Fumarole migration and fluid geochemistry at Poas volcano (Costa Rica) from 1998 to 2001

We report the results of a geochemical survey of fumaroles, thermal springs, and gas discharges from areas in and around the active crater lake of Pods volcano (Costa Rica) from February 1998 to February 2001. The springs are highly acidic-sulphate waters with temperatures approaching boiling point, whereas gas chemistry is characterized by typical magmatic species, such as SO2, HF HCl, H-2, and CO. From February 1998 new fumarolic fields formed inside the southern part of the crater. They moved anticlockwise from the S to the NE inner walls of the crater, while those located in the southern part of the crater and close to the pyroelastic cone south of the crater lake diminished or disappeared altogether, during 1999 and 2000. This shift was also characterized by chemical variation of the magmatic gas species. In spite of the chemical changes of fumaroles, the composition of the lake changed little during this time. This fact, together with the chemical profile with depth of the lake, suggests that the lake is a very efficient condenser of magmatic fluids. An apparent chemical stratification of the lake suggests that dilution with meteoric water is not complete, due to the presence of liquid sulphur at the lake bottom and/or due to the continuous influx of new magmatic components.