Contrasting hydrothermal activity at Sierra Negra and Alcedo volcanoes, Galapagos Archipelago, Ecuador
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 100ka. 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 <=210°C (January to February 1995). Water content of the total gas is approximately 75mol.%, and noncondensible gases consist of approximately 97mol.% CO2 and approximately 85% SO2 of the total sulfur gas. Relative amounts of He, Ar, and N2 show a distinct hot-spot signature ( ). The δ13C-CO2 is approximately -3.6‰ and δ34ST is approximately +3.3‰. The δD/δ18O of fumarole H2O indicates steam separation from local meteoric waters whose estimated minimum mean residence time from 3H analyses is <=40years. 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 15m3/s of steam, yet discharge temperatures are <=97°C. Water content of the total gas is 95-97mol.%, noncondensible gas is 92-98mol.% CO2, and sulfur gas is dominated by H2S. Relative amounts of He, Ar, and N2 show extensive mixing between hot spot and air or air-saturated meteoric water components but the average . The δ13C-CO2 is approximately -3.5‰ and δ34ST is approximately -0.8‰. The δD/δ18O of fumarole steam indicates separation from a homogeneous reservoir that is enriched 3-5‰ in 18O compared with local meteoric water. 3H indicates that this reservoir water has a maximum mean residence time of approximately 400years and empirical gas geothermometry indicates a reservoir temperature of 260-320°C. The intracaldera hydrothermal reservoir in Alcedo is probably capable of producing up to 150MW however, environmental concerns as well as lack of infrastructure and power users will limit the development of this resource.
- Publication:
-
Bulletin of Volcanology
- Pub Date:
- 2000
- DOI:
- 10.1007/s004450050289
- Bibcode:
- 2000BVol...62...34G