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

The well-constrained hydrology of the Furnas caldera permits the quantifica tion of the precipitation, evapotranspiration, runoff and the groundwater d ischarge out of the volcanic structure. Chemical composition of the dischar ge in Furnas has been stable for at least a century. A hydrogeochemical sur vey and discharge measurement of the springs and runoff in Furnas allow est imates of chemical fluxes from the volcanic structure. A tectonically contr olled Lineament of high carbon dioxide flux has been identified across the caldera floor. Three water types are encountered in Furnas: hydrothermal, c arbonated and cold waters. The hydrothermal waters originate from shallow a quifers which derive their heat from local intrusions at shallow depth (100 -200 m) at about 160 degrees C. The carbonated waters, some of which are st eam heated by discharge from underlying thermal aquifers occurs above the c arbon dioxide anomaly across the caldera floor. Extensive leaching, in some cases stoichiometric, of glassy Volcanic rocks by these mildly acid carbon ated waters supplies most of the dissolved solids in Furnas waters. Chemica l flux estimates for the volcano show that carbon dioxide (9358 tons/yr), l eached silica (3994 tons/yr) and leached sodium (2628 tons/yr) are the domi nating mobile components of the system. (C) 1999 Elsevier Science B.V. All rights reserved.