Hydrothermal Processes at Aluto-Langano (Ethiopia): Insights From the Stable Carbon Isotope Composition of Fluid Inclusions

Aluto-Langano is a high-temperature, liquid-dominated geothermal field located in the Lake District, Ethiopia, associated with the active rifting of the Main Ethiopian Rift. The system has been investigated since early 1980s’ for geology, present-day fluid composition, mineralogical alteration, and fluid inclusion systematics. In this contribution, we present new data on stable carbon isotope composition of fluid inclusions (δCΣC) that complement the set of information already published about microthermometry and noble gas composition (He isotope composition and He/Ne ratios) of fluid inclusions from deep wells. Large variations of δCΣC (–2.08 to –15.69‰ vs. V-PDB) point to the existence of different carbon sources and to the occurrence of complex processes during the migration of fluids from the roots of the system into the geothermal reservoir. A simplified, zero-dimensional, non-isothermal model, has been implemented to process the stable isotope composition of the fluid inclusions. The model accounts for different degrees of gas/liquid mixing, possible isotopic equilibration with hydrothermal calcite, and the entrainment of C-gases (CO2 and CH4) with variable carbon isotope signature. Numerical outputs suggest that the carbon signature of fluid inclusions is compatible with the existence of a two-phase, gas-rich fluid at the time of their formation, with PCO2 values and CH4/CO2 ratios likely buffered by “typical” Ca-Al-silicate-clay-calcite hydrothermal assemblages and Fe(II)-Fe(III) redox couple. Most negative δCΣC values were reproduced by accommodating in the model a significant contribution of thermogenic gases, possibly originated by thermal degradation of lacustrine sediments present in the local stratigraphic column. Introduction Exploration of geothermal resources in Ethiopia started in 1969 as a joint effort between the Ethiopian Government and the United Nations Development Program (UNDP, 1973). This large Figure 1. Map of the Main Ethiopian Rift (MER) with location of the Aluto-Langano geothermal area (Box a, red contoured square), of geothermal wells (yellow dots) and main faults (suspected and recognized; red traces) within the area of deep drillings (Box b). Box c shows a EW geological section of the geothermal field, with indication of main faults (red traces), well profiles (black downward arrows), and underground temperatures (orange contours; modified after ELC, 1986). Box b, additional features: WFB (Wonji Fault belt) is a NNE-SSW belt of Quaternary faulting and fissure volcanism, suspected to control the upflow of deep, geothermal fluids at Aluto-Langano. Box c, lithology details: 1 = pyroclastics and lava flows; 2 = lacustrine sediments; 3 = tuffs, breccias and basalts;