Effect of Fe³⁺ on Phase Relations in the Lower Mantle: Implications for Redox Melting in Stagnant Slabs

Abstract

Recent studies have revealed that Earth's deep mantle may have a wider range of oxygen fugacities than previously thought. Such a large heterogeneity might be caused by material subducted into the deep mantle. However, high‐pressure phase relations are poorly known in systems including Fe3+ at the top of the lower mantle, where the subducted slab may be stagnant. We therefore conducted high‐pressure and high‐temperature experiments using a multi‐anvil apparatus to study the phase relations in a Fe3+‐bearing system at 26 GPa and 1573–2073 K, at conditions prevailing at the top of the lower mantle. At temperatures below 1923 K, MgSiO3‐rich bridgmanite, an Fe3+‐rich oxide phase, and SiO2 coexist in the recovered sample. Quenched partial melt was observed above 1973 K, which is significantly lower than the solidus temperature of an equivalent Fe3+‐free bulk composition. The partial melt obtained from the Fe3+‐rich bulk composition has a higher iron content than coexisting bridgmanite, similar to the Fe2+‐dominant system. The results suggest that strong mantle oxygen fugacity anomalies might alter the subsolidus and melting phase relations under lower mantle conditions. We conclude that (1) a small amount of melt may be generated from an Al‐depleted region of a stagnant slab, such as subducted former banded‐iron‐formation, and (2) Fe3+ is not transported into the deep part of the lower mantle because of its incompatibility during melting.

Stability of Fe5O6 and its relation to other Fe-Mg-oxides at high pressures and temperatures

“…In all cases, the run products contained (Mg,Fe) 2 Fe 2 O 5 coexisting with magnesiowüstite and either wadsleyite or ringwoodite. This is consistent with the results of Sinmyo et al (2019) The Fe 5 O 6 endmeber. In spite of repeated attempts, the crystal quality of samples of Fe 5 O 6 proved insufficient for a full single-crystal structural analysis.…”

“…These presses have been cross-calibrated over a wide range in temperature (at least 800-1800°C, see Keppler and Frost 2005). In addition, two experiments were performed at 28 GPa using a 1500 t Kawai-type multi-anvil press with the Osugi-type guide block system, IRIS-15 (Ishii et al 2016;2019).…”

Oxygen Vacancy Substitution Linked to Ferric Iron in Bridgmanite at 27 GPa

“…Hummer and Fei (2012) investigated Fe 3+ substitution mechanisms using multianvil experiments; however, their experiments did not reach chemical equilibrium as demonstrated by the coexistence of unreacted MgO and SiO 2 phases. Essentially, previous studies (Andrault & Bolfan-Casanova, 2001;Catalli et al, 2010;Hummer & Fei, 2012;Liu et al, 2018;Sinmyo et al, 2019) used starting materials without saturation of MgO (atomic Mg/Si = 1.0 or lower), which may prohibit the formation of MgFeO 2.5 based on observations that MgAlO 2.5 decreases with decreasing Mg/Si ratio in the MgO-SiO 2 -Al 2 O 3 system because of the reaction 2MgO + Al 2 O 3 = 2MgAlO 2.5 (Liu, Nishi, et al, 2017). In contrast, the Earth's lower mantle contains ferropericlase, and the concentration of MgFeO 2.5 should thus be maximized.…”

Pressure Destabilizes Oxygen Vacancies in Bridgmanite

“… Center shift and quadrupole splitting derived from fits of the Mössbauer spectra. There is no detectable Fe 2+ in bridgmanite, which should have center shift and quadrupole splitting of about 0.9–1.2 and 1.5–2.5 mm/s, respectively, based on the literature data as shown by open squares (Huang, Boffa‐Ballaran, McCammon, Miyajima, Dolejš & Frost, 2021; Lauterbach et al., 2000; McCammon, 1998; Sinmyo et al., 2019). …”

“…Center shift and quadrupole splitting derived from fits of the Mössbauer spectra. There is no detectable Fe 2+ in bridgmanite, which should have center shift and quadrupole splitting of about 0.9-1.2 and 1.5-2.5 mm/s, respectively, based on the literature data as shown by open squares (Huang, Boffa-Ballaran, McCammon, Miyajima, Dolejš & Frost, 2021;Lauterbach et al, 2000;McCammon, 1998;Sinmyo et al, 2019). (Frost & Langenhorst, 2002;Frost et al, 2004;Huang, Boffa-Ballaran, McCammon, Miyajima, Dolejš & Frost, 2021;Hummer & Fei, 2012;Lauterbach et al, 2000), all of which are lower than this study because their experimental temperatures are lower, and/or MgFe 2 O 4 -phase did not appear (namely Fe 3+ is not saturated).…”