase (from 92°C to 960°C) does not have any implication with magma ascent.