Abstract:
A split-sphere anvil apparatus (USSA-2000) was used to determine the melting curve of jadeite (NaAlSi2O6) from 2.4 to 16.5 GPa, and the melting 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 material and exceed the melting temperatures of enstatite around 13 GPa. The melting curve of jadeite was fitted with the Simon equation. Melting on the enstatite-jadeite join is peritectic in the investigated pressure range, with the peritectic composition becoming more magnesium-rich with increasing pressure. The melting changes to azeotropic at pressures higher than 8 GPa. Thus, while the fractionation at lower pressures can proceed to the most sodium-rich compositions, the fractionation at pressures greater than 8 GPa is controlled by the composition of the minimum on the azeotrope. This could have important implications for the fractionation of the upper mantle, applicable to fractional crystallization in a magma ocean.