He, Ar, Sr, Nd and Pb isotopes in volcanic rocks from Afar: Evidence for a primitive mantle component and constraints on magmatic sources

Abstract

He, Ar, Sr, Nd and Pb isotopic ratios have been determined for a set of submarine basaltic glasses from the Gulf of Tadjoura and subaerial lavas from Afar, Republic of Djibuti. Rare gases were recovered by vacuum crushing and analysed with a new analytical system. Helium isotopic ratios up to 15 Ra show the occurrence of a “high 3 He” hot spot beneath Afar and indicate the occurrence of a plume originating from a gas-rich, presumably deep, region of the mantle. He-Ar isotope systematics indicate a mixing between three geochemical end-members: the plume component, enriched in 3He, a radiogenic, possibly crustal, component, and the atmosphere. Low 40Ar/36Ar ratios are interpreted as the result of selective atmospheric contamination of Afar magmas, but the process of contamination is unclear. In a He-Sr-Nd-Pb space, Afar data are plotted in a field similar to basic lavas emitted in Iceland (Djibuti territory) and in the Reykjanes Ridge (Gulf of Tadjoura) and differ by their He isotopic ratios from lavas emitted in EM (e.g., Gough and Tristan da Cunha) and HIMU (e.g., Saint Helena and Tubuaii) hot spots analysed so far. © 1993, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.