Orthopyroxenes oceanic mantle

Normal mantle melting, such as that which leads to the formation of oceanic crust and oceanic islands, produces magma of basaltic to picritic composition and leaves a residue consisting of olivine, orthopyroxene, chnopyroxene and an aluminous phase. The compositions of minerals in this residue, and their relative proportions, are unlike those in old subcontinental hthosphere Mg-Fe ratios of the ferromagnesian minerals are too low, and the amount of chnopyroxene and spinel or garnet is too high. 

Although this idea has now been largely superceded by the polybaric melting model a new possibility has recently emerged whereby the uppermost mantle may behave as a chemical filter. Midocean ridge basalt (MORB) is silica saturated and is in equilibrium with orthopyroxene at pressures greater than about 8 kb. At the lower pressures commensurate with the base of the oceanic crust MORB melts are undersaturated in silica and have the capacity to dissolve orthopyroxene (Braun Kelemen, 2002). There is field evidence from... 

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