Compositional Effects on the Akimotoite–Bridgmanite Phase Transition and Their Significance for Subducting Slab Behavior

The Mantle Transition Zone (MTZ) is a geophysically and geochemically significant yet incompletely constrained region of Earth’s interior. Among the high-pressure mineral phases stable under MTZ conditions, akimotoite is especially relevant in the context of cold subducting slabs. The phase transition between akimotoite and bridgmanite near the 660 km discontinuity is thought to influence slab behaviour and associated mantle features. Experimental and meteoritic studies have shown that akimotoite can incorporate a range of cations, such as Fe and Al, which may significantly affect its phase stability and the pressure–temperature conditions governing its transformation to bridgmanite. In this study, we employ first-principles calculations within the quasi-harmonic approximation to quantify the thermodynamic and thermoelastic effects of cationic substitution on the akimotoite-to-bridgmanite transition. To capture realistic mantle compositional variability, we construct a two-phase coexisting region for Fe- and Al-bearing systems to better constrain the solid solution effect in this regime. Our results demonstrate that increasing Fe²⁺ content significantly decreases the akimotoite–bridgmanite transition pressure and enhances the acoustic velocity contrast across the boundary. The associated modification of the Clapeyron slope implies possible changes in slab buoyancy and stagnation behaviour near the 660-km discontinuity (Pandit et al., 2025). These results underscore the importance of compositional effects in modulating phase stability and provide new constraints on the role of the akimotoite–bridgmanite transition in MTZ subduction dynamics.

 

Reference:

Pandit, P., Chandrashekhar, P., Sharma, S., & Shukla, G. (2025). Effect of Fe²⁺ on akimotoite to bridgmanite transition: Its implication on subduction dynamics. Geochemistry, Geophysics, Geosystems, 26(3), e2024GC012010.