The Composition of Near-solidus Partial Melts of Fertile Peridotite at 1 and 1·5 GPa: Implications for the Petrogenesis of MORB

We have determined the near-solidus melt compositions for peridotite MM-3, a suitable composition for the production of mid-ocean ridge basalt (MORB) by decompression partial melting, at 1 and 1·5 GPa. At 1 GPa the MM-3 composition has a subsolidus plagioclase-bearing spinel lherzolite assemblage, and a solidus at ∼ 1270°C. At only ∼ 5°C above the solidus, 4% melt is present as a result of almost complete melting of plagioclase. This melting behaviour in plagioclase lherzolite is predicted from simple systems and previous experimental work. The persistence of plagioclase to > 0·8 GPa is strongly dependent on bulk-rock CaO/Na2O and normative plagioclase content in the peridotite. At 1·5 GPa the MM-3 composition has a subsolidus spinel lherzolite assemblage, and a solidus at ∼ 1350°C. We have determined a near-solidus melt composition at ∼ 2% melting within 10°C of the solidus. Near-solidus melts at both 1 and 1·5 GPa are nepheline normative, and have low normative diopside contents; also they have the highest TiO2, Al2O3 and Na2O, and the lowest FeO and Cr2O3 contents compared with higher degree partial melts. Comparison of these near-solidus melts with primitive MORB glasses, which lie in the olivine-only field of crystallization at low pressure, indicate that petrogenetic models involving aggregation of near-fractional melts formed during melting at pressures of 1·5 GPa or less are unlikely to be correct. In this study we use an experimental approach that utilizes sintered oxide mix starting materials and peridotite reaction experiments. We also examine some recent studies using an alternative approach of melt migration into, and entrapment within ‘melt traps’ (olivine, diamond, vitreous carbon) and discuss optimal procedures for this method.