[S-IT20] Deep Earth Sciences

[SIT20-P11] Effect of iron on thermal conductivity of dry ringwoodite: implications for mantle dynamics and evolution of terrestrial planets

*Youyue Zhang¹, Takashi Yoshino², Masahiro Osako³, Steeve Gréaux¹ (1.Geodynamics Research Center, Ehime University, 2.Institute for Planetary Materials, Okayama University, 3.National Museum of Nature and Science)

Keywords:Thermal conductivity, Ringwoodite, High pressure, Planetary mantle

Thermal conductivity of planetary materials is a critical parameter that governs heat transfer within a planet, which in turn influences its thermal state and geodynamics. Ringwoodite, a high-pressure polymorph of olivine, constitutes approximately 60 vol% of the Earth's mantle transition zone (MTZ) [1] and about 80 vol% of the harzburgitic slab [2], making it a key role in heat and mass transfer between the upper and lower mantle. Recent seismological observations also suggest the presence of Fe-rich ringwoodite in the deep Martian mantle [3], indicating that iron-rich ringwoodite may significantly influence the core-mantle boundary heat flux on Mars. Despite its importance, the heat transport properties of ringwoodite remain poorly understood. Few experimental studies have investigated the thermal properties of ringwoodite with Earth mantle compositions, by measuring thermal diffusivity or thermal effusivity [4][5][6]. The effect of iron on thermal conductivity of ringwoodite has not been experimentally constrained yet. Moreover, the fact that ringwoodite easily bears water complicates previous findings, as these studies may involve the complex effects of lattice or grain boundary water, which obscure our understanding of the mineral’s role in the thermal history and dynamics of terrestrial planets.
In this study, dry ringwoodite with varying Fe contents, specifically (Mg_0.9Fe_0.1)₂SiO₄ and (Mg_0.7Fe_0.3)₂SiO₄, was synthesized at 20 GPa and 1775 K using a Kawai-type multi-anvil apparatus. The thermal conductivity and thermal diffusivity of Fe-bearing ringwoodite were determined simultaneously by combining the multi-anvil high-pressure experimental technique with the pulse heating method, up to 20 GPa and 1100 K. The pressure and temperature dependencies of thermal conductivity for ringwoodite with different iron contents were obtained. The experimental results show that (Mg_0.9Fe_0.1)₂SiO₄ ringwoodite exhibits higher thermal conductivity values than previous studies, which may suggest a negative effect of water on the thermal conductivity of ringwoodite. (Mg_0.7Fe_0.3)₂SiO₄ ringwoodite has a thermal conductivity of approximately half that of (Mg_0.9Fe_0.1)₂SiO₄ under the same P-T conditions, indicating a strong iron effect on the thermal properties of ringwoodite. The contrast in thermal conductivity between ringwoodite and olivine is much larger in Earth’s mantle compared to Mars, suggesting that the thermal evolution of these two terrestrial planets may differ.
Reference: [1] Frost, Elements 4 (3): 171–176 (2008). [2] Irifune & Ringwood, Geophys. Res. Lett. 14, 1546–1549 (1987). [3] Huang et al., Proceedings of the National Academy of Sciences 119.42 (2022). [4] Hofmeister, Science, 283 (5408), 1699–1706 (1999). [5] Xu et al., Physics of the Earth and Planetary Interiors, 143-144, 321–336 (2004). [6] Marzotto, et al., Geophysical Research Letters 47.13 (2020).