Lattice thermal conductivity of MgSiO3 perovskite and post-perovskite at the core–mantle boundary

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Abstract

Thermal conductivity is essential for controlling the rate of core heat loss and long-term thermal evolution of the Earth, but it has been poorly constrained at the high pressures of Earth's lowermost mantle. We measured the lattice component of thermal diffusivity, heat transport by scattering of phonons, of both MgSiO3 perovskite (Pv) and post-perovskite (PPv) at high pressures of up to 144 GPa and at room temperature. Lattice thermal conductivity of Pv-dominant lowermost mantle assemblage obtained in this study is about 11 W/m/K, while PPv-bearing rocks exhibit ∼60% higher conductivity. Since such Pv value is comparable to the conventionally assumed lowermost mantle conductivity, our findings do not significantly alter but support the recent notion of high core–mantle boundary heat flow along with a young inner core and high temperatures in the early deep Earth.

Highlights

► We measured lattice thermal diffusivity of MgSiO3 perovskite and post-perovskite up to 144 GPa. ► Obtained thermal conductivity of perovskite-dominant lowermost mantle is about 11 W/m/K. ► Post-perovskite exhibits ∼70% higher conductivity than perovskite. ► Our findings support the recent estimates of high heat flux across the core–mantle boundary.

Introduction

Heat in the Earth's interior is transported dominantly by convection in the mantle and core, and by conduction at thermal boundary layers. The thermal conductivity of the bottom thermal boundary layer of the solid mantle determines the magnitude of heat flux from the core, and is intimately related to instability of the boundary layer and the formation of mantle plumes, the long-term thermal evolution of both mantle and core, and the driving force for generation of the geomagnetic field (see Lay et al. (2008) for a review). Core heat loss has been estimated quantitatively from the temperature gradient in the boundary layer, inferred from recent seismological and mineral physics studies, and the thermal conductivity of the lowermost mantle (Hernlund et al., 2005, Lay et al., 2006, van der Hilst et al., 2007, Tateno et al., 2009a). However, the thermal conductivity (κ) and diffusivity (D=(κ/ρCp), where ρ is density and Cp is specific heat at constant pressure) have been poorly constrained at high pressure.

The lattice thermal diffusivity of MgSiO3 Pv, a primary mineral in the Earth's lower mantle, has only been measured at 1 bar and 26 GPa (Osako and Ito, 1991, Manthilake et al., 2011). Its pressure dependence therefore remains uncertain, although some predictions have been made based on theoretical modeling (Hofmeister, 2008, Manthilake et al., 2011) and first-principles calculations (de Koker, 2010). The value of MgSiO3 PPv, a high-pressure polymorph of MgSiO3 Pv, has only been speculated (Hofmeister, 2007a). Recent experiments on Ca1−xSrxIrO3 analogs showed that the thermal conductivity (or diffusivity) of PPv is nearly twice as high as that of the Pv phase (Keawprak et al., 2009, Cheng et al., 2011, Hunt et al., 2012). Previous estimates of the lower mantle thermal conductivity ranged widely between 5 and 30 W/m/K (e.g., Manga and Jeanloz, 1997, Hofmeister, 2008), and it has been often assumed to be 10 W/m/K (Stacey, 1992).

Here we measured the lattice thermal diffusivity of MgSiO3 Pv and PPv over the entire lower mantle pressure range of up to 144 GPa and at room temperature using newly developed pulsed light heating thermoreflectance technique in a diamond-anvil cell (DAC) (Fig. 1). Combined with an externally heated DAC technique, high-temperature diffusivity measurements on Pv was also performed at 70 GPa up to 436 K. Our results yield a Pv-dominant lowermost mantle conductivity of about 11 W/m/K, comparable to the conventionally assumed value of 10 W/m/K proposed by Stacey (1992). We also found that PPv-bearing rock exhibits ∼60% higher conductivity than Pv-dominant one, in good agreement with the recent results on Ca1−xSrxIrO3 analogs (Keawprak et al., 2009, Cheng et al., 2011, Hunt et al., 2012). However, enhanced conductivity of PPv does not significantly affect the estimate of core heat flow based on the double-crossing model. Our findings support the recent estimates of high heat flux across the core–mantle boundary (Labrosse et al., 2007, Lay et al., 2008).

Section snippets

Sample preparation and high-pressure generation

We used polycrystalline MgSiO3 Pv as a sample, which was synthesized at 25 GPa and 2073 K in a multi-anvil apparatus prior to the thermal diffusivity measurements. The Pv structure was confirmed by Raman spectroscopy. It was polished on both sides and thinned to about 10 μm in thickness by using Ion Slicer Ar ion-milling device (JEOL EM-09100IS) (Tateno et al., 2009b). Since complete transformation from Pv to PPv was found to be difficult when using Pv as the starting material, we also used MgSiO3

Lattice thermal diffusivities and conductivities of MgSiO3 Pv and PPv at high pressure

We conducted ten separate experiments to measure the lattice thermal diffusivities of MgSiO3 Pv from 11 to 144 GPa at room temperature (Table 1). As plotted in Fig. 5, the thermal diffusivity of Pv varied with increasing pressure; it increased from 2.5±0.2 mm2/s at 11 GPa to 11.6±2.5 mm2/s at 144 GPa. The datum at 108 GPa, obtained for Pv sample synthesized from gel starting material in a DAC, is consistent with other data collected for that prepared in a multi-anvil apparatus prior to DAC

Acknowledgments

We thank A.M. Hofmeister, D.A. Yuen, and N. de Koker for helpful comments, and R. Sinmyo and H. Ozawa for technical supports. Comments from two anonymous reviewers helped to improve the manuscript. X-ray diffraction measurements were conducted at BL10XU, SPring-8 (proposals no. 2010A0099 and 2010B0099). Sample section was prepared at the Foundation for Promotion of Material Science and Technology of Japan. K.O. was supported by the Japan Society for the Promotion of Science.

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    Present address: Center for Quantum Science and Technology under Extreme Conditions, Osaka University, Toyonaka, Osaka 560-8531, Japan.

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