Helium and neon isotopes in Sao Miguel island basalts, Azores Archipelago: New constraints on the "low 3He" hotspot origin

Lavas from the Sao Miguel Island, Azores Archipelago, have peculiar isotopic compositions, including radiogenic lead and strontium and un-radiogenic neodymium. The peculiar isotopic trend of Sao Miguel is evident in the lead-lead diagram where both Pb-207/Pb-204 and Pb-208/Pb-204 ratios are high for a given Pb-206/Pb-204 ratio compared to other oceanic island basalts. This signature is unique among OIBs and is particularly evident in the Nordeste area, the oldest part of Sao Miguel island (>= 1 Ma). Only a few olivine samples from the Nordeste volcanic complex have been analyzed for helium. They show radiogenic helium signatures with He-4/He-3 up to 174,000 (R/Ra similar to 4) [Moreira et al., Helium and lead isotope geochemistry in the Azores archipelago, Earth Planet. Sci. Lett. 169: 189-205, 1999]. However, because the Nordeste volcano has an age between 1 and 4 Ma and because these samples have low helium concentrations, these radiogenic helium isotopic ratios must be considered with caution as they can also reflect post eruptive radiogenic production. In this paper we present a detailed study of the helium and neon isotopic ratios obtained from 17 Nordeste samples in order to better constrain the helium isotopic signature of the Sao Miguel mantle source. By coupling helium and the other isotopic systems, we propose that the Sao Miguel source contains non-degassed material, enriched in U and Th, that was stored in the mantle for the last similar to 3 Ga. As suggested by Elliot et al. [Elliott et al., The origin of enriched mantle beneath Sao Miguel, Azores, Geochim. Cosmochim. Acta 71: 219-240, 2007], underplated magma intruded into oceanic lithosphere and subducted similar to 3 Ga ago is a possible explanation for the peculiar Sao Miguel source isotopic signatures. (C) 2012 Elsevier B.V. All rights reserved.