Spreading-rate dependence of the extent of mantle melting beneath ocean ridges

Abyssal peridotites and mid-ocean-ridge basalts (MORBs) are complementary products of the mantle melting and melt-extraction processes that create the ocean crust. Studies of abyssal peridotites1–4 and MORBs2 have showed that the extent of mantle melting is high beneath hotspot-influenced shallow ridges, and is low beneath deep ridges away from hotspots. These results have led to the recognition of a global correlation of MORB composition with ridge depth5, and to the notion that mantle temperature variation exerts the primary control on the extent of melting beneath ocean ridges2,5–8. This conclusion is, however, based largely on data from slow-spreading ridges in the Atlantic and Indian oceans. At the fast-spreading East Pacific Rise (EPR), there is little correlation between MORB chemistry and ridge depth9,10, an observation that has proved puzzling8–12. Here we show that abyssal peridotites from the EPR13–21 are extremely depleted in basaltic major-element components—significantly more so than peridotites from ridges away from hotspots in the Atlantic and Indian oceanss—indicating that the EPR peridotites are residues of the highest extents of melting. These abyssal peridotite data and existing MORB major-element data12 both suggest that the extent of mantle melting beneath normal ocean ridges increases with increasing spreading rate.