Isotopic Characterization of Lavas from the Great Meteor, Hyeres and Plato Seamounts: Influence of the Azores Mantle Anomaly

Dredging operations were performed south of the Azores Archipelago, during LEG 2 of cruise EMEPC/Açores/G3/2007, and lava samples (basalts s.l.) were recovered on the Great Meteor, Hyeres and Plato seamounts (Lourenço et al., 2008). The resulting new isotopic data (Sr, Nd, Pb) is reported and compared with the existing isotopic signature of the Azores Region. The studied lavas exhibit porphyritc and vesicular textures and a phenocryst assemblage of olivine, clinopyroxene, plagioclase and oxides in variable proportions, which are immersed in a microcrystalline matrix dominated by plagioclase and oxides. The samples with MgO > 6% are alkaline basalts and basanites, presenting enrichment in LREE, with average (La/Sm)n ratios of 2.4 and (La/Yb)n ratios ranging between 8.1 and 15.1 (Madureira et al., 2008) . Also spiderdiagrams evidences Nb and Ta enrichment over LILE, typical of OIB with HIMU affinity (Weaver, 1991). The Sr and Nd isotopic ratios fall within the Azores isotopic signature, in particular that of the central island group. Such ratios follow a compositional trend extending from enriched MAR basalts towards lavas with a stronger HIMU component. The lead isotopes plot below the NHRL, with 7/4 ranging between -1.45 and -5.72 and 8/4 between -1.0 and -8.63. It is also noteworthy, that the 206Pb/204Pb isotopic ratio increases with latitude (19.44-19.87 in Great Meteor basalts to 20.11-20.17 in Plato samples). The increase in 206Pb/204Pb isotopic ratio with latitude is interpreted as the result of an increase of the influence of the Azores mantle domain (White et al. 1976) in agreement with Gente et al., (2003) proposal of a long-lived interaction (85 Ma) between the MAR and the Azores enriched mantle source, with the migration of the later towards NNE. Lourenço et al. 2008 GRA, vol. 10 Gente et al., G3, vol. 10. Madureira et al., 2008 GRA, vol. 10. Weaver, B.L. 1991 EPSL 104, 381-397. White et al. 1976 Nature, 263.