Pure bridgmanite lower mantle composition

Research using GSECARS 13-IDD and XSD 3-IDB beamlines rules out possibility of a pure bridgmanite lower mantle composition.

The high-pressure equations of state for 13% Fe and Fe-free bridgmanite
are incorporated into an ideal lattice mixing model enabling the estimation
of thermophysical properties for a large range of ferrous iron compositions. Using this mixture model, we examine the range of plausible values in temperature composition space relevant to the deep mantle. Through this analysis, we demonstrate that there is no combination of temperature and composition capable of matching the Earth’s bulk properties near the base of the mantle, ruling out the possibility of a pure bridgmanite lower mantle composition. Furthermore, we explore the buoyancy properties of bridgmanite-dominated piles in the deep mantle, directly relevant to Large Low-Shear Velocity Provinces. Using plausibility arguments, we show that metastable bridgmanite domes are marginally possible, given our knowledge of the equation of state but represent a sensitive balance between iron content and temperature and are therefore unlikely. Instead,we find the passive chemical pile explanation more compelling, as it allows for a broad range of composition and temperature values in the deep mantle but may require external forces to sweep them into coherent structures.

Aaron S. Wolf, Jennifer M. Jackson, Przemeslaw Dera and Vitali B. Prakapenka, “The Thermal Equation of State of (Mg, Fe)SiO3 Bridgmanite (perovskite) and Implications for Lower Mantle Structures,” Journal of Geophysical Research: Solid Earth, Vol. 120, Issue 11, pp. 7460–7489, November 2015, DOI: 10.1002/2015JB012108