SEDIMENTS on mid-ocean ridges associated with ocean floor spreading and high heat flow are poor in Al, Ti, Si, Cu, Co, Ni and rare earth elements, and enriched in Fe, Mn, V, Cd, Hg, As, B, Cr, Ba and P, compared with other pelagic sediments. The origin of these distribution patterns is not fully understood but is most likely the result of volcanic processes associated with the degassing of the mantle under spreading ridges1–5.