The miscibility of Calcium Silicate Perovskite and Bridgmanite: A single phase perovskite in hot, iron-rich regions

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Authors

Joshua Martin Richard Muir , Feiwu Zhang

Abstract

Calcium silicate perovskite and bridgmanite are two phases which coexist in the lower mantle but at some temperature will dissolve into each other to form a single phase. In this work we use DFT to calculate the temperature at which this occurs at 3 pressures- 25, 75 and 125 GPa. At these three pressures we find solubility of Ca in Mg of 0.41/0.52/0.92 % at 2000 K and 0.72/1.07/3.30 % at 2500 K and of Mg in Ca of 0.18/0.13/0.06 % and 0.37/0.43/0.65 % for the pure MgSiO3-CaSiO3 system. We thus conclude that in the absence of other elements pyrolytic compositions of bridgmanite and Calcium silicate perovskite do not mix in the lower mantle but could mix near the bottom (~90 GPa) of hotter mantle patches such as potentially in LLSVPs.
We build a simple model to test the effects of other elements and find that a large concentration (>~1 atomic%) is needed of any element before miscibility is strongly affected. Of the elements likely to be present in these kinds of concentrations ferrous iron increases miscibility while aluminium decreases it and ferric iron can increase or decrease it. With expected Fe and Al compositions pyrolite mixes at around ~90 GPa along a lower mantle geotherm but will remain unmixed if the Fe partitions largely to ferropericlase. Harzburgitic compositions are predicted to mix throughout the lower mantle with sufficient iron while basaltic compositions are predicted to remain unmixed at both geotherm and slab temperatures. We suggest therefore that phase mixing of Calcium silicate perovskite and bridgmanite is likely to be important in deeper, hotter and more iron enriched parts of the lower mantle.
Using static (~0 K) calculations we also predict that phase mixing of the perovskites will have strong seismic effects. For a pyrolytic mixture these are a small decrease in density (-0.14-0.25%), a significant decrease (-1.5-3.5%) in Vs, a small decrease in Vp (-0.5-1.2%) and a small increase in bulk sound (+0.00-0.75%). These seismic changes, while preliminary, are similar to those seen in LLSVPs which are also regions that are hotter and thus promote the formation of a single phase.

DOI

https://doi.org/10.31223/X56309

Subjects

Earth Sciences, Mineral Physics, Physical Sciences and Mathematics

Keywords

Bridgmanite, Calcium-silicate perovskite

Dates

Published: 2020-11-02 06:28

Last Updated: 2020-11-02 14:28

License

CC BY Attribution 4.0 International

Additional Metadata

Conflict of interest statement:
None

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