Phase transformation of ferric-iron-rich silicate in Earth’s mid-mantle

Abstract

Incorporation of ferric iron in mantle silicates stabilizes different crystal structures and changes phase transition conditions, thus impacting seismic wave speeds and discontinuities. In MgSiO3-Fe2O3 mixtures, recent experiments indicate the coexistence of fully oxidized iron-rich (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 with Fe-poor silicate (wadsleyite or bridgmanite) and stishovite at 15 to 27 GPa and 1773 to 2000 K, conditions relevant to subducted lithosphere in the Earth’s transition zone and uppermost lower mantle. X-ray diffraction measurements show that (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 recovered from these conditions adopts the R3c LiNbO3-type structure, which transforms to the bridgmanite structure again between 18.3 GPa and 24.7 GPa at 300 K. Diffraction observations are used to obtain the equation of state of the LiNbO3-type phase up to 18.3 GPa. These observations combined with multi-anvil experiments suggest that the stable phase of (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 is bridgmanite at 15-27 GPa, which transforms on decompression to LiNbO3-type structure. Our calculation revealed that ordering of the ferric ion reduces the kinetic energy barrier of the transition between (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 LiNbO3 structure and bridgmanite relative to the MgSiO3 akimotoite-bridgmanite system. Dense Fe3+-rich bridgmanite structure is thus stable at substantially shallower depths than MgSiO3 bridgmanite and would promote subduction.