Helium and lead isotope geochemistry of the Azores Archipelago

New helium and lead isotopic data for basalts from the Azores archipelago ( North Atlantic) show that the Azores have He-4/He-3 ratios both higher and lower than MORE values. Good covariations of helium and lead isotopes are o bserved at the scale of the archipelago, and suggest the coexistence of two mantle components in the Azores which are identified by data from Sao Migu el and Terceira. The eastern part of Sao Miguel island displays radiogenic helium (He-4/He-3 > 140,000, R/R-a < 5.1) and lead (20.00, 15.75 and 40.33 for Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204). Th, Pb-207/Pb-204 and Pb-208/Pb-204 ratios for Sao Miguel are unusually radiogenic for oceanic ba salts. Terceira basalts contain relatively unradiogenic/primitive He-4/He-3 ratios, with a minimum value of 64,000 (R/R-a = 11.3), and relatively high lead isotopic ratios (Pb-206/Pb-204 = 20.02, Pb-207/Pb-204 = 15.64 and Pb- 208/Pb-204 = 39.35). We propose that the Terceira source has a composition produced by a mixing between recycled oceanic crust (high Pb-206/Pb-204) an d entrained lower mantle (high He-3) material. The Sao Miguel island isotop ic signature may be due to sampling of local (km-size) heterogeneity locate d at relatively shallow depth. The preferred origin of this heterogeneity i s the Jurassic delamination of subcontinental lithosphere, which occurred d uring rifting and opening of the North Atlantic. The primitive helium ratio s were also observed on the Mid Atlantic ridge at 38.5 degrees N, reflectin g plume-ridge interaction, whereas radiogenic ratios (>100,000) were observ ed at latitude higher than 40 degrees N and may reflect the influence of th e Sao Miguel component at the ridge. (C) 1999 Elsevier Science B.V. All rig hts reserved.