THE SYSTEM DIOPSIDE – FORSTERITE – ENSTATITE AT 5.1 GPa: A TERNARY MODEL FOR MELTING OF THE MANTLE¶

Data determined using a multianvil press are given for liquidus phase relations in the system diopside – forsterite – enstatite (CaMgSi2O6 – Mg2SiO4 – MgSiO3) and extend for a limited distance into the larger ternary system diopside – forsterite – quartz at 5.1 GPa. In the system diopside–enstatite, which cuts across the system diopside – forsterite – quartz and divides it into two smaller ternary systems, the peritectic between clinopyroxene and orthopyroxene occurs at Di43En57 and is defined by the reaction 100 cpx = 64 opx + 36 liq. An azeotropic minimum occurs on the clinopyroxene liquidus at Di67En33. In the ternary system diopside – forsterite – enstatite, a peritectic occurs at Di43Fo46Qtz11 and is defined by the reaction 69 opx + 31 liq = 95 cpx + 5 fo (wt.%). These ternary phase relations can be used to model three important aspects of the volatile-free phase relations and melting behavior of natural peridotite with a high degree of accuracy. (1) Olivine, orthopyroxene, and clinopyroxene occur at the solidus at 2 GPa, but orthopyroxene is absent at the solidus at 5.1 GPa. (2) During equilibrium melting at 5.l GPa, orthopyroxene appears just above the solidus and then disappears again at higher temperatures. At both 2 and 5.1 GPa, the proportions of phases in the ternary system at various degrees of melting are close to the proportions observed in the melting of natural peridotite by Walter (1998). (3) As pressure increases, MgO increases and SiO2 decreases in initial melts.