Contrasting hydrothermal activity at Sierra Negra and Alcedo volcanoes, Galapagos Archipelago, Ecuador

Sierra Negra and Alcedo volcanoes are adjacent tholeiitic shields with summ it calderas located on Isabela Island in the Galapagos Archipelago. Althoug h basaltic eruptions have characterized the evolution of these volcanoes, A lcedo has produced minor volumes of rhyolite lavas and tephras dated at app roximately 100 ka. Fumaroles and ephemeral acid-sulfate seeps occur within the calderas of both volcanoes, whereas neutral-chloride and diluted steam- heated hot springs are absent. Fumarolic activity inside Sierra Negra (Mina Azufral) is fault controlled along the west margin of a horst and discharg e temperatures are less than or equal to 210 degrees C (January to February 1995). Water content of the total gas is approximately 75 mol.%, and nonco ndensible gases consist of approximately 97 mol.% CO2 and approximately 85% SO2 of the total sulfur gas. Relative amounts of He, Ar, and N-2 show a di stinct hot-spot signature (He-3/ He-4=17.4+/-0.3 R-A) The delta(13)C-CO2 is approximately -3.6 parts per thousand and delta(34)S(T) is approximately 3.3 parts per thousand. The delta D/ delta(18)O of fumarole H2O indicates s team separation from local meteoric waters whose estimated minimum mean res idence time from H-3 analyses is less than or equal to 40 years. Fumarolic activity at. Alcedo is controlled by a caldera-margin fault containing at l east: seven hydrothermal explosion craters, and by an intracaldera rhyolite vent. Two explosion craters which formed in 1993-1994 produce approximatel y 15 m(3)/s of steam, yet discharge temperatures are less than or equal to 97 degrees C. Water content of the total gas is 95-97 mol.%, noncondensible gas is 92-98 mol.% CO2, and sulfur gas is dominated by H2S. Relative amoun ts of He, Ar, and N-2 show extensive mixing between hot spot and air or air -saturated meteoric water components but the average >He-3/ He-4=15.5+/-0.4 R-A. The delta(13)C-CO2 is approximately -3.5 parts per thousand and delta (34)S(T) is approximately -0.8 parts per thousand. The delta D/ delta(18)O of fumarole steam indicates separation from a homogeneous reservoir that is enriched 3-5 parts per thousand in O-18 compared with local meteoric water . H-3 indicates that this reservoir water has a maximum mean residence time of approximately 400 years and empirical gas geothermometry indicates a re servoir temperature of 260-320 degrees C. The intracaldera hydrothermal res ervoir in Alcedo is probably capable of producing up to 150 MW; however, en vironmental concerns as well as lack of infrastructure and power users will limit the development of this resource.