The Ne isotopic signature of Terceira lavas (Azores): evidence for Ne recycling?

Pedro Madureira pedro@uevora.pt (Departamento Geociências da Univ. de Évora/Centro de Geofísica de Évora, 7000-671 Évora, Portugal)
João Mata jmata@fc.ul.pt (GeoFCUL: Departamento de Geologia da Faculdade de Ciências da Univ. de Lisboa/ CeGUL:Centro de Geologia Univ. Lisboa, 1749-016 Lisboa, Portugal)
Manuel Moreira moreira@ipgp.jussieu.fr (Laboratoire de Géochimie et Cosmochimie, IPGP-Université Paris VII-Denis Diderot, 75005 Paris, France)

e-Terra
volume 5 - nº 11 - 2008

e-terra.geopor.pt

Key-words: Noble gas, Neon, Recycling, Olivine, Azores.

Abstract: The available noble gas database shows that the presence of an atmospheric component in oceanic basalts is common. Several models have been considered and will be discussed here in order to explain the Ne isotopic data obtained for olivine phenocrysts sampled in Terceira lavas. Analysed samples are porphyritic olivine+clinopyroxene±plagioclase bearing lavas that were erupted from the Fissural and Santa Bárbara eruptive vents. Different incompatible element ratios (e.g. Ba/Nb, La/Nb) were obtained for lavas from these two volcanic systems, which can be related with the presence of crustal recycled components in the Terceira mantle source. Ne isotopic ratios are also distinct for Fissural and Santa Bárbara olivine crystals, the later being invariably similar to air. Indirect barometric data from clinopyroxene-liquid equilibrium point to olivine crystallisation at mantle depths, thus precluding the existence of direct atmospheric contamination during magma ascent and cooling. Alternatively, we argue for a recycled origin of atmospheric Ne in accordance with geochemical data.

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