An in situ method for measuring CO 2 flux from volcanic-hydrothermal fumaroles

Abstract

The theory of an in situ method to measure CO 2 flux from hydrothermal fumarolic vents using a tracer gas technique is established and applied to vents and wells at three sites: Izu-Oshima (Japan), Kirishima (Japan) and Teide (Canary Islands, Spain) volcanoes. The method is done by releasing the tracer gas into the vent at known flux and by measuring CO 2 to the tracer gas ratio in situ. In Izu-Oshima, we carried out flux measurements at a low-temperature steam well (90°C) with three different tracer gas species (H 2, CH 4 and SF 6) and three methods (GC, portable analyzers, FTIR) to evaluate this method. The calculated CO 2 flux showed an average of 2.3 t/day. In the case of Kirishima volcanic area, the flux measurement was tested at two low-temperature fumaroles on Iwoyama cone (<100°C). The CO 2 fluxes were 9×10 −2 and 2×10 0 kg/day, respectively. The total CO 2 flux from the cone including both fumarolic and ground CO 2 discharges was estimated to be less than several tons per day. In Teide volcano, CO 2 flux measurements were performed in the fumarolic system of the summit crater (85°C). CO 2 flux from one fumarole reached levels up to 30.2 kg/day. We estimated that the CO 2 discharge from all fumaroles is roughly 30 t/day or less at maximum. These results imply that the total output of CO 2 flux from Teide's fumarolic activity is much lower than the observed levels of CO 2 diffuse degassing from the summit crater.