UTh isotope disequilibria and ocean island basalt generation in the Azores

A suite of 23 alkalic lavas from the Azores islands of Sao Jorge, Pico, Faial, Terceira and São Miguel have been analysed for their major and tram element and Sr, Nd, Pb and Th isotopic compositions. Previous studies have shown that lavas along the MAR show increases in La/Sm and 87Sr/86 Sr coupled with decreasing Fe towards the Azores platform inferred to reflect increasing dominance of a plume component over a depleted MORB-mantle component. The La/Sm and Sr, Nd and Pb isotope data of lavas from São Jorge, Pico, Faial and Terceira overlap the high La/Sm, 87Sr/86Sr end of the MAR lava array inferred to represent the deep seated plume and they have low Fe8.0∗ compared with for example, Iceland basalts. When compared with the MAR array, higher Fe8.0∗ and Na8.0 and lower Si8.0 probably reflect lower degrees of melting at higher average pressures beneath a lithospheric lid. The combination of enrichment in incompatible elements (including volatiles) with depletion in Fe in the regional plume component is a feature most likely to have developed within the lithospheric mantle. Lavas from São Miguel require an additional high 87Sr/86Sr, 207Pb/204Pb, low 143Nd/144Nd component with high Th/Nb which is suggested to result from incorporation of sediment in the source of these lavas. UTh isotope data indicate 9–29% 230Th excesses similar to those observed in alkalic basalts from Iceland or Hawaii. This, combined with the observation that lavas from Sao Miguel at the periphery of the inferred plume have smaller 230Th excesses (average 14%) than those nearer the centre, including the recent measurements from the MAR-ridge (average 23%), suggests that there is no simple correlation between buoyancy flux or lithospheric lid thickness and 230Th excesses. Long-term source Th/U ratios inferred from Pb isotopes are greater than those measured indicating U addition some time prior to partial melting. The large variations in (230Th/232Th) and (238U/232Th) are negatively correlated with 87Sr/86Sr indicating that much of the variation in U/Th reflects long-lived source heterogeneity. Moreover, the degree of 230Th excess is also negatively correlated with 87Sr/86Sr suggesting a link between melting process and source heterogeneity. The variations in 230Th excess are unlikely to result from thermally controlled variations in solidus pressure since this would require temperature differences (250°C) that are improbable at the length scale between São Miguel and Terceira (200 km). Instead, it is suggested that the incorporation of sediment and volatiles beneath São Miguel has led to increased rates of melting, resulting in smaller 230Th excesses, consistent with the higher volcanic output on Sao Miguel compared with Terceira.