He-CO2-N2 Isotope and Relative Abundance Characterization of Geothermal Fluids from the Ethiopian Rift

Abstract

We report He-CO2-N2 isotopic and relative abundances in free gases and dissolved gas phase of geothermal fluids from the Ethiopian Rift. Fluid samples were collected from ~30 geothermal localities from three key regions throughout rifted and non-rifted areas of Ethiopia. The majority of samples, including off-rift samples, indicate a strong contribution of mantle-derived He-C-N to the fluid samples. Helium (3He/4He) and 15N-(N2) isotope anomalies are highest ( more than 15.9RA and more than +5.0‰, respectively) at a single locality in south Afar (Sodere), but the maximum 13C-(CO2) (-0.78‰) is found east of Lake Shalla in the Lake District of the Main Ethiopian Rift. High 3He/4He values, consistent with mantle plume contributions, are also evident in fluids from the Lake District, where fluids from the Lake Shalla site extend up to 15.5RA. CO2/3He values span over four orders of magnitude while δ13C-(CO2) values cluster mostly between mantle-like values of -4 and -7‰; only samples east of Lake Shalla display more positive values. Atmospheric-derived nitrogen has likely influenced a number of measured 15N-(N2) values but following a correction for atmospheric-contamination, the majority of samples reveal positive values (up to 6.5‰) which appear to be coupled to high 3He/4He values. In regions affected by upwelling mantle plumes, such high values have been interpreted to reflect deep mantle inputs of recycled nitrogen.