Valence and spin states of iron are invisible in Earth’s lower mantle

Abstract

Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging. Here we show through a study of a ferric-iron-only (Mg0.46Fe3+0.53)(Si0.49Fe3+0.51)O3 Bdg that Fe3+ in the octahedral site undergoes a spin transition between 43 and 53 GPa at 300 K. The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions, the valence state of iron has minor effects on density and sound velocities relative to major cation composition.

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.…”

Abnormal Elasticity of Fe‐Bearing Bridgmanite in the Earth's Lower Mantle

“…A noticeable volume collapse of 0.5(±0.1)% has been observed across the spin transition between 18 and 25 GPa from P-V data at 300 K (1) O 3 (Bgm5) as a function of pressure at 300 K. Solid circles are experimental data from two runs, where the cell was loaded with neon in the first run (Run 1, green circles) and loaded with helium in the second run (Run 2, red circles), and blue circles are results from Run 2 during decompression. Data from Run 2 in He medium were used in the thermoelastic modeling (Run 1 data were not used because of suspected nonhydrostaticity at higher pressures; see supporting information for details (Liu et al, 2018). Data from Run 2 in He medium were used in the thermoelastic modeling (Run 1 data were not used because of suspected nonhydrostaticity at higher pressures; see supporting information for details (Liu et al, 2018).…”

“…Geophysical Research Letters in bridgmanite with composition (Mg 0.9 Fe 0.1 )SiO 3 containing 20% Fe 3+ (Mao et al, 2015), and recent experimental study observed 1.9% volume decrease within the spin transition of B-site Fe 3+ in ferric-iron-only bridgmanite (Mg 0.46 Fe 3+ 0.53 )(Fe 3+ 0.51 Si 0.49 )O 3 (Liu et al, 2018). In contrast, both Fe 3+ and Fe 2+ in the A site have been reported to remain in the HS state throughout the entire lower-mantle pressure range (24-130 GPa) (Dorfman et al, 2015;Hsu et al, 2010Hsu et al, , 2011Lin et al, 2016).…”

“…Of particular relevance to interpreting the aforementioned thermal and chemical variations are the V P , V S and density profiles of (Al,Fe)-bearing bridgmanite at the relevant high P-T conditions of the lower mantle. The incorporation of Fe in different spin and valence states in bridgmanite has been reported to influence its physical properties, such as elastic and transport properties (Chantel et al, 2012;Hsieh et al, 2017;Kurnosov et al, 2017;Li & Zhang, 2005;Liu et al, 2018;Mao et al, 2017;Murakami et al, 2012;Shukla et al, 2016;Wentzcovitch et al, 2004). The incorporation of Fe in different spin and valence states in bridgmanite has been reported to influence its physical properties, such as elastic and transport properties (Chantel et al, 2012;Hsieh et al, 2017;Kurnosov et al, 2017;Li & Zhang, 2005;Liu et al, 2018;Mao et al, 2017;Murakami et al, 2012;Shukla et al, 2016;Wentzcovitch et al, 2004).…”

“…Previous studies have shown that bridgmanite can accommodate as much as 10 mol.% Al, together with a substantial coupled substitution of Fe, about 11 mol.%, in its structure at mid-lower mantle conditions (Irifune et al, 2010;Xu et al, 2017). The incorporation of Fe in different spin and valence states in bridgmanite has been reported to influence its physical properties, such as elastic and transport properties (Chantel et al, 2012;Hsieh et al, 2017;Kurnosov et al, 2017;Li & Zhang, 2005;Liu et al, 2018;Mao et al, 2017;Murakami et al, 2012;Shukla et al, 2016;Wentzcovitch et al, 2004). While the thermal equation of state (EoS) of bridgmanite has been studied extensively (Boffa Ballaran et al, 2012;Dorfman et al, 2013;Lundin et al, 2008;Mao et al, 2017;Wolf et al, 2015), acoustic velocity results at high P-T are rare (Chantel et al, 2012;Jackson et al, 2005;Kurnosov et al, 2017;Li & Zhang, 2005;Murakami et al, 2007;Murakami et al, 2012).…”