MELTING OF FORSTERITE, MG2SIO4, FROM 9.7 TO 16.5 GPA

A multianvil apparatus has been used to determine the pressure-tempera ture melting curve of forsterite from 9.7 to 16.5 GPa. At 10.1 GPa a s ingular point occurs that marks the change from congruent melting at l ower pressures to incongruent melting (forsterite = periclase + liquid ) at higher pressures. The melting curve also passes through two invar iant points. At one (15.6 GPa, 2310-degrees-C), the phases forsterite, periclase, anhydrous B, and liquid coexist and the melting reaction c hanges from forsterite = periclase + liquid at lower pressures to fors terite = anhydrous B + liquid at higher pressures. At the other (16.7 GPa, 2315-degrees-C), the phases forsterite, modified spinel, anhydrou s B, and liquid coexist, and the melting reaction changes from forster ite = anhydrous B + liquid at lower pressures to modified spinel = anh ydrous 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 low er dT/dP slope of the forsterite melting curve. This causes the forste rite-enstatite eutectic to shift toward forsterite as pressure increas es, but our data are consistent with earlier findings that the shift i s not sufficient to support an origin for the mantle by eutectic-like melting at high pressures.