[SIT22-P05] Thermal conductivity and compressibility of iron and aluminum-bearing bridgmanite: implications for spin-crossover of iron
Keywords:bridgmanite, lower mantle, spin transition
Bridgmanite (Bdg), iron (Fe)- and aluminum (Al)-bearing magnesium silicate perovskite is the most abundant mineral in the Earth’s lower mantle. Thus, its thermal conductivity governs the lower mantle thermal conductivity that critically controls the thermo-chemical evolution of both the core and the lower mantle. While there is extensive research for the lattice thermal conductivity of MgSiO3 Bdg, the effects of Fe and Al incorporation on its lattice thermal conduction are still controversial.
We measured the lattice thermal conductivity of Bdg with chemical compositions of Mg0.832Fe0.209Al0.060Si0.916O3, Mg0.793Fe0.075Al0.217Si0.914O3 and Mg0.718Fe0.123Al0.281Si0.878O3 up to 142 GPa, 180 GPa and 74 GPa, respectively, at 300 K using the pulsed light heating thermoreflectance technique in a diamond anvil cell. The results show that the lattice thermal conductivity of Mg0.832Fe0.209Al0.060Si0.916O3 Bdg is 25.5 ± 2.2 W/m/K at 135 GPa and 300 K, which is 19% lower than that of Fe and Al-free Bdg at identical conditions. Considering the temperature effect on the lattice thermal conductivity and the contribution of radiative thermal conductivity, the total thermal conductivity of Fe and Al-bearing Bdg does not change very much with temperature at 135 GPa, and could be higher than that of post-perovskite with identical chemical composition. Our results imply that the compositional variation of bridgmanite do not induce heterogeneity of thermal conductivity in the lateral direction at the core-mantle boundary. The present results also revealed that the lattice thermal conductivities of Mg0.793Fe0.075Al0.217Si0.914O3 Bdg and Mg0.718Fe0.123Al0.281Si0.878O3 Bdg showed abnormal reduction in the pressure range of 20-40 GPa at 300 K, which may be due to the spin crossover of octahedral Fe3+. This indicates that temperature in the subducted slab could be significantly lower than previously thought, which have a great potential to the lower mantle dynamics, and water transportation to the deeper part of the Earth’s lower mantle.
We measured the lattice thermal conductivity of Bdg with chemical compositions of Mg0.832Fe0.209Al0.060Si0.916O3, Mg0.793Fe0.075Al0.217Si0.914O3 and Mg0.718Fe0.123Al0.281Si0.878O3 up to 142 GPa, 180 GPa and 74 GPa, respectively, at 300 K using the pulsed light heating thermoreflectance technique in a diamond anvil cell. The results show that the lattice thermal conductivity of Mg0.832Fe0.209Al0.060Si0.916O3 Bdg is 25.5 ± 2.2 W/m/K at 135 GPa and 300 K, which is 19% lower than that of Fe and Al-free Bdg at identical conditions. Considering the temperature effect on the lattice thermal conductivity and the contribution of radiative thermal conductivity, the total thermal conductivity of Fe and Al-bearing Bdg does not change very much with temperature at 135 GPa, and could be higher than that of post-perovskite with identical chemical composition. Our results imply that the compositional variation of bridgmanite do not induce heterogeneity of thermal conductivity in the lateral direction at the core-mantle boundary. The present results also revealed that the lattice thermal conductivities of Mg0.793Fe0.075Al0.217Si0.914O3 Bdg and Mg0.718Fe0.123Al0.281Si0.878O3 Bdg showed abnormal reduction in the pressure range of 20-40 GPa at 300 K, which may be due to the spin crossover of octahedral Fe3+. This indicates that temperature in the subducted slab could be significantly lower than previously thought, which have a great potential to the lower mantle dynamics, and water transportation to the deeper part of the Earth’s lower mantle.