| Abstract |
The fumarolic manifestations of Gebiba and Finkilo discharge gas mixtures practically free of air and Air-Saturated Water (ASW) with high CO2, relatively high CH4, and low H2S, H2, CO, He, Ar and N2, indicating that the deep CO2-rich gases do not interact with shallow aquifer(s) in these sites. Gas geothermometers indicate initial (reservoir) temperatures of 295-330°C for the Finkilo fumaroles and 200-250°C for the Gebiba fumaroles. Gas geothermometers and stable water isotopes indicate occurrence of vapor/liquid separation close to 100°C at Gebiba. Due to the possible occurrence of steam condensation, gas geothermometers do not constrain the vapor/liquid separation temperature of Finkilo fumarolic effluents. Stable water isotopes indicate vapor/liquid separation temperatures in the order of 155-170°C for the two best samples from Finkilo fumaroles. All the fumarolic manifestations other than Finkilo and Gebiba release gas mixtures dominated by ASW with low (or relatively low) CO2, low CH4, and undetectable H2S, H2, He and CO. The abundance of ASW in these fumarolic gases suggests that the deep CO2-rich gases interact with a shallow aquifer which is evidently in contact with the atmosphere. These fumarolic manifestations containing significant proportions of ASW and unsuitable for the application of gas geothermometers have the following vapor/liquid separation temperatures, based on stable water isotopes: 150-180°C for Kure, 150°C for Oitu, 135-95°C Bobessa, 100°C for Auto and ~95°C Hulo. Summing up, the highest vapor/liquid separation temperatures are found: (a) along the NNE-trending Jawe Fault, passing through Finkilo and possibly Oitu, and (b) along the NNE-trending tectonic structure passing through Kure and possibly Bobessa. Consequently, these tectonic structures locally act as main upflow zones of the Aluto-Langano geothermal system. In contrast, the peripheral fumarolic manifestations of Gebiba, Auto and Hulo have low vapor/liquid separation temperatures, indicating that they may represent lateral discharge zones of the Aluto geothermal system. |