Keywords:Lowermost mantle, Lattice thermal conductivity , Ferropericlase, Ab initio calculations
Determination of lattice thermal conductivity (κlat) of lower mantle (LM) minerals is a key to understanding the dynamics and evolution of the earth’s deep interior. (Mg, Fe)O Ferropericlase (Fp) is believed to be the second most abundant LM mineral after (Fe, Al) bearing MgSiO3 bridgmanite and postpervskite (PPv). Recent experimental and theoretical studies under LM pressures showed that κlat) of MgO is substantially reduced by Fe incorporation (Ohta et al., 2017; Hsieh et al., 2018; Song et al., 2019). However, the temperature condition is limited to a low temperature of 300 K, which is far from the actual LM condition. Therefore, the role of FP on the heat transport at the deepest mantle is still poorly understood. In this study, we determined κlat of FP at the LM pressure and temperature conditions, based on the ab initio anharmonic lattice dynamics techniques with fully solving the phonon Boltzmann transport equation (Dekura and Tsuchiya, 2017; 2019) combined with the internally consistent LDA+U technique for more precisely describing the Fe-O bond (Wang et al., 2015). Calculations demonstrate strong negative solid solution effects (~-70-80% for ~13-19 mol% FeO) of low-spin Fe on κlat of MgO owing to decreases both in phonon group velocity and lifetime. An effective κlat of the lowermost mantle is then estimated for pyrolytic aggregate (FP+PPv) with pyrolytic ratio to be ~4 Wm-1K-1, which produces a net heat flow from the core to mantle ~6-7 TW. This value is more than ~50% smaller than that estimated from the core with high thermal conductivity of iron (~15 TW). This discrepancy could be reconciled by the thermally or chemically stratified layer at the top of the outer core observed seismologically (Helffrich and Kaneshima, 2010) if subadiabatic temperature gradient exists there.