Stability and Solubility of the FeAlO₃ Component in Bridgmanite at Uppermost Lower Mantle Conditions

Abstract

We report the stability and solubility of the FeAlO3 component in bridgmanite based on phase relations in the system MgSiO3‐FeAlO3 at 27 GPa and 2000 K using a multi‐anvil apparatus combined with in situ synchrotron X‐ray diffraction measurements. The results demonstrate that the FeAlO3 component dominates Fe3+ and Al3+ substitution in bridgmanite, although trace amounts of oxygen‐ and Mg‐site vacancy components are also present. Bridgmanite with more than 40 mol% FeAlO3 transforms into the LiNbO3‐type phase upon decompression. The FeAlO3 end‐member decomposes into corundum and hematite and does not form single‐phase bridgmanite. We determined the maximum solubility of the FeAlO3 component in bridgmanite at 27 GPa and 2000 K to be 67 mol%, which is significantly higher than previously reported values (25–36 mol%). We determined the partial molar volume (27.9 mol/cm3) and bulk modulus (197 GPa) of hypothetical FeAlO3 bridgmanite, which are significantly higher and lower than those of AlAlO3 and FeSiO3 bridgmanite, respectively. The non‐ideality of MgSiO3‐FeAlO3 solid solution (W = 13 kJ/mol, where W is the interaction parameter) is significantly larger than that for MgSiO3‐AlAlO3 (5 kJ/mol) and MgSiO3‐FeSiO3 (3 kJ/mol) solid solutions. The rapid decrease in abundance of the MgAlO2.5 component in bridgmanite with increasing pressure is enhanced by the presence of the FeAlO3 component. The FeAlO3 content in pyrolite and mid‐ocean ridge basalt is far below its solubility limit in bridgmanite and provides new insight into the mineralogy of the lower mantle.

The Effect of Fe‐Al Substitution on the Crystal Structure of MgSiO3 Bridgmanite

“…The three data points from Liu et al. (2020) have larger molar volumes than our Fe 3+ AlO 3 substitution trend, which may result from inaccuracy in the determination of the Brg volumes due to the use of a micro‐focused XRD. The V 0 reported for a very Fe‐rich Brg with the composition (Mg 0.46 Fe 3+ 0.53 ) (Si 0.49 Fe 3+ 0.51 )O 3 (Liu et al., 2018) was 2% larger than predicted from our Fe 3+ Fe 3+ O 3 trend line, most likely because the room pressure volume in Liu et al.…”

“…The large deviation of the Fe 3+ AlO 3 -rich samples reported by Vanpeteghem et al (2006) is most likely due to the presence of additional Na and Ti in some of these samples (Figure 2b). The three data points from Liu et al (2020) have larger molar volumes than our Fe 3+ AlO 3 substitution trend, which may result from inaccuracy in the determination of the Brg volumes due to the use of a micro-focused XRD. The V 0 reported for a very Fe-rich Brg with the composition (Mg 0.46 Fe 3+ 0.53 ) (Si 0.49 Fe 3+ 0.51 ) O 3 (Liu et al, 2018) was 2% larger than predicted from our Fe 3+ Fe 3+ O 3 trend line, most likely because the room pressure volume in Liu et al (2018) was determined by extrapolating back to room pressure using a second order Birch-Murnaghan EoS.…”

Pressure Destabilizes Oxygen Vacancies in Bridgmanite

“…The upper limit of Fe 3+ content should be obtained in the system with coexistence of bridgmanite and hematite, which was not investigated in this study. Additionally, the formation of the FeAlO 3 component will also increase the Fe 3+ content in Al-bearing bridgmanite, which causes the high Fe 3+ content (about 0.7 pfu) in Liu, Dubrovinsky, et al (2019) and Liu et al (2020).…”

“…Bridgmanite is stabilized in the pressure range 23–125 GPa (e.g., Ishii et al., 2018; Murakami et al., 2004) and is the dominant mineral in Earth (e.g., Irifune & Ringwood, 1987a, 1987b). It can incorporate large amounts of trivalent elements such as Al 3+ and Fe 3+ in its crystal structure (e.g., Andrault et al., 1998; McCammon, 1997; Navrotsky, 1999; Navrotsky et al., 2003; Shim et al., 2017) by the formation of XXO 3 and MgXO 2.5 components (X is Fe 3+ or Al 3+ ) via charge‐coupled and oxygen‐vacancy mechanisms, respectively (e.g., Huang, Boffa‐Ballaran, McCammon, Miyajima, Dolejš & Frost, 2021; Huang, Boffa‐Ballaran, McCammon, Miyajima, & Frost, 2021; Lauterbach et al., 2000; Liu, Akaogi, & Katsura, 2019; Liu, Ishii, & Katsura, 2017; Liu, Boffa‐Ballaran, et al., 2019; Liu et al., 2020; Navrotsky, 1999; Navrotsky et al., 2003; Nishio‐Hamane et al., 2005, 2008; O'Neill & Jeanloiz, 1994). The different substitution mechanisms thus produce different types of defect species.…”

Vibrational and Thermodynamic Properties of Hydrous Iron-Bearing Lowermost Mantle Minerals

“…While generally, the lower mantle is reductive, Fe can be incorporated into bridgmanite and post-perovskite in both the +2 (ferrous) and +3 (ferric) state, with the latter being favored in the presence of Aluminum at the high pressures and temperatures near the D" [12]. This is created through a disproportionation reaction of Fe 2+ into Fe 3+ and metallic Fe [13][14][15]. Experimental and theoretical studies demonstrate that Fe 3+ can be incorporated into the lattice of lower mantle minerals either as Fe 3+ -Fe 3+ pairs on the Mg (A) and Si (B) sites or as Fe 3+ -Al 3+ pairs with Fe 3+ on the A and Al 3+ on the B though above~80 GPa these preferences can be changed as some Fe 3+ swaps to the B site and Al 3+ to the A site [2,[16][17][18][19].…”