U-series constraints on melting mineralogy beneath ocean islands

U-series nuclides are fractionated during mantle melting, with a magnitude that is in part controlled by the dynamics of the melting process. This had lead to an interest in using U-series measurements in ocean island basalts (OIB) as a means of assessing mantle upwelling rates at hotspot localities. Yet Hawaii aside, there is only modest variability in buoyancy fluxes beneath ocean islands and instead we suggest that U- series measurements on OIB inform more on the mineralogy of the basalt source. This notion is based on the very different melting rates of the main mantle mineralogies that are invoked to be important components in heterogeneous OIB sources. In the simplest scenario, such an analysis ought to be able to distinguish between a pure pyroxenitic source, such as has recently been inferred from elevated Ni contents of primitive OIB olivines and a more conventional peridotitic mantle. The latter should have a lower melting rate and higher U-series disequilibria than the former. In order to make such as assessment, however, an additional key unknown, namely mantle porosity, needs to be constrained. Here we use combined 238U- 230Th and 235U-231Pa analyses to solve simultaneously for melting rate and porosity, thereby deriving the melt productivities and ultimately constraints on the mineralogy, for two OIB suites (Pico, Azores and La Palma, Canaries). Our results are consistent with melting of a peridotitic source, with melt productivities too low for a pure pyroxenite mineralogy. We suggest that higher melting rates that should be evident from a contribution of eclogitic heterogeneities are lost in a multi-stage stage melting process, during which initial, deep melts of eclogitic pods are dispersed into surrounding peridotite and essentially buffered by repeated melt-solid reaction.