Ab Initio Molecular Dynamics Investigation of Molten Fe–Si–O in Earth's Core
Résumé
Silicon and oxygen are potential light elements in Earth's core because their stronger affinity to metal observed with increasing temperature posits that significant amounts of both can be incorporated into the core. It was proposed that an Fe-Si-O liquid alloy could expel SiO 2 at the core-mantle boundary during secular cooling, leaving the core with either silicon or oxygen, not both. This was recently challenged in a study showing no exsolution but immiscibility in the Fe-Si-O system. Here we investigate the liquidus field of Fe-Si and Fe-O binaries and Fe-Si-O ternaries at core-mantle boundary pressures and temperatures using ab initio molecular dynamics. We find that the liquids remain well mixed with ternary properties identical to mixing of binary properties. Two-phase simulations of solid SiO 2 and liquid Fe show dissolution at temperatures above 4100 K, suggesting that SiO 2 crystallization as well as liquid immiscibility in Fe-Si-O is unlikely to occur in Earth's core. Plain Language Summary The standard hypothesis is that Earth's core inherited its composition (iron-nickel alloy + lighter elements such as Si, O, S, and C) during core formation by interaction with Earth's silicate magma ocean and has since remained fixed and well mixed through geological time. It was recently proposed, on the basis of high-pressure experiments, that silicon and oxygen dissolved in the core could crystallize as SiO 2 at the core-mantle boundary during secular cooling, challenging our understanding of core composition and evolution. While other experimental studies have not corroborated these findings, a recent study involving experiments and calculations proposed no SiO 2 exsolution but rather liquid-liquid immiscibility in the iron-silicon-oxygen system. In this study, we use ab initio molecular dynamics simulations to theoretically investigate the liquidus field of Fe-Si, Fe-O and Fe-Si-O alloys. We find that (1) the liquid alloys remain stable and well mixed, with the volumes of iron, silicon, and oxygen mixing ideally at core conditions; (2) no sign of SiO 2 crystallization or phase separation (i.e., immiscibility); and (3) simulations of solid SiO 2 in contact with liquid Fe show mixing of both phases, ruling out silica precipitation out of the core.
Domaines
Sciences de la Terre
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