Rheological Weakening of Olivine + Orthopyroxene Aggregates Due To Phase Mixing: Part 2. Microstructural Development

Abstract

To understand the processes involved in phase mixing during deformation and the resulting changes in rheological behavior, we conducted torsion experiments on samples of iron‐rich olivine plus orthopyroxene. The experiments were conducted at a temperature, T, of 1200°C and a confining pressure, P, of 300 MPa using a gas‐medium deformation apparatus. Samples composed of olivine plus 26% orthopyroxene were deformed to outer radius shear strains up to γ ≈ 26. In samples deformed to lower strains of γ ≲ 4, elongated olivine and pyroxene grains form a compositional layering. Already by this strain, mixtures of small equant grains of olivine and pyroxene begin to develop and continue to evolve with increasing strain. The ratios of olivine to pyroxene grain size in deformed samples follow the Zener relationship, indicating that pyroxene grains effectively pin the grain boundaries of olivine and inhibit grain growth. Due to the reduction in grain size, the dominant deformation mechanism changes as a function of strain. The microstructural development forming more thoroughly mixed, fine‐grained olivine‐pyroxene aggregates can be explained by the difference in diffusivity among Me (Fe or Mg), O, and Si, with transport of MeO significantly faster than that of SiO2. These mechanical and associated microstructural properties provide important constraints for understanding rheological weakening and strain localization in upper mantle rocks.

Diffusion Creep and Grain Growth in Forsterite +20 vol% Enstatite Aggregates: 2. Their Common Diffusional Mechanism and Its Consequence for Weak‐Temperature‐Dependent Viscosity

“…They proposed that at forsterite/enstatite interfaces perpendicular to the compressional direction, a reaction of Mg 2 SiO 4 − MgO = MgSiO 3 occurs, while a reaction of MgSiO 3 + MgO = Mg 2 SiO 4 occurs at interfaces perpendicular to the tensile direction to accommodate sample strain. Tasaka et al () postulated the same reaction for the phase mixing microstructure observed in their experimentally deformed olivine + orthopyroxene aggregates. This mixing process only requires MgO diffusion while leaving Si, the slowest ion, at its original locations.…”

“…O Diffusion Control CaseSundberg and Cooper (2008) proposed that strain can be accommodated without diffusion of all of ions that compose forsterite and enstatite during diffusion creep. They proposed that at forsterite/enstatite interfaces perpendicular to the compressional direction, a reaction of Mg 2 SiO 4 À MgO = MgSiO 3 occurs, while a reaction of MgSiO 3 + MgO = Mg 2 SiO 4 occurs at interfaces perpendicular to the tensile direction to accommodate sample strain Tasaka et al (2017). postulated the same reaction for the phase mixing microstructure observed in their experimentally deformed olivine + orthopyroxene aggregates.…”

Rheological Weakening of Olivine + Orthopyroxene Aggregates Due to Phase Mixing: 1. Mechanical Behavior

“…In contrast, in samples deformed to strains of γ ≈ 26, olivine and pyroxene grains are small, thoroughly mixed, and equant (Figure c). Detailed microstructural analyses including grain size, CPO, and grain boundary length are described in our companion paper (Tasaka et al, ).…”

Fluid‐Assisted Deformation and Strain Localization in the Cooling Mantle Wedge of a Young Subduction Zone (Semail Ophiolite)

“…Different mechanisms have been invoked to account for these combined processes of grain size reduction and phase mixing in ductile mantle shear zones: dislocation-mediated dynamic recrystallization and GBS-related grain switching events (e.g., Linckens et al, 2014), dynamic recrystallization and stress-induced differential cation diffusion (Tasaka et al, 2017), or dispersed nucleation of fine grains through dissolutionprecipitation processes (Hidas et al, 2016;Précigout et al, 2017;Précigout & Stünitz, 2016), associated with metamorphic reactions (De Ronde & Stünitz, 2007;De Ronde et al, 2005;Newman et al, 1999;Toy et al, 2010;Vissers et al, 1995) or melt-rock interaction (Dijkstra et al, 2002;Kaczmarek & Tommasi, 2011). The evidence for dislocation mobility in olivine and pyroxene (undulose extinctions and subgrain walls; Figures 3 and 4) and the occurrence of olivine triple junctions suggest that dynamic recrystallization by subgrain rotation partly contributed to reducing grain size.…”