Iron Partitioning and Oxidation State in Earth’s Lower Mantle

Abstract:

Valence state and concentrations of iron in lower mantle phases have strong effects on their chemical and physical properties. Experimental studies have reported stark differences in iron partitioning between bridgmanite (Brg) and ferropericlase (Fp) for San Carlos olivine [1] and pyrolite [2] systems. We recently performed experiments at lower mantle conditions for an Al-rich olivine system [3] and observed an iron enrichment of the silicate phase very similar to that in pyrolite. Mössbauer studies [4] have shown that in the presence of aluminum non negligible amounts of Fe³⁺ could be incorporated in bridgmanite explaining the observed iron enrichment.

Non negligible amounts of Fe³⁺ in the lower mantle could influence transport properties of the phases [5]. The evaluation of ferrous and ferric iron concentrations in lower mantle mineral assemblages is then key to a thorough understanding of geophysical observations and associated mantle dynamics.

We used electron energy loss spectroscopy technique to quantify the proportions of Fe²⁺ and Fe³⁺ iron in Brg and Fp phases previously synthesized from Al-rich olivine composition [3]. The oxidation state of iron in the lower mantle will be discussed as well as ensuing implications on transport properties for relevant lower mantle compositions.

References

[1] Sakai et al., 2009

[2] Prescher et al., 2014

[3] Piet et al., submitted

[4] McCammon et al., 1996

[5] Xu et al., 1998