Single-crystal X-ray diffraction study of Fe2SiO4 fayalite up to 31 GPa

Olivine is widely believed to be the most abundant mineral in the Earth's upper mantle. Here, we report structural refinement results for the Fe-end-member olivine, Fe2SiO4 fayalite, up to 31 GPa in diamond-anvil cell, using single-crystal synchrotron X-ray diffraction. Unit-cell parameters a, b, c and V, average Si-O Fe-O bond lengths, as well as Si-O Fe-O polyhedral volumes continuously decrease with increasing pressure. The pressure derivative of isothermal bulk modulus is determined to be 4.0 (2) using third-order Birch-Murnaghan equation of state with ambient isothermal bulk modulus fixed to 135 GPa on the basis of previous Brillouin measurements. The Si-O tetrahedron is stiffer than the Fe-O octahedra, and the compression mechanism is dominated by Fe-O bond and Fe-O octahedral compression. Densities of olivine along 1600 and 900 K adiabats are calculated based on this study. The existence of metastable olivine inside the cold subduction slab could cause large positive buoyancy force against subduction, slow down the subduction and possibly affect the slab geometry.