The influence of deep mantle thermal conductivity on the long-term thermal evolution of Earth's mantle and core

Abstract

Constraining the heat flow across the core-mantle boundary (CMB) is crucial for understanding the thermal history of Earth’s mantle and the core. The primary mechanism governing heat transfer at the CMB is conduction, with lattice vibration (lattice thermal conductivity) commonly considered to be the dominant mechanism of thermal conduction in the lower mantle. However, there are large uncertainties in current estimates of lattice thermal conductivity of mantle material under CMB condition, due to the influence from mineral composition and the post-perovskite phase transition (e.g., Hsieh et al., 2018 PNAS; Ohta et al., 2017 EPSL). On the other hand, the role of radiative contribution (radiative thermal conductivity) remains less well understood. Several recent studies have attempted to measure the radiative thermal conductivity of bridgmanite and pyrolitic materials under lower mantle conditions, but the resulting experimental data have yielded divergent estimations for the radiative thermal conductivity of average mantle material at CMB conditions, ranging from 0.35 W/(m K) to 4.2 W/(m K) (Lobanov et al., 2020 EPSL; Murakami et al., 2022 EPSL). Adopting the highest estimate could result in an approximate 50% increase in the estimated bulk thermal conductivity compared to conventionally assumed values.   To address the implications of these thermal conductivity uncertainties on mantle convection, we have incorporated variable thermal conductivities into a global thermochemical geodynamic model, StagYY. The simulations use a 2D spherical annulus geometry and extend over a 4.5 Gyr timespan. The geodynamic model includes parameterized core cooling, heat-producing elements partitioning, and crust formation, but it does not include an initial primordial reservoir at CMB. Preliminary findings from our study reveal that the relationship between thermal conductivity and CMB heat flux is not always straightforward. For models with stagnant-lid tectonics, higher thermal conductivity leads to higher CMB heat flux in the initial 1 Gyr and lower CMB heat flux at 4.5 Gyr. However, in models with mobile-lid tectonics, the CMB heat flux also increases with higher thermal conductivity in the first 1 Gyr, but CMB heat flux varies more and becomes unrelated to thermal conductivity at 4.5 Gyr. In summary, deep mantle thermal conductivity has little effect on the present-day CMB heat flux due to plate tectonics on Earth. Varying thermal conductivity mainly influences the amount of core cooling, particularly in early planetary evolution.