Laboratory investigation of mechanisms for phase mixing in olivine + ferropericlase aggregates

Abstract

To investigate the role of grain boundary pinning and the mechanisms by which phase mixing occurs during deformation of polymineralic rocks, we conducted high-strain torsion experiments on samples consisting of olivine plus 30 vol% ferropericlase. Experiments were performed in a gas-medium deformation apparatus at 1524 K and 300 MPa. Samples were deformed to outer radius shear strains of up to γ ( R ) = 14.1. The value of the stress exponent and the small grain sizes of our samples indicate that our two-phase material deformed by dislocation-accommodated grain boundary sliding. In samples deformed to 1 < γ < 7, elongated clusters of ferropericlase grains form thin layers in the olivine matrix, and small grains of ferropericlase appear at olivine grain boundaries and three- and four-grain junctions. By γ ≈ 14, a well-distributed mixture of small ferropericlase grains among the olivine grains developed. Microstructures exhibit similarities to both mechanical and chemical models proposed to describe the processes leading to phase mixing. Our results provide evidence for grain size reduction during phase mixing that results in a grain size significantly smaller than the value predicted by the single-phase recrystallization piezometer for olivine. Thus, phase mixing provides a mechanism for the persistent weakening of rocks that is important for developing and maintaining shear zones necessary for plate tectonics. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics'.

The Generation of Plate Tectonics From Grains to Global Scales: A Brief Review

“…At lithosphere like stress and temperature conditions, the model predicts the formation of rapidly deforming strongly localized ultramylonitic bands embedded within coarser grained, poorly mixed or monomineralic, slowly deforming regions (Figure c). The coexistence of the well‐mixed weak regions and the poorly mixed strong regions is in agreement with the hysteretic branches predicted by Bercovici and Ricard (), as well as the experimental observations of Cross & Skemer (; see also Figure d) and Wiesman et al ().…”

“…The effectiveness of Zener pinning, however, is controlled by the degree to which the different phases are interspersed or mixed with each other at the grain scale. The process of phase mixing, and its effect on rock rheology and shear localization, have been the focus of numerous recent experimental and theoretical studies (Bercovici & Mulyukova, ; Bercovici & Ricard, ; Bercovici & Skemer, ; Cross & Skemer, ; Tasaka et al, ; Tasaka et al, ; Wiesman et al, ). At low pressures (i.e., <1 GPa), intergrain mixing may occur by chemical or metamorphic reactions (Platt, ; Czertowicz et al, ; Précigout & Stünitz, ) and brittle processes (Dimanov et al, ), all of which typically involving cavitation.…”

“…At low pressures (i.e., <1 GPa), intergrain mixing may occur by chemical or metamorphic reactions (Platt, ; Czertowicz et al, ; Précigout & Stünitz, ) and brittle processes (Dimanov et al, ), all of which typically involving cavitation. At higher pressures representative of the middle and lower lithosphere, grain mixing may instead be accommodated by dissolution and precipitation of one phase on the grain boundaries of the other phase (Tasaka et al, , ; Wiesman et al, ), or percolation of one phase along the grain boundaries of the other phase, driven by capillary forces and deviatoric stress acting to dilate the grain boundaries (Bercovici & Skemer, ).…”

“…For self‐weakening and shear localization by grain damage to be effective, and specifically for the Zener pinning to be effective, the different mineral phases that make up the rock must be well mixed. The newly developed theories (Bercovici & Mulyukova, ; Bercovici & Skemer, ) and experimental studies (Cross & Skemer, ; Tasaka et al, , ; Wiesman et al, ) provide a physical basis for the grain scale mixing processes by which the fine‐grained mylonites and ultramylonites may form. The microphysical mixing models need to be further incorporated into the larger‐scale mechanical models, in which the small‐scale units of ultramylonites can be mixed with coarser grained monomineralic fabric, for example, by folding and stretching of the deforming lithosphere, in order to be applied to the geodynamics of plate boundary formation.…”

Rheological Weakening of Olivine + Orthopyroxene Aggregates Due to Phase Mixing: Effects of Orthopyroxene Volume Fraction

“…Previous deformation experiments using two-phase aggregates demonstrated that strain weakening is associated with grain size reduction resulting from phase mixing in olivine-pyroxene systems (Farla et al, 2013;Hirauchi et al, 2016;Precigout & Stunitz, 2016;Tasaka, Zimmerman, Kohlstedt, Stunitz, & Heilbronner, 2017), olivine-ferropericlase systems (Wiesman et al, 2018), and calcite-halite systems (Cross & Skemer, 2017). Especially for peridotites, several mixing mechanisms have been proposed including dynamic recrystallization, geometrical mixing due to grain boundary sliding, formation of cavities leading to nucleation of new grains under hydrous conditions, formation of new hydrous phases due to reaction of minerals, severing of new grains formed at triple junctions coupled with their transport along grain boundaries, severe shear thinning of individual phases, and stress-driven diffusion processes (Bercovici & Skemer, 2017;Cross & Skemer, 2017;Farla et al, 2013;Hirauchi et al, 2016;Linckens et al, 2014;Precigout & Stunitz, 2016;Tasaka, Zimmerman, Kohlstedt, Stunitz, & Heilbronner, 2017).…”