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 observed at the scale of the archipelago, and suggest the coexistence of two mantle components in the Azores which are identified by data from Sao Miguel 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 basalts. 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) and entrained lower mantle (high He-3) material. The Sao Miguel island isotopic signature may be due to sampling of local (km-size) heterogeneity located at relatively shallow depth. The preferred origin of this heterogeneity is the Jurassic delamination of subcontinental lithosphere, which occurred during rifting and opening of the North Atlantic. The primitive helium ratios were also observed on the Mid Atlantic ridge at 38.5 degrees N, reflecting plume-ridge interaction, whereas radiogenic ratios (>100,000) were observed at latitude higher than 40 degrees N and may reflect the influence of the Sao Miguel component at the ridge. (C) 1999 Elsevier Science B.V. All rights reserved.