Chemistry of waters from Furnas volcano, Sمo Miguel, Azores: fluxes of volcanic carbon dioxide and leached material

The well-constrained hydrology of the Furnas caldera permits the quantification of the precipitation, evapotranspiration, runoff and the groundwater discharge out of the volcanic structure. Chemical composition of the discharge in Furnas has been stable for at least a century. A hydrogeochemical survey and discharge measurement of the springs and runoff in Furnas allow estimates of chemical fluxes from the volcanic structure. A tectonically controlled lineament of high carbon dioxide flux has been identified across the caldera floor. Three water types are encountered in Furnas: hydrothermal, carbonated and cold waters. The hydrothermal waters originate from shallow aquifers which derive their heat from local intrusions at shallow depth (100–200 m) at about 160°C. The carbonated waters, some of which are steam heated by discharge from underlying thermal aquifers occurs above the carbon dioxide anomaly across the caldera floor. Extensive leaching, in some cases stoichiometric, of glassy volcanic rocks by these mildly acid carbonated waters supplies most of the dissolved solids in Furnas waters. Chemical flux estimates for the volcano show that carbon dioxide (9358 tons/yr), leached silica (3994 tons/yr) and leached sodium (2628 tons/yr) are the dominating mobile components of the system.