Partial melting and upwelling rates beneath the Azores from a U-series isotope perspective

We report U-series systematics (U-238-Th-230-Pa-231-Ra-226) for basalts from the Azores islands and the nearby mid-Atlantic ridge with the aim of constraining melting processes in this region of plume-ridge interaction. Both Th-230 and Pa-231 excess show rough negative trends with indices of source enrichment (Sr, Nd and Pb isotopes) which cannot be explained by melting of a source variably enriched by mafic components (e.g. pyroxenite). However, there is evidence for a distinct source component, possibly sediment, beneath Sao Miguel which may give rise to these trends. Pa-231 excess and Sm-Nd fractionation are positively and negatively correlated (respectively) with distance from the centre of the plume, while 230Th excesses show no clear trend. This suggests that mantle upwelling velocity exerts a strong control on U-Pa fractionation and that melting is initiated deeper in the centre of the plume. These observations are used to infer melting conditions beneath the mid-Atlantic ridge and Azores islands. The mantle upwelling velocity is constrained to be 3-4 cm/a which is smaller than that inferred for Hawaii or Iceland. Predicted Ra-226-excesses are similar to, or lower than those measured, suggesting high velocity melt ascent in channels and rapid differentiation prior to eruption. The effect of the presence of volatiles in the source is also investigated and we show that the presence of water could explain the Th-230 and Pa-231 trends as a function of distance across the plume. A 3-D model for the Azores plume can reproduce the U-series isotope observations. (c) 2005 Elsevier B.V. All fights reserved.