Comments on 'Mantle Melting and Melt Extraction Processes beneath Ocean Ridges: Evidence from Abyssal Peridotites' by Yaoling Niu

Phase equilibrium data for melting of lherzolite in simplified and natural systems are used to show that the ‘polybaric melting reaction’ for mid-ocean ridge basalt proposed by Niu (1997, Journal of Petrology 38, 1047–1074) is erroneous for two reasons. First, the modal abundances of minerals in abyssal peridotites that have equilibrated at low pressures and temperatures cannot be used to calculate high-pressure and -temperature melting reactions. This is because the observed low-temperature mineral modes do not faithfully reproduce the changing abundances of clinopyroxene and orthopyroxene as a function of temperature and pressure that occur at the conditions of melting. Using abyssal peridotite mineral modes to calculate melting reactions results in a severe overestimation in the amount of orthopyroxene, and an underestimation in the amount of clinopyroxene, that dissolves into the melt in polybaric, near-fractional melting paths. The magnitude of these errors can be quantitatively significant for trace element melting models. Second, experimental data show that the complexity of phase relations as a function of pressure and temperature leads to a rich variety of melting reactions in polybaric melting paths. A single ‘polybaric melting reaction’ is not realistic. Only high-pressure and -temperature phase relations can yield reliable melting reactions.