High-pressure melting phase equilibria of mantle lithologies and genesis of magmas

Abstract

Partial melting of mantle material in the Earth’s interior is one of the essential processes responsible for the thermal and chemical evolution of the Earth. Since peridotite is the predominant lithology in the upper mantle, peridotite is thought to be the principal source for various types of magmas and there have been a number of experimental studies on partial melting of peridotite. Recent technical progresses in high-pressure melting experiments have allowed precise determination of chemical composition of peridotite partial melts produced at fairly low degrees (＜5 wt%) of melting up to ～1.5GPa, which will in turn allow developing more realistic models for genesis of magmas in the context of fractional melting, especially at mid-oceanic ridges. On the other hand, partial melting of pyroxenite, the subdominant lithology in the mantle, has recently been recognized to play important roles in magma genesis. Melting interval of pyroxenite is much smaller than that of peridotite, and therefore pyroxenite-derived melt contributes to magma generation in much higher proportion than the pyroxenite proportion in the source. An important aspect of pyroxenite partial melting is the existence of garnet-pyroxene thermal divide defined by the enstatite -Ca-Tschermaks pyroxene - diopside plane in CaO-MgO-Al₂O₃-SiO₂ projections at pressures higher than ～2GPa. Pyroxenites that plot on either side of this thermal divide produce distinct types of partial melts, which are quite similar to either alkali-basaltic or tholeiitic magmas generated at hotspot volcanoes.