MELTING OF FORSTERITE, MG2SIO4, FROM 9.7 TO 16.5 GPA

A multianvil apparatus has been used to determine the pressure-temperature melting 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, 2310-degrees-C), the phases forsterite, periclase, anhydrous B, and liquid coexist and the melting reaction changes from forsterite = periclase + liquid at lower pressures to forsterite = anhydrous B + liquid at higher pressures. At the other (16.7 GPa, 2315-degrees-C), the phases forsterite, modified spinel, 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. The Simon equation, P(GPa) = 2.44[(T(degrees-K)/2171)11.4 - 1], fits both our melting curve data and the lower pressure data of Davis and England (1964). At low pressures, the melting curve of forsterite lies at higher temperatures than that of enstatite, but the two curves cross at 13.3 GPa because of the lower dT/dP slope of the forsterite melting curve. This causes the forsterite-enstatite eutectic to shift toward forsterite as pressure increases, but our data are consistent with earlier findings that the shift is not sufficient to support an origin for the mantle by eutectic-like melting at high pressures.