[S-IT14] Deep Earth Sciences

[SIT14-06] Phase Stability of MgSiO₃ Silicate Minerals at the Mantle TransitionZone (MTZ)

*Priyanka Pandit¹, Prathibha Chandrashekar¹, Gaurav Shukla^1,2 (1.Indian Institution of Science Education and Research Kolkata , 2.National Center for High Pressure Studies Kolkata)

Keywords:Multiple 660 km discontinuity, Slab stagnations, Lower Mantle transition Zone related phase changes

The post-spinel phase transition from ringwoodite to bridgmanite and ferropericlase is generally attributed to the seismic discontinuity at 660km depth. However, seismic observations reveal multiple discontinuities and depressions by ~30 to ~90 km at this boundary in the cold subduction zones (e.g., Tonga subduction, Mariana slab, etc.). Several high-pressure experimental and first-principles studies suggest that the akimotoite to bridgmanite phase transition may play a significant role in unravelling the complexity of the 660 km boundary. The recent discovery of iron-rich natural analogues of akimotoite and bridgmanite in the Suizhou L6 chondrite raises the possibility that iron incorporation in akimotoite and bridgmanite may have a significant impact on the phase stability of these minerals. Using first-principles computational methods, we investigated the stability field of iron-rich analogues of akimotoite and bridgmanite, (Mg_1-xFe_x²⁺)SiO₃. The static transition pressure decreases significantly with the incorporation of iron. We find that at the transition point, the contrast in compressional velocity decreases, whereas the shear velocity contrast increases with increasing concentration of Fe²⁺ from x = 0 to 0.5. We also investigated the effects of temperature on the transition within the quasiharmonic approximation and found that the overall steepness of the Clapeyron slope decreases with an increase in iron concentration.