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 in affecting mantle viscosity, melting and the generation of crust at mid-ocean ridges. The concentration of water in oceanic basalts1,2 has been observed to stay below 0.2 wt%, except for water-rich basalts sampled near hotspots and generated by ‘wet’ mantle plumes3,4,5. Here, however, we report unusually high water content in basaltic glasses from a cold region of the mid-ocean-ridge system in the equatorial Atlantic Ocean. These basalts are sodium-rich, having been generated by low degrees of melting of the mantle, and contain unusually high ratios of light versus heavy rare-earth elements, implying the presence of garnet in the melting region. We infer that water-rich basalts from such regions 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 view supported by numerical modelling. We therefore conclude that oceanic basalts are water-rich not only near hotspots, but also at ‘cold spots’.