Degassing pathways through the shallow magmatic-hydrothermal system of Poás

Abstract

We report results from a multidisciplinary campaign carried out at Poás crater-lake (Costa Rica) on 17-18 March 2009. Thermal imagery of fumaroles on the north side of the dome and the lake surface revealed mean apparent temperatures of 25-40°C (maximum of 80°C), and 30-35°C (maximum of 48°C), respectively. Mean radiative heat output of the lake, uncorrected for downwelling flux, was estimated as ~230 MW. The mean SO2 flux emitted by the crater measured by walking-traverses was 76 tonnes day-1, with approximately equal contributions from both the dome and the lake and fumarole plumes. Gas measurements by active open-path FTIR spectroscopy indicated molar ratios of H2O/SO2 = 151 and CO2/SO2 = 1.56. HCl and HF were not detected in measured spectra but based on the detection limits of these species, we calculate SO2/HCl > 40, and SO2/HF > 200. Particles were sampled from the plume by air filtration. The filters were analysed using ion chromatography, which revealed an abundance of K+ and SO42-, with smaller amounts of Ca2+, Mg2+ and Cl-. We discuss here the implications of the results for degassing pathways through the shallow magmatic-hydrothermal system.