The hafnium paradox and the role of garnet in the source of mid-ocean-ridge basalts

MID-OCEAN-RIDGE basalts (MORBs) are thought to result from melting in the mantle at depths of less than 60 km, in the spinel stability field1–3. MORBs have 176Hf/177Hf ratios indicating derivation from a mantle reservoir with a long-term Lu/Hf ratio greater than that of Cl chondrite meteorites, yet the measured Lu/Hf ratios in MORB are lower than in Cl chondrites: this is the 'hafnium paradox'4. Here we show that the Lu–Hf and Sm–Nd systematics of MORBs require garnet to be a residual phase in MORB melt genesis. This places the onset of melting beneath a mid-ocean ridge at depths greater than 80 km. A sequential melting model, in which melting starts in the garnet stability field and then continues at shallower levels, best explains the combined Nd and Hf isotope systematics, and is compatible with our present geophysical and geochemical knowledge of mid-ocean-ridge magmatism.