Measurements of the Lamb-Moessbauer factor at simultaneous high-pressure-temperature conditions and estimates of the equilibrium isotopic fractionation of iron

Isotopic fractionation has been linked to the lattice vibrations of materials through their phonon spectra. The Lamb-Mossbauer factor (fLM) has the potential to provide information about the lattice vibrations in materials. We constrain the temperature evolution of the fLM of gamma - and epsilon -Fe at in situ high-P-T conditions between 1650 K and the melting point. We find that the vibrations of gamma - and epsilon -Fe can be described using a quasiharmonic model with a pressure- and temperature-dependent Debye temperature computed from the measured fLM. From the Debye temperature, we derive the equilibrium isotopic fractionation beta -factor of iron. Our results show that the quasiharmonic behavior of metallic iron would lower the value of lnbeta (sub Fe) (super 57/54) by 0.1 ppm at 1600-2800 K and 50 GPa when compared to the extrapolation of room temperature nuclear resonant inelastic X-ray scattering data. Our study suggests that anharmonicity may be more prevalent in Fe metal than in lower mantle minerals at 2800 K and 50 GPa, a relevant condition for the core formation, and the silicate mantle may be isotopically heavy in iron.