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 relat ions in the system diopside - forsterite - enstatite (CaMgSi2O6 - Mg2SiO4 - MgSiO3) and extend for a limited distance into the larger ternary system d iopside - 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 Di(43)En(57) and is defined by the reaction 100 cp x = 64 opx + 36 liq. An azeotropic minimum occurs on the clinopyroxene liqu idus at Di(67)En(33). In the ternary system diopside - forsterite - enstati te, a peritectic occurs at Di(43)Fo(46)Qtz(11) and is defined by the reacti on 69 opx + 31 liq = 95 cpx + 5 fo (wt.%). These ternary phase relations ca n be used to model three important aspects of the volatile-free phase relat ions and melting behavior of natural peridotite with a high degree of accur acy. (1) Olivine, orthopyroxene, and clinopyroxene occur at the solidus at 2 GPa, but orthopyroxene is absent at the solidus at 5.1 Cpa. (2) During eq uilibrium melting at 5.1 GPa, orthopyroxene appears just above the solidus and then disappears again at higher temperatures. At both 2 and 5.1 GPa, th e 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 Waiter (1998). (3) As pressure increases, MgO increases and SiO2 decrea ses in initial melts.