Deformation of Post‐Spinel Under the Lower Mantle Conditions

Abstract

To study the viscosity of bridgmanite and ferropericlase aggregate, uniaxial compression deformation experiments on pre‐synthesized post‐spinel phase and bridgmanite two‐layered samples were conducted under top lower mantle pressure and 1773 K utilizing DT‐Cup apparatus. Up to the strain of 0.25 ± 0.05, the observed comparable strain of the bridgmanite and post‐spinel samples suggests the bridgmanite dominates the bulk viscosity of the post‐spinel without strain localization in periclase. The microstructures of the deformed post‐spinel samples show evidence of a similar strain of periclase with the bulk strain without strain partitioning. Texture analyses of bridgmanite indicate a dominant slip plane (100), with a steady state fabric strength achieved within the strain of 0.12 ± 0.01. The current experiment has provided no evidence about an onset of strain localization of ∼30 vol.% periclase at 0.25 strain. Our observations provide direct experimental verification of bridgmanite controlled rheology under low strain magnitude, which should be considered in geodynamical models which include mantle compositional and rheological evolution in the lower mantle.

Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump

“…However, using these scenarios to explain an increase in viscosity of one to two orders of magnitude requires an interconnected framework of ferropericlase (ferropericlase-controlled lower mantle rheology) 5 , 22 , which is unlikely because the electrical conductivity of the lower mantle is comparable to that of bridgmanite 27 , 28 , but three orders of magnitude smaller than that of ferropericlase 27 . In particular, recent atomic modelling 29 shows periclase has a slower creep rate than that of bridgmanite under mantle conditions, whereas deformation experiments 30 suggest that bridgmanite has an identical creep rate to that of post-spinel (70% bridgmanite + 30% ferropericlase); both of these findings indicate a bridgmanite-controlled lower-mantle rheology. Moreover, the oxygen vacancies in bridgmanite formed by the substitutions of Si 4+ with Al 3+ and Fe 3+ have been proposed to cause an increase in bridgmanite strength with depth 31 – 33 .…”

A Common Diffusional Mechanism for Creep and Grain Growth in Polymineralic Rocks: Application to Lower Mantle Viscosity Estimates

“…In this study, we approximate lower mantle material as a two‐phase aggregate of either bridgmanite + ferropericlase or post‐perovskite + ferropericlase in the upper to middle and the lowermost portions of the lower mantle (i.e., D″ layer), respectively, following previous mantle viscosity estimates (F. Xu et al., 2022; Yamazaki & Karato, 2001). These two minerals account for 90% of lower mantle material (Ita & Stixrude, 1993) and effectively represent the mantle as consisting of a mechanically strong primary phase and a weak secondary phase (Yamazaki & Karato, 2001).…”