[S-IT20] 地球深部科学

[SIT20-22] Our understanding of the CMB heat transport

*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：Lower mantle、Thermal conductivity、CMB heat flux、Fe solid solution effect、Ab initio calculation

The lattice thermal conductivity (κ_lat) is critical for understanding the energy transport in Earth's deep interior. High-pressure experiments report that only ~7 mol% Fe incorporation drastically reduces the κ_lat of MgSiO3 bridgmanite (Brg) by ~40% [e.g., Edmund et al., 2024]. However, the experimental pressure (P, T) conditions are limited compared to the actual lower mantle (LM) P, T. Recently, we developed an advanced computational framework to predict the κ_lat of Fe-bearing systems integrating density-functional theory, anharmonic lattice dynamics, the internally consistent LDA+U method, and the phonon Boltzmann transport theory [e.g., Dekura et al., 2023; 2024]. In this study, we apply this parameter-free approach to κ_lat of Fe-bearing Brg and post-perovskite (PPv) and construct the LM conductivity profile. The results reveal that ~6 mol% Fe incorporation significantly reduces the κ_lat of endmembers by ~60% due to drops in phonon group velocity and phonon lifetimes. The effective thermal conductivity (κ_eff) for the pyrolitic LM aggregate (Brg/PPv:FP = 8:2) is found to vary from approximately ~1 Wm^-1K^-1 at the uppermost LM to approximately ~4 Wm^-1K^-1 at the CMB. The spatial CMB heat flux modeled using the obtained κ_eff shows a significant lateral heterogeneity (~1.0 × 10^-2 to ~9.0 × 10^-2 Wm^-2) and the net CMB heat flow of approximately 7 TW. Our estimation is significantly lower than that calculated for the Fe-free system (approximately 17 TW) [e.g., Dekura et al., 2019], which highlights the critical role of Fe in the LM conductivity. The present estimation for the net flow is substantially lower than that estimated for the outer core (~12-18 TW) [e.g., de Koker et al., 2012]. Potential mechanisms, such as the formation of a thermally stratified layer at the uppermost outer core, contributions from radiative heat transfer, or the effects of chemical heterogeneity near the CMB, may help reconcile the significant discrepancy.