Journal of Petrology, Volume 40, Issue 7: July 1999.

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

MICHAEL J. WALTER

INSTITUTE FOR STUDY OF THE EARTH'S INTERIOR, OKAYAMA UNIVERSITY, MISASA, TOTTORI-KEN 682-01, JAPAN

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.

Keywords: mantle melting reactions;abyssal peridotite;mid-ocean ridge basalt

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