Impact of Fe²⁺ incorporation on the structural and thermodynamic characteristics of the akimotoite-to-bridgmanite phase transition

The 660 km discontinuity serves as an important boundary for elucidating the dynamics of subducting slabs at this depth. Subducting slabs, characterized by their lower temperatures compared to the surrounding mantle, undergo distinct phase transitions. Harzburgitic compositions are the most gravitationally stable within the lower mantle transition zone, potentially incorporating up to 15 vol% akimotoite in their mineral assemblage. Recent experimental studies indicate that the transition from akimotoite to bridgmanite may be a critical factor in deciphering the complexities inherent to this region.  Additionally, discoveries of iron-rich natural analogs of akimotoite and bridgmanite in Suizhou L6 chondrite have attracted considerable scientific attention regarding the stability of these iron-rich phases. These analogs, identified as Hemleyite (Fe²⁺_0.48Mg_0.37Ca_0.04Na_0.04Mn²⁺_0.03Al_0.03Cr³⁺_0.01) SiO₃ and Hiroseite (Fe²⁺_0.44Mg_0.37Fe³⁺_0.1Al_0.04 Ca_0.03Na_0.02) (Si_0.89Al_0.11) O₃​, provide critical insights into the geochemical behavior and phase stability of iron-bearing silicates under extreme conditions. In this study, first-principles computational methods based on density functional theory (DFT) were employed to investigate the stability fields of these iron-rich phases under high-pressure and high-temperature conditions, aiming to elucidate the effects of Fe²⁺ incorporation on slab stagnation behavior. The analysis demonstrates a positive correlation between acoustic velocity contrasts and increasing Fe²⁺ concentration at the phase transition boundary, while the transition pressure decreases significantly. Additionally, the findings suggest that the overall steepness of the Clapeyron slope remains largely invariant with increasing iron content.