Helium and lead isotope geochemistry of the Azores Archipelago

Abstract

New helium and lead isotopic data for basalts from the Azores archipelago (North Atlantic) show that the Azores have 4He/ 3He ratios both higher and lower than MORB 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 São Miguel and Terceira. The eastern part of São Miguel island displays radiogenic helium ( 4He/ 3He > 140,000, R/ R a<5.1) and lead (20.00, 15.75 and 40.33 for 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb). The 207Pb/ 204Pb and 208Pb/ 204Pb ratios for São Miguel are unusually radiogenic for oceanic basalts. Terceira basalts contain relatively unradiogenic/primitive 4He/ 3He ratios, with a minimum value of 64,000 ( R/ R a=11.3), and relatively high lead isotopic ratios ( 206Pb/ 204Pb = 20.02, 207Pb/ 204Pb = 15.64 and 208Pb/ 204Pb = 39.35). We propose that the Terceira source has a composition produced by a mixing between recycled oceanic crust (high 206Pb/ 204Pb) and entrained lower mantle (high 3He) material. The São 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°N, reflecting plume–ridge interaction, whereas radiogenic ratios (>100,000) were observed at latitude higher than 40°N and may reflect the influence of the São Miguel component at the ridge.