Crystallization of silicon dioxide and compositional evolution of the Earth’s core

Abstract

The Earth's core is about ten per cent less dense than pure iron (Fe), suggesting that it contains light elements as well as iron. Modelling of core formation at high pressure (around 40-60 gigapascals) and high temperature (about 3,500 kelvin) in a deep magma ocean predicts that both silicon (Si) and oxygen (O) are among the impurities in the liquid outer core. However, only the binary systems Fe-Si and Fe-O have been studied in detail at high pressures, and little is known about the compositional evolution of the Fe-Si-O ternary alloy under core conditions. Here we performed melting experiments on liquid Fe-Si-O alloy at core pressures in a laser-heated diamond-anvil cell. Our results demonstrate that the liquidus field of silicon dioxide (SiO) is unexpectedly wide at the iron-rich portion of the Fe-Si-O ternary, such that an initial Fe-Si-O core crystallizes SiO as it cools. If crystallization proceeds on top of the core, the buoyancy released should have been more than sufficient to power core convection and a dynamo, in spite of high thermal conductivity, from as early on as the Hadean eon. SiO saturation also sets limits on silicon and oxygen concentrations in the present-day outer core.

Ab Initio Molecular Dynamics Investigation of Molten Fe–Si–O in Earth's Core

“…This comes with the change of bonding environments for the elements that are going to precipitate. In particular, the well-equilibrated liquid Fe 0.79 Si 0.08 O 0.13 ternary at 4000 K and 136 GPa from Pozzo et al (2013) not only provides almost identical diffusion coefficients of Fe, Si, and O as in our calculations but also is at odds with the proposed SiO 2 exsolution isotherms (Hirose et al, 2017) by having higher concentrations of Si and O than what the precipitation model allows for ( Figure S3). Another evidence comes from diffusion properties in liquid iron alloy.…”

“…The incorporation of both Si and O into the core during core-mantle differentiation provides a mechanism to account for the core's density deficit. Recently, Hirose et al (2017) proposed a scenario that relaxes that hypothesis, where dissolved Si and O in the core would crystallize during secular cooling and float out due to its negative buoyancy, hence changing core composition over time. The conditions (pressure, temperature, and composition) of metal-silicate equilibration during core formation set the composition of the bulk core by the end of accretion, and this is considered frozen from then onward.…”

The solubility of heat-producing elements in Earth’s core

“…Recently, MgO exsolution (Badro et al, 2016) and SiO 2 crystallisation (Hirose et al, 2017) have been proposed as an alternative source of chemical buoyancy to drive a dynamo prior to inner core growth. These are potent sources of energy and dissipation, and while they both eradicate the need for a hot initial core, their effect is not predicted to kick in until 1 to 2 Gyr after Earth's formation for MgO, and even later for SiO 2 , leaving the Earth devoid of a geodynamo, assuming a young inner core age.…”

Feasibility of a magma ocean dynamo on Mars

“…No computationally simulated dynamos have yet been constructed to operate in this regime, though Hori et al (2012) note that volumetric buoyancy sources, which could characterize exsolutiondriven dynamos, yield more multipolar fields and hence less spatial regularity to the field, perhaps mimicking Mars' surface magnetism. If, however, the Earth's dynamo operates this way (Hirose et al 2017), its strong dipolarity contradicts the volumetric source field morphology inference.…”

“…Breuer et al (2010), summarizing earlier suggestions (Stevenson 1983), note that saturation of metal in light elements might occur at the lower pressures of a magma ocean that might later exsolve from the metal due to oversaturation at higher pressures after the metal segregates to form the core. Flow of exsolved material upwards in the core could drive a dynamo, which Hirose et al (2017) show that it is quite efficient and possibly operates the present-day Earth's. No computationally simulated dynamos have yet been constructed to operate in this regime, though Hori et al (2012) note that volumetric buoyancy sources, which could characterize exsolutiondriven dynamos, yield more multipolar fields and hence less spatial regularity to the field, perhaps mimicking Mars' surface magnetism.…”