A heterogeneous subcontinental mantle under the African-Arabian plate boundary revealed by boron and radiogenic isotopes

Abstract

The north and northwest margins of the Arabian plate are locus of a diffuse and long-lasting (early Miocene to Pleistocene) Na-alkali basaltic volcanism, sourced in the asthenosphere mantle. The occurrence and 20-Ma persistence of magma supply under the study area is explained considering that upwelling asthenosphere produces very limited magma volumes in the axial zone and portions of hot, fertile mantle continue their eastward migration and are stored at shallower depths under the 100-km thick Arabian lithosphere, which is much thinner than the African one (≈175 km). Erupted basalts sampled a continuous variation of the mantle source, with a striking correlation among potential temperature, pressure and isotopic composition shifting between two end members: a 140 km-deep, more depleted source, and a 100 km-deep, more enriched one. In particular, we observed an unusual variation in boron isotopes, which in theory should not vary between more depleted and more enriched domains. This study shows that subcontinental mantle is more heterogeneous than the suboceanic one, and able to record for very long times recycling of shallow material.