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

Citation:
Goff, F, McMurtry GM, Counce D, Simac JA, Roldan-Manzo AR, Hilton DR.  2000.  Contrasting hydrothermal activity at Sierra Negra and Alcedo volcanoes, Galapagos Archipelago, Ecuador. Bulletin of Volcanology. 62:34-52.

Date Published:

May

Keywords:

Alcedo, carbon relationships, Galapagos, gas geochemistry, gases, geothermal system, geothermal systems, isotope geochemistry, isotope ratios, kilauea-volcano, mantle plume, mount st-helens, new-mexico, Sierra Negra, valles caldera, volcanic gases, waters

Abstract:

Sierra Negra and Alcedo volcanoes are adjacent tholeiitic shields with summit calderas located on Isabela Island in the Galapagos Archipelago. Although basaltic eruptions have characterized the evolution of these volcanoes, Alcedo has produced minor volumes of rhyolite lavas and tephras dated at approximately 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 discharge 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 noncondensible 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 distinct 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 steam separation from local meteoric waters whose estimated minimum mean residence 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 least: seven hydrothermal explosion craters, and by an intracaldera rhyolite vent. Two explosion craters which formed in 1993-1994 produce approximately 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 amounts 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 reservoir temperature of 260-320 degrees C. The intracaldera hydrothermal reservoir in Alcedo is probably capable of producing up to 150 MW; however, environmental concerns as well as lack of infrastructure and power users will limit the development of this resource.

Notes:

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DOI:

10.1007/s004450050289