[S-IT14] Deep Earth Sciences

[SIT14-P17] Solid-liquid partitioning of ferrous iron and ferric iron under the mantle transition zone and lower mantle pressures

*Hideharu Kuwahara¹, Ryoichi Nakada² (1.Geodynamics Research Center, Ehime University, 2.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

Keywords:Early Earth, Magma ocean, Redox state, High pressure experiment

Iron is the most abundant multivalent element in the mantle of rocky bodies. Thus, the oxidation state of the mantle is thought to be determined by the valence state of iron oxides [e.g., 1]. During the core-mantle differentiation, a magma ocean would have been very reducing because of the chemical equilibration with the metallic core. However, Ce anomaly in mantle-derived zircons crystallized from melting of the Earth’s upper mantle 4 billion years ago suggests that the upper mantle was already oxidized to the same degree as today [2]. This indicates the great mantle oxidation during or immediately after the formation of the Earth.
The redox disproportionation of ferrous iron (Fe²⁺) in a magma ocean under high pressures has been proposed as a possible mechanism of the great mantle oxidation of the Earth's mantle [3-5]. Although the redox disproportionation of Fe²⁺ in a magma ocean may have mainly controlled the valence state of iron in the bulk silicate Earth, solid-liquid partitioning of ferrous iron (Fe²⁺) and ferric iron (Fe³⁺) during magma ocean solidification is also important to constrain the distribution of Fe²⁺ and Fe³⁺ in the mantle. However, the partition coefficients of Fe²⁺ and Fe³⁺ between mantle minerals and melt have been poorly constrained.
Here we report our recent experimental study on the partitioning of Fe²⁺ and Fe³⁺ between the lower mantle minerals and melts [6]. Bridgmanite, the most abundant mineral in the lower mantle, is compatible with Fe³⁺, but our experimental results at pressures of 23-27 GPa show limited fractionation of Fe²⁺ and Fe³⁺ between bridgmanite and coexisting melts. In addition, majorite, a major mantle mineral at the mantle transition zone, the solid-liquid partitioning experiment at 18 GPa shows insignificant difference of Fe³⁺/Fe²⁺ ratios between majorite and melt, similar to bridgmanite. In this presentation, we will also discuss the effects of Al content and oxygen fugacity, which are thought to affect the compatibility of bridgmanite and majorite with Fe³⁺.

References: [1] Frost and McCammon, 2008, Annual Reviews of Earth and Planetary Sciences 36, 389-420 [2] Trail et al., 2011, Nature 480, 79-82. [3] Hirschmann, 2012, Earth and Planetary Science Letters 341-344, 48-57. [4] Armstrong et al., 2019, Science 365, 903-906. [5] Kuwahara et al., 2023, Nature Geoscience 16, 461-465. [6] Kuwahara and Nakada, 2023, Earth and Planetary Science Letters 615, 118197.