Rapid decrease of MgAlO2.5 component in bridgmanite with pressure

Abstract

The solubility of the MgAlO 2.5 component in bridgmanite was measured at pressures of 27, 35 and 40 GPa and a temperature of 2000 o K using an ultra-high pressure multi-anvil press. Compositional analysis of recovered samples demonstrated that the MgAlO 2.5 component decreases with increasing pressure, and approaches virtually zero at 40 GPa. Above this pressure, the MgAlO 2.5 component, i.e. the oxygen-vacancy substitution, becomes negligible, and Al is incorporated in bridgmanite by the charge-coupled substitution only. These results are supported by the volume change associated with the change from the oxygen-vacancy substitution to charge-coupled substitution. The present result may explain the seismically observed slab stagnation in the mid-lower mantle. Although bridgmanite has been put forward as a potential host for water and argon in the lower mantle by trapping them in oxygen vacancies, such capabilities will rapidly decrease with depth and be lost in regions deeper than 1000 km.

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

“…Previous studies demonstrated that Al 2 O 3 -rich bridgmanite transforms to the LN-type phase upon decompression at ambient temperature (Funamori et al, 1997;Ishii et al, 2017;Liu et al, 2016;Liu, Ishii, & Katsura, 2017;Liu, Dubrovinsky, et al, 2019;Miyajima et al, 1999). In situ XRD observations (Figure 4) demonstrate that the same phase transition occurs in the En 50 FA 50 sample.…”

“…Journal of Geophysical Research: Solid Earth studies on K 0 of AlAlO 3 -bridgmanite (e.g., Zhang et al, 1999;Daniel et al, 2004;Yagi, et al, 2004;Walter et al, 2004Walter et al, , 2006, the reported data are highly scattered due to the strong variation of AlAlO 3 and MgAlO 2.5 components with pressure (Brodholt, 2000;Liu, Ishii, & Katsura, 2017), temperature (Brodholt, 2000;Liu, Akaogi, & Katsura, 2019), and Mg/Si ratio of the bulk composition (Andrault et al, 2001;Liu, Boffa Ballaran, et al, 2019) in the lower mantle. We therefore used dK 0 /dX = − 0.010 ± 0.003 GPa/mol for the AlAlO 3 component based on a theoretical study by Panero et al (2006).…”

“…Significant amounts of the oxygen vacancy component MgAlO 2.5 have been proposed for bridgmanite in the uppermost part of the lower mantle (Brodholt, 2000;Grüninger et al, 2019;Liu, Akaogi, & Katsura, 2019;Liu, Boffa Ballaran, et al, 2019;Liu, Ishii, & Katsura, 2017). Therefore, we also consider the incorporation of FeAlO 3 by consumption of MgAlO 2.5 as follows:…”

“…The content of FeAlO 3 in bridgmanite changes from 9 to 22 mol% in going from bulk pyrolitic mantle to basaltic slabs. Furthermore, a decrease in ΣFe 3+ /Fe ratio and oxygen vacancy component in bridgmanite has been proposed to explain slab stagnation in the mid-lower mantle (Liu, Ishii, & Katsura, 2017;Shim et al, 2017). Although the presence of the FeAlO 3 component may decrease with increasing depth based on the molar volume change for reaction (8), it should also suppress the oxygen vacancy component in bridgmanite.…”

“…Although the presence of the FeAlO 3 component may decrease with increasing depth based on the molar volume change for reaction (8), it should also suppress the oxygen vacancy component in bridgmanite. Furthermore, the dominant FeAlO 3 component may stabilize a dry bridgmanite because the charge-coupled component cannot provide cation sites to stabilize water in the crystal structure (e.g., Bolfan-Casanova et al, 2003;Litasov et al, 2003;Liu, Ishii, & Katsura, 2017;Navrotsky, 1999). These considerations suggest that subducted basaltic slabs dominated by bridgmanite may become stiffer than the bulk lower mantle.…”