Shielding of periclase by bridgmanite during compression of a polycrystalline mantle assemblage

Abstract Earth’s lower mantle consists predominantly of two minerals, bridgmanite and Fe-bearing periclase, forming polycrystalline aggregates. Interpretation of seismic observations to constrain mantle temperature and composition requires a detailed understanding of the elastic behavior of mantle rocks. While the elastic properties of single-phase bridgmanite and periclase have been studied extensively, the bulk elastic properties of multiphase materials depend on how stress and strain are partitioned between the constituent phases, a process poorly understood. Here, we present high-pressure synchrotron X-ray diffraction data on a sintered polycrystal of bridgmanite and periclase with estimated volume proportions of 7:3 and a bulk composition of (Mg0.91Fe0.09)2SiO4, approximating a lower-mantle rock. We compare the compression behavior of the two-phase assemblage in quasi-hydrostatic conditions with and without high-temperature stress annealing. We find that bridgmanite forms a load-bearing framework, partially shielding periclase from the external confining pressure. During compression without annealing, this results in the development of under-pressure in the more compressible periclase. Consequently, the pressure-volume curve appears shallower relative to single-phase periclase and the onset of the iron spin crossover is suppressed, occurring at about 8 GPa higher external pressure. In contrast, when the two-phase assemblage is heated at 1800 K for around 1 minute, stresses are relaxed through plastic deformation and pressure is homogenized, producing compression curves consistent with single-phase measurements. Our results indicate that stress-strain partitioning in multiphase materials is highly temperature-dependent, with compression behavior moving from a regime close to the iso-strain bound towards the iso-stress bound with increasing temperature. The relative timescales of pressure variations and stress-relaxation likely determine the bulk elastic behavior of a multiphase mantle rock when a seismic wave passes. Future experimental investigations of the temperature- and frequency-dependence of stress-strain partitioning in bridgmanite-periclase aggregates are key to accurately model seismic properties of lower-mantle rocks.