Helium isotopic composition of fluid inclusions from two Ethiopian geothermal fields

He isotopic analyses have been carried out on the gas extracted from fluid inclusions which trapped geothermal fluids of the Aluto-Langano and Tendaho geothermal fields (Ethiopia). The results suggest an important He contribution to the past geothermal fluids from the high He-3 Ethiopian plume that also determined the high He-3/He-4 ratios of the present-day volcanic and geothermal gas and of the Ethiopian Rift and Afar basalts. The He-3/He-4 ratios of both present-day and past geothermal fluids were also influenced by crustal and/or upper mantle sources. The He/Ne ratios indicate a moderate or very low input of atmospheric He from meteoric waters. The exceptions are represented by a fluid discharged from a shallow-depth geothermal well and by the gas extracted from fluid inclusions of a shallow-depth sample from Tendaho, which show the lowest He/Ne ratios.