Water-rich basalts at mid-ocean-ridge cold spots

Although water is only present in trace amounts in the suboceanic upper mantle, it is thought to play a significant role inaffecting mantle viscosity, melting and the generation of crust atmid-ocean ridges. The concentration of water in oceanic basaltshas been observed to stay below 0.2wt%, except for water-richbasalts sampled near hotspots and generated by ‘wet’ mantle plumes. Here, however, we report unusually high water contentin basaltic glasses from a cold region of the mid-ocean-ridgesystem in the equatorial Atlantic Ocean. These basalts aresodium-rich, having been generated by low degrees of meltingof the mantle, and contain unusually high ratios of light versusheavy rare-earth elements, implying the presence of garnet in the melting region. We infer that water-rich basalts from suchregions of thermal minima derive from low degrees of ‘wet’melting greater than 60 km deep in the mantle, with minor dilution by melts produced by shallower ‘dry’ melting—a viewsupported by numerical modelling. We therefore conclude thatoceanic basalts are water-rich not only near hotspots, but also at‘cold spots’.