Phase stability and thermal equations of state of Fe3S and Fe2S polymorphs to Earth's core pressures and high temperatures
Abstract
The seismologically-determined density profile and dynamics of Earth's iron-rich core can be explained by incorporating a light element alloying component such as sulfur. Accordingly, understanding the phase stability and pressure-temperature-volume relationships of iron-rich sulfides to core conditions is critical for assessing the core's composition. In this in-situ X-ray diffraction and ex-situ chemical analysis study, we determined the high P-T stability fields of Fe3 S and Fe2 S polymorphs to outer core pressures and high temperatures and fit their thermal equations of state. Between 26 and 140 GPa and at moderate temperatures, a Pnma Fe2 S structure coexists with Fe and FeO. Upon heating, below 85 GPa, the Pnma Fe2 S phase transitions to I-4 Fe3 S. Above 85 GPa, the Pnma Fe2 S phase transitions to a P-62m Fe2 S structure at high temperatures. This phase is stable on the liquidus to 142 GPa. Chemical analysis of a recovered sample from 170 GPa and high temperatures reveals an Fe2 S phase coexisting with FeO and quenched melt. We fit thermal equations of state for I-4 Fe3S and Pnma Fe2S to 75 GPa and 137 GPa respectively and determined the zero pressure volumes, isothermal bulk moduli, pressure derivatives and gamma values. Fe3S was found to compress isotropically while the Pnma Fe2S phase displayed anisotropic compressibility. Extrapolating the adiabatic density curves of Fe3S and Pnma Fe2S to outer core conditions, we conclude that 16-17 wt% S is required to account for the density deficit at the core-mantle boundary. Based on the phase relations determined in this study, Fe2S, not Fe3S, is stable at outer core pressures and high temperatures and could play a critical role in the thermodynamics of Earth's core.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMDI13A..05Z
- Keywords:
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- 1507 Core processes;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1510 Dynamo: theories and simulations;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 7207 Core;
- SEISMOLOGY