Metastability of enstatite in deep subducting lithosphere

OLIVINE and (Mg,Fe)SiO₃ pyroxene, the most abundant minerals in the Earth's upper mantle, are believed to transform to high-pressure phases at ∼400 km depth^1-5. The possible metastable persistence of olivine to greater depths in some subduction zones - with consequences for the origin of deep-focus earthquakes and the dynamics of subduction - has been discussed extensively^6-14, but the role of other mantle minerals 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-olivine, Mg₂SiO₄) transforms rapidly to β-phase at 1,200 °C and 16 GPa, the transformation of enstatite to β-phase plus stishovite on the same timescale requires much higher temperatures. At lower temperatures, enstatite transforms directly to the ilmenite structure, but only at pressures greater than 20 GPa. Enstatite should therefore persist metastably to greater depths than olivine in subduction zones, transforming directly to the ilmenite structure. The enstatite-ilmenite transformation is accompanied by a large decrease in volume, which should increase the stresses in subducting slabs, change the buoyancy forces that drive subduction, and might also contribute to the origin of deep-focus earthquakes.