Mantle Dynamics and Melting at the Ultraslow Spreading Terceira Axis, Azores.

The ultraslow spreading Terceira axis (full-spreading rate of 2-4mm/year) in the Azores exhibits an along-axis pattern of deep amagmatic basins and large volcanic centres comparable to the ultraslow Arctic Gakkel Ridge and Southwest Indian Ridge (SWIR). From west to east the volcanic centres of the Terceira axis include the islands of Graciosa and Terceira, the seamount João de Castro, and the island of São Miguel. In contrast to the Arctic Gakkel Ridge and the SWIR, the Terceira axis is situated within a large oceanic plateau that formed from a melting anomaly possibly associated with a hot- or wetspot. Lavas from the eastern volcanoes João de Castro and on São Miguel are more enriched in several incompatible elements (e.g. K2O) and have higher Th/U and Nd/Sm compared to Graciosa and Terceira. This trace element enrichment is correlated to an enrichment of the 208Pb/204Pb and 143Nd/144Nd isotope ratios suggesting a time-integrated enrichment of Nd and Th relative to Sm and U, respectively. Consequently, two different mantle domains underlie the Terceira axis and only limited mixing occurs between material beneath the eastern and western volcanic systems. The Terceira-Graciosa mantle material flows towards the Mid-Atlantic Ridge causing a geochemical anomaly at the spreading axis. The fractionation-corrected Terceira axis lavas define a negative trend in Na2O versus TiO2 contents, with low Na2O-high TiO2 at São Miguel and high Na2O-low TiO2 at Graciosa/Terceira. Modelling suggests that similar degrees of partial melting (2-3 %) of variably re-enriched depleted mantle sources can reproduce this negative trend. The FeOT and heavy rare earth elements ratios (e.g. Dy/Yb) are lower in the western lavas from Graciosa and Terceira than beneath the eastern volcanoes. On the other hand, Al2O3 contents are higher in the lavas from Graciosa and Terceira than in the eastern part of the Terceira axis. Thus, from west to east the depth of partial melting increases. Trace element modelling (e.g. Ce/Yb, Sm/Yb) suggests that the melting columns beneath Graciosa and Terceira are situated within the garnet/spinel transition zone, whereas the lavas from the eastern volcanoes were generated within the garnet peridotite stability field. The observed variation in melting depth corresponds to an increasing lithospheric age and hence lithospheric thickness to the east, i.e. away from the Mid-Atlantic Ridge. The lithospheric ages (~40 Ma) in the vicinity of the João de Castro volcanic system suggest a lithospheric thickness of approximately 70 km, well within the garnet stability field.