HELIUM ISOTOPE RATIOS IN ETHIOPIAN RIFT BASALTS

Helium isotope ratios were measured in olivine and pyroxene phenocryst s from basalts of the Ethiopian Rift Valley and Afar Depression betwee n 6 degrees and 15 degrees N and 37 degrees and 43 degrees E. He-3/He- 4 ratios range from 6 to 17 times the atmospheric value (R(A) = 1.4 x 10(-6)), that is, from ratios less than typical MORE (depleted mantle) helium (R/R(A) = 8 +/- 1) to ratios similar to high-He-3 hotspots and to the Yellowstone hotspot (R/R(A) = 16.5). The high He-3/He-4 ratios occur all along the Ethiopian Rift and well up into the Afar Depressi on, with a maximum value of 17.0 R(A) at 8 degrees N in the Rift Axis and a high value of 14.2 R(A) in the central Tat'Ali sector of the Afa r Depression. The ratios decrease to MORB-like values near the edge of the Red Sea, and to sub-MORE ratios (5-6 R(A)) at the northern end of the Rift (Zula Peninsula) and at the southern end, at lakes Abaya and Chamo. The Ethiopian Rift provides the only continental hotspot terra in in which helium isotope ratios can be compared in detail between vo lcanic lavas and associated geothermal and volcanic gases, a primary m otivation for this work. Comparison with our previously measured ratio s in fluids and gases (range 2-15 R(A)) shows excellent agreement in t he areas sampled for both lavas and fluids, and indicates that high-te mperature volcanic fluids can be used for establishing helium isotope signatures in such terrains. The high-He-3 values in both fluids and b asalts show that a Primitive Mantle (PM) component is required and tha t a Lower Mantle High-He-3 plume is strongly involved as a driving for ce in the rifting process of the East African Rift System.