[S-IT18] GEOPHYSICAL PROPERTIES AND TRANSPORT PROCESSES IN THE DEEP CRUST AND MANTLE

[SIT18-P02] Simultaneous measurements of thermal conductivity and thermal diffusivity of bridgmanite and post-spinel: implications for planetary mantle dynamics and evolution

*Youyue Zhang¹, Takashi Yoshino¹, Masahiro Osako² (1.Institute for Planetary Materials, Okayama University, 2.National Museum of Nature and Science)

Keywords:Thermal conductivity, Bridgmanite, Post-spinel, High pressure, Mantle

Aluminium (Al)-, iron (Fe)-bearing bridgmanite is thought to be the dominant phase in the Earth’s lower mantle. Its thermal conductivity plays important roles in mantle dynamics and thermal evolution of Earth and Venus. Although a number of studies have reported thermal properties of bridgmanite, the absolute values and pressure dependence of thermal conductivity of the end member and impurity influence on its thermal conductivity are still controversial. The temperature dependence of bridgmanite thermal conductivity is also poorly constrained. In this study, simultaneous measurements of thermal conductivity (λ) and thermal diffusivity (κ) of post spinel and bridgmanite with five different compositions (MgSiO₃, Mg_0.97Fe_0.03SiO₃, Mg_0.91Fe_0.09SiO₃, Mg_0.97Al_0.05Si_0.98O₃ and Mg_0.89Fe_0.09Al_0.05Si_0.97O₃) were performed up to 24 GPa and 1100 K by a pulse heating method in a large volume multi-anvil press. The results show that the addition of a small amount of impurity (5 mol% Al or 3 mol% Fe) to the end member bridgmanite leads to significant decrease of λ, to almost half values. Addition of impurities will decrease not only the absolute value of λ, but also result in smaller pressure, temperature dependence of λ. The impurity influence is not simple linear relationships with substitution amount, additional Al impurity (5 mol%) leads to higher absolute value, pressure dependence and temperature dependence of Fe-bearing bridgmanite (Mg_0.91Fe_0.09SiO₃), possibly due to increase of ferric iron content. The impurity effect on λ could influence the delayed phase transition of a subduction slab at 660 km discontinuity by controlling the conductive heating efficiency, thus, control the residence time of slab stagnancy. The sensitive temperature dependence of the end member λ may provide clues for sharp divergence of secular cooling rate between Earth and Venus.