On the increase in thermal diffusivity caused by the perovskite to post-perovskite phase transition and its implications for mantle dynamics

Simon A Hunt,D Rhodri Davies,Andrew M Walker,Richard J Mccormack,Andrew S Wills,David P Dobson,Li Li

doi:10.1016/j.epsl.2011.12.009

Simon A Hunt, D Rhodri Davies + Show 5 more

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https://doi.org/10.1016/j.epsl.2011.12.009

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Abstract

The thermal diffusivity (κ) of perovskite and post-perovskite CaIrO3 has been measured, at elevated pressure and temperatures up to 600°C, using the X-radiographic Ångström method. At high temperatures we find that the thermal diffusivity of post-perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated. Assuming a similar effect occurs in MgSiO3 post-perovskite, the effect of the contrasting thermal transport properties between perovskite and post-perovskite on mantle dynamics has been investigated using simple two-dimensional convection models. These show a reduction in extent and increase in depth of post-perovskite lenses, as well as increased core–mantle-boundary heat-flux, broader upwellings and more vigorous downwellings when compared to the reference, constant a, case.

Highlights

The D′′ region, at the base of Earth's mantle, is known to be both anomalous and crucial to our understanding of mantle dynamics and Earth's evolution
At all temperatures investigated in this study the thermal diffusivity of post-perovskite is greater than that of perovskite
The ratio of the difference between the measured values is reasonably constant above 275 °C; between 300 and 600 °C the thermal diffusivity of post-perovskite has a weighted average 1.8 ± 1 times greater than that of perovskite

Summary

Introduction

The D′′ region, at the base of Earth's mantle, is known to be both anomalous and crucial to our understanding of mantle dynamics and Earth's evolution. This ~200 km thick layer contains superadiabatic thermal gradients in excess of 1000 K, is the region in which some mantle upwellings are thought to originate (Schubert et al, 2001), and may act as a graveyard for subducted slabs. In order to model and understand the dynamics of D′′ and its effect on the rest of the planet, it is necessary to determine the rheology, density, thermal expansion and thermal diffusivity of both perovskite and post-perovskite. Whilst the unit cell volumes, density, and thermal expansion of both MgSiO3–perovskite and post-perovskite have been measured (e.g. Guignot et al, 2007; Komabayashi et al, 2008), the absolute or even relative thermal transport properties of MgSiO3–perovskite and postperovskite are, at best, poorly constrained

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