Phase transitions in MgSiO3 post-perovskite in super-Earth mantles

TL;DR

This study explores how the Mg/Si ratio influences phase transitions of MgSiO3 post-perovskite at TPa pressures, crucial for modeling super-Earth mantles and their internal structures.

Contribution

It introduces the importance of the Mg/Si ratio in determining phase transition sequences and internal layering in super-Earth mantles, a factor previously overlooked.

Findings

Identified new phase transformation sequences dependent on Mg/Si ratio.

Recombination reactions vary with compositional changes.

Highlights the Mg/Si ratio's role in mantle modeling.

Abstract

The highest pressure form of the major Earth-forming mantle silicate is MgSiO3 post-perovskite (PPv). Understanding the fate of PPv at TPa pressures is the first step for understanding the mineralogy of super-Earths-type exoplanets, arguably the most interesting for their similarities with Earth. Modeling their internal structure requires knowledge of stable mineral phases, their properties under compression, and major element abundances. Several studies of PPv under extreme pressures support the notion that a sequence of pressure induced dissociation transitions produce the elementary oxides SiO2 and MgO as the ultimate aggregation form at ~3 TPa. However, none of these studies have addressed the problem of mantle composition, particularly major element abundances usually expressed in terms of three main variables, the Mg/Si and Fe/Si ratios and the Mg#, as in the Earth. Here we show that the critical compositional parameter, the Mg/Si ratio, whose value in the Earth's mantle is still debated, is a vital ingredient for modeling phase transitions and internal structure of super-Earth mantles. Specifically, we have identified new sequences of phase transformations, including new recombination reactions that depend decisively on this ratio. This is a new level of complexity that has not been previously addressed, but proves essential for modeling the nature and number of internal layers in these rocky mantles.