Ab initio investigation of high-pressure phase relation and elasticity in the NaAlSi2O6 system

Abstract

Subducted crusts contain aluminous phases with incompatible elements such as sodium. In order to understand the fate of subducted crusts and how they recycle within the Earth, it is important to investigate high-pressure stability and elastic properties of host phases. Here we have studied NaAlSi 2O6 jadeite, a NaAlSiO4 calcium ferrite (CF)-type phase, and SiO2 stishovite by means of a first principles computation method. Jadeite was found to dissociate into an assemblage of the CF-type phase and stishovite at about 18 GPa, associated with remarkable increases of compressional wave (18.7%), shear wave (26.4%), bulk sound (13.3%) velocities, and density (15.1%). Proximity of this transition condition to the 660 km discontinuity in the mantle transition zone suggests that seismically observed splitting of the reflection could be related to this phase transition, implying subduction of crustal materials down to the mantle transition zone. © 2010 by the American Geophysical Union.