[S-IT17] TRANSPORT PROPERTIES AND PROCESSES IN THE EARTH

[SIT17-P02] Simultaneous measurements of thermal conductivity and thermal diffusivity of MgSiO₃ glass under high pressure

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

キーワード：Thermal conductivity, MgSiO3 glass, High pressure, Mantle

Silicate magma ocean may have played a crucial role in the evolution of the early Earth. On present-day Earth, geophysical observations indicate the presence of liquid silicates in the crust and at the top of the transition zone, and silicate melts are also thought to exist at depths as deep as the core-mantle boundary (CMB) [1]. Therefore, the high-pressure properties of silicate melts in the deep mantle are essential for understanding the dynamic processes and evolution of the Earth's interior. Of particular interest are the thermal properties of silicate melts, which control the rate of heat transport through the magma. The value of thermal conductivity is important for understanding the process from double-diffusive convection in the magma chamber to the earliest thermal evolution of the presumed molten Earth [2]. However, the thermal conductivity of silicate melts at high pressures is largely unknown. Since it remains extremely challenging to study the physical and chemical properties of silicate melts under extreme P-T conditions in the deep mantle, silicate glasses are commonly used as analogues to understand the properties of silicate melts in extreme environments [3][4].
In this study, the thermal conductivity and thermal diffusivity of MgSiO₃ glass were determined simultaneously by combining a multi-anvil high-pressure experimental technique and pulse heating method up to 12 GPa. The pressure dependence of the thermal conductivity of MgSiO₃ glass was obtained. The experiment results show that MgSiO₃ glass has much lower thermal conductivity (λ) values than bridgmanite. λ of MgSiO₃ glass is lower than MgSiO₃ bridgmanite by a factor of 6 and is lower than Al, Fe-bearing bridgmanite by a factor of 3 at ambient conditions. The pressure dependence of λ of MgSiO₃ glass is also smaller than MgSiO₃ bridgmanite by a factor of 4. It may suggest that the structural influence on thermal conductivity can be larger than the impurity effect in a MgSiO₃-FeAlO₃ system. The low thermal conductivity of silicate melts may indicate a positive feedback mechanism in the mantle melt region: the formation of partial melts largely reduces the thermal conductivity, further trapping heat and stabilizing the melting pocket.

Reference: [1] Petitgirard S, et al., Proc. Natl. Acad. Sci. U.S.A, 112, 14186–14190, (2015). [2] Elkins-Tanton, Annual Review of Earth and Planetary Sciences, 40, 113–139, (2012). [3] Lee et al., Proc. Natl. Acad. Sci. U.S.A., 105(23), 7925–7929, (2008). [4] Murakami & Bass, Proc. Natl. Acad. Sci. U.S.A., 108(42), 17286–17289, (2011).