MELTING OF JADEITE TO 16.5 GPA AND MELTING RELATIONS ON THE ENSTATITE-JADEITE JOIN

A split-sphere anvil apparatus (USSA-2000) was used to determine the m elting curve of jadeite (NaAlSi2O6) from 2.4 to 16.5 GPa, and the melt ing relations on the enstatite (Mg2Si2O6)-jadeite join at 2.8, 4, and 6 GPa. The melting temperatures of jadeite increase more rapidly with pressure than the known melting temperatures of any other mantle mater ial and exceed the melting temperatures of enstatite around 13 GPa. Th e melting curve of jadeite was fitted with the Simon equation, as foll ows: P (GPa) = 2.5 + 2.01 [T (K)/1543)3.7 - 1]. Melting on the enstati te-jadeite join is peritectic in the investigated pressure range, with the peritectic composition becoming more magnesium-rich with increasi ng pressure. The melting changes to azeotropic at pressures higher tha n 8 GPa. Thus, while the fractionation at lower pressures can proceed to the most sodium-rich compositions, the fractionation at pressures g reater than 8 GPa is controlled by the composition of the minimum on t he azeotrope. This could have important implications for the fractiona tion of the upper mantle, applicable to fractional crystallization in a magma ocean.