Helium isotope systematics in the vicinity of the Azores triple junction: Constraints on the Azores geodynamics

Abstract

We present new He isotopic data from subaerial and submarine samples collected along the Terceira Rift (Azores Plateau) in order to constrain the sources involved in the Azores magmatism in the unique geodynamic context of the Azores triple junction. Lower than MORB 4He/3He isotopic ratios were determined on olivines from the Terceira Island and the adjacent Serreta submarine ridge (down to ≈69,000; ≈10.5 R/Ra), which in addition to the Ne isotopes previously published in Madureira et al. (2005) [Primitive helium and neon isotopes in Terceira Island: Constraints on the origin of the Azores archipelago. Earth and Planetary Science Letters 233, 429–440], support the geochemical models that consider the involvement of the lower mantle in the Azores magmatism. The analysis of He data published for the Mid-Atlantic Ridge (MAR) points to the existence of chemically distinct mantle domains underneath Eurasia and Nubia lithospheric plates, which can result from an heterogeneous distribution of asthenosphere metasomatism during the initial stages of plume activity in the Azores region. At the Terceira Rift, lavas from Graciosa Island, as well as from the western end of São Miguel Island and D. João de Castro Bank, display 4He/3He ratios similar to those observed along the MAR segments located to the north of the Azores triple junction area. Conversely, samples from the south Hirondelle Basin and also Faial display a He isotopic signature similar to that of MORB erupted along MAR segments located to the south of the Azores Plateau. The Terceira Rift and the Azores triple junction area are thus characterized by the mingling of two different asthenospheric domains referred as “Eurasia” and “Nubia” type and by the presence of mantle plume derived material having primitive helium signature.