Anomalous thermal properties and spin crossover of ferromagnesite (Mg,Fe)CO3

TL;DR

This study uses first-principles calculations to reveal how iron spin crossover in ferromagnesite drastically alters its thermal properties, impacting Earth's deep carbon cycle and lower mantle dynamics.

Contribution

It provides new insights into the thermal property changes of ferromagnesite caused by iron spin crossover at high pressures and temperatures.

Findings

Volume reduction and elastic anomalies due to spin crossover.

Significant increase in thermal expansion and heat capacity.

Persistent heat capacity anomaly up to high temperatures.

Abstract

Ferromagnesite (Mg,Fe)CO3, also referred to as magnesiosiderite at high iron concentration, is a solid solution of magnesite (MgCO3) and siderite (FeCO3). Ferromagnesite is believed to enter the Earth's lower mantle via subduction and is considered a major carbon carrier in the Earth's lower mantle, playing a key role in the Earth's deep carbon cycle. Experiments have shown that ferromagnesite undergoes a pressure-induced spin crossover, accompanied by volume and elastic anomalies, in the lower-mantle pressure range. In this work, we investigate thermal properties of (Mg,Fe)CO3 using first-principles calculations. We show that nearly all thermal properties of ferromagnesite are drastically altered by iron spin crossover, including anomalous reduction of volume, anomalous softening of bulk modulus, and anomalous increases of thermal expansion, heat capacity, and Guneisen parameter. Remarkably, the anomaly of heat capacity remains prominent (up to 40%) at high temperature without smearing out, which suggests that iron spin crossover may significantly affect the thermal properties of subducting slabs and the Earth's deep carbon cycle.