STABILITY OF NA2MG2SI2O7 AND MELTING RELATIONS ON THE FORSTERITE-JADEITE JOIN AT PRESSURES UP TO 22 GPA

Abstract

Melting experiments on the forsterite-jadeite join (Mg2SiO4-NaAlSi2O6) were carried out at 4-22 GPa in a split-sphere anvil apparatus (USSA-2000). Forsterite-rich compositions produced below the solidus a divariant assemblage of forsterite, clinopyroxene, garnet, and Na2Mg2Si2O7 (NMS), with NMS being the solidus phase. The melting curves of NMS and Na2MgSiO4 (N2MS) were determined from 1 atm to 22 GPa. The results were used to solve the basic topology of the phase diagrams relevant to assemblages containing NMS and N2MS. It is expected that NMS is present in the Earth's mantle in nepheline-normative compositions due to the instability of nepheline at higher pressures. However, at pressures higher than 13 GPa, NMS-bearing assemblages could potentially form even at lower sodium contents more common in the mantle, and cause a major decrease in the temperatures of the anhydrous solidus in the transition zone (400-670 km). Such a decrease in the solidus temperatures is likely to have a profound effect on the evolution of the Earth's mantle in any scenario involving a magma ocean.