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

摘要

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 (δ~(13)C_(∑C)) 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 δ~(13)C_(∑C) (-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 (CO_2 and CH_4) 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 P_(CO2) values and CH_4/CO_2 ratios likely buffered by "typical" Ca-Al-silicate-clay-calcite hydrothermal assemblages and Fe(Ⅱ)-Fe(Ⅲ) redox couple. Most negative δ~(13)C_(∑C) 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.