Melting of Forsterite, Mg₂SiO₄, From 9.7 to 16.5 GPa

A multianvil apparatus ha been used to determine th pressure-temperature mel ing curve of forsterite from 9.7 to 16.5 GPa. At 10.1 GPa a singular point occurs that marks the change from congruent melting at lower pressures to incongruent melting (forsterite periclase + liquid) at higher pressures. The melting curve also passes through two invariant points. At one (15.6 GPa, 2310øC), the phases forsterite, p riclase, anhydrous B, and liquid coexist and the melting reaction changes from forsterite = periclase + liquid at lower pressures to forsterite = anhydrous B + liquid at higher p essures. At the other (16.7 GPa, 2315øC), the phases forsterite, modified spine!, anhydrous B, and liquid coexist, and the melting reaction changes from forsterite = anhydrous B + liquid at lower pressures to modified spinel = anhydrous B + liquid at higher pressures. TheSimon equation, P(GPa) = 2.44[(T(øK)/2171) ll'41], fits both our melting curve data nd the lower pressure data of Davis and England (1964). At low pressures, the melting curve of forsterite l es at higher temperatures han that of enstatite, but he two curves cross at 13.3 GPa because of the lower dT/dP slope ofthe forsterite m lting curve. This causes the forsterite-enstatite eutectic oshift toward forsterite as pressure increases, but our data re consistent withearlier findings that the shift is not sufficient to support an origin for the mantle by eutectic-like m lting at high pressures.