[S-IT21] 核-マントルの相互作用と共進化

[SIT21-09] Ab initio anharmonic lattice dynamics for Fe-bearing lower mantle minerals

*出倉 春彦¹、土屋 卓久¹ (1.愛媛大学地球深部ダイナミクス研究センター)

キーワード：下部マントル鉱物、格子熱伝導率、第一原理計算

Determination of lattice thermal conductivity (κ_lat) of lower mantle (LM) minerals is a key to understanding the dynamics and evolution of the earth’s deep interior. Some recent experimental studies have shown that κ_lat of MgO and MgSiO₃ bridgmanite (Brg) are substantially reduced by Fe incorporation (Manthilake et al., 2012; Goncharov et al., 2015; Ohta et al., 2017; Hsieh et al., 2017). In contrast, Okuda et al. (2017) reported a very different result on Brg with a marginal effect of Fe. Besides, the effect of Fe on MgSiO₃ post-perovskite (PPv) has never been reported. Therefore, in this study, we investigate κ_lat of Fe-bearing LM minerals (ferropericlase (FP), Brg, and PPv) in 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) 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 of low-spin Fe on κ_lat of FP and high-spin Fe on κ_lat of Brg and PPv, as a linear decrease in logκ_lat of FP with increasing the Fe concentration. Our detailed analyses indicate that such strong effects occur primarily due to the substantial changes in harmonic properties and are found to be Brg > PPv > FP. The present results improve the conventional estimation of the effective LM conductivity (e.g., Stacey 1992). It is estimated to be ~2-3 Wm^-1K^-1 for the pyrolytic aggregate (FP + Brg) and ~4-5 Wm^-1K^-1 (FP+PPv) at 136 GPa and 4000 K, which are ~60-80% smaller than the conventional value of 10 Wm^-1K^-1.