METASTABILITY OF ENSTATITE IN DEEP SUBDUCTING LITHOSPHERE

OLIVINE and (Mg,Fe)SiO3 pyroxene, the most abundant minerals in the Ea rth's upper mantle, are believed to transform to high-pressure phases at similar to 400 km depth(1-5). The possible metastable persistence o f olivine to greater depths in some subduction zones-with consequences for the origin of deep-focus earthquakes and the dynamics of subducti on-has been discussed extensively(6-14) but the role of other mantle m inerals has not been considered. We report here an experimental study of the kinetic behaviour of the magnesian pyroxene enstatite (MgSiO3) at upper-mantle conditions. We find that, whereas forsterite (Mg-olivi ne, Mg2SiO4) transforms rapidly to beta-phase at 1,200 degrees C and 1 6 GPa, the transformation of enstatite to beta-phase plus stishovite o n the same time-scale requires much higher temperatures. At lower temp eratures, enstatite transforms directly to the ilmenite structure, but only at pressures greater than 20 GPa. Enstatite should therefore per sist metastably to greater depths than olivine in Subduction zones, tr ansforming directly tb the ilmenite structure. The enstatite-ilmenite transformation is accompanied by a large decrease in volume, which sho uld increase the stresses in subducting slabs, change the buoyancy for ces that drive Subduction, and might also contribute to the origin of deep-focus earthquakes.