Partitioning of oxygen between the Earth's mantle and core

Abstract

[1] Experiments to investigate the partitioning of oxygen between liquid iron and (Mg,Fe)O magnesiowüstite were conducted at 30-70 GPa and 2800-3500 K using a laser-heated diamond anvil cell. A thin foil was prepared from the reacted regions in the recovered samples using a focused ion beam. The compositions of coexisting quenched iron and magnesiowüstite were measured using a transmission electron microscope equipped with energy dispersive X-ray spectroscopy and electron energy-loss spectroscopy. In order to understand and model the results, additional experiments were performed to determine the activity of oxygen, or rather FeO, in liquid Fe metal. Multianvil experiments to measure the oxygen contents of coexisting immiscible metallic and ionic liquids in the Fe-FeO system were performed up to 25 GPa. The results were used to extract excess mixing properties for Fe-FeO liquids at high pressure and temperature. These properties were used to derive a model that describes oxygen partitioning in the Fe-Mg-O system that is independent of the actual experimental partitioning data. The model indicates that the oxygen content of liquid Fe becomes a strong nonlinear function of the FeO content of magnesiowüstite at pressures greater than 25 GPa. This prediction is in excellent agreement with the experimental partitioning data, which is faithfully reproduced in most instances. The new results confirm that the Earth's core is undersaturated in oxygen with respect to the FeO content of the bulk mantle, which will result either in FeO being depleted from the very base of the mantle or lead to the development of an FeO-enriched outer layer of the core. These possibilities are not mutually exclusive.

High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O

“…We also ran 15 experiments in a multianvil press at pressures of 15 (Frost et al, 2010) results in a value that is higher by ~0.4 log units, in agreement with previous studies (e.g., Cottrell et al, 2009;Siebert et al, 2011).…”

“…Incorporating activities for Fe and FeO that differ from unity based on a thermodynamic model (Frost et al, 2010) results in a calculated oxygen fugacity that is higher by ~0.4 log units, in agreement with previous studies Siebert et al, 2012). Two experiments were run at almost identical conditions, with the only difference being the silicon content of the metal.…”

“…If n is known, the effects of oxygen fugacity can be removed through use of the exchange coefficient, which is defined as K D = D M / D Fe n/2 . In the case of oxygen, the exchange coefficient is defined as (Frost et al, 2010). We have calculated D and K D for each element that is present in both the metal and silicate melts in our experiments.…”

Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle

“…While the proposed reaction products have not been found in later studies 85 , recent experiments have shown that the lowermost mantle may be depleted in FeO due to the loss to the oxygen-undersaturated outer core [85][86][87][88] . Addition of oxygen to the core could have been provided by an oxidized lower mantle basal magma ocean early in Earth's history 88 .…”

A seismologically consistent compositional model of Earth’s core

“…Because the inner core is growing from the outer core, and these are in chemical equilibrium, then the outer core should also contain silicon, between 1.2 and 3.6% [assuming a liquid/ solid partition coefficient (7) of 1.5 ± 0.3 for silicon]. Moreover, core formation experiments place a tight constraint (12,(31)(32)(33)(34)(35) on oxygen solubility in molten iron: incorporating large amounts of oxygen in a core-forming metal in equilibrium with a molten silicate requires very high temperatures. In those conditions, silicon will always be incorporated in the metal as well (31,34,35).…”