The role of second phases for controlling microstructural evolution in polymineralic rocks: A review

Herwegh, Marco; Linckens, Jolien; Ebert, Andreas W.; Berger, Alfons; Brodhag, Sabine

doi:10.1016/j.jsg.2011.08.011

Cited by 150 publications

(145 citation statements)

References 150 publications

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“…In ultramylonites that deform by GSS creep, grain growth is usually inhibited by the presence of secondary phases, a process called Zener pinning (Herwegh et al, 2011). When synkinematic creep cavities control fluid transport in ultramylonites (Fusseis et al, 2009;Menegon et al, 2015), cavities should also control secondary phase precipitation and hence directly influence Zener pinning (Herwegh and Jenni, 2001).…”

Section: The Role Of Creep Cavities In the Activation Of Gss Creepmentioning

confidence: 99%

“…We speculate that fluidfilled creep cavities will have affected grain-boundary migration directly by acting as pinning phases, therefore arresting grain sizes at sub-equilibrium dimensions and promoting the transition to VGBS. This idea can be further explored by combining our data with the Zener parameter, which quantifies the influence of second phases on rheology (Herwegh et al, 2011):…”

Section: The Role Of Creep Cavities In the Activation Of Gss Creepmentioning

confidence: 99%

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Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing

et al. 2017

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Abstract. Establishing models for the formation of wellmixed polyphase domains in ultramylonites is difficult because the effects of large strains and thermo-hydro-chemomechanical feedbacks can obscure the transient phenomena that may be responsible for domain production. We use scanning electron microscopy and nanotomography to offer critical insights into how the microstructure of a highly deformed quartzo-feldspathic ultramylonite evolved. The dispersal of monomineralic quartz domains in the ultramylonite is interpreted to be the result of the emergence of synkinematic pores, called creep cavities. The cavities can be considered the product of two distinct mechanisms that formed hierarchically: Zener-Stroh cracking and viscous grain-boundary sliding. In initially thick and coherent quartz ribbons deforming by grain-size-insensitive creep, cavities were generated by the Zener-Stroh mechanism on grain boundaries aligned with the Y Z plane of finite strain. The opening of creep cavities promoted the ingress of fluids to sites of low stress. The local addition of a fluid lowered the adhesion and cohesion of grain boundaries and promoted viscous grain-boundary sliding. With the increased contribution of viscous grainboundary sliding, a second population of cavities formed to accommodate strain incompatibilities. Ultimately, the emergence of creep cavities is interpreted to be responsible for the transition of quartz domains from a grain-size-insensitive to a grain-size-sensitive rheology.

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Section: The Role Of Creep Cavities In the Activation Of Gss Creepmentioning

confidence: 99%

Section: The Role Of Creep Cavities In the Activation Of Gss Creepmentioning

confidence: 99%

Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing

et al. 2017

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“…In order to avoid second-phase effects on the grain-size analysis of recrystallized quartz, only grains completely surrounded by grain boundaries were considered in the construction of grain boundary maps. Grains in contact with different phases were excluded from the data set, since their recrystallized size might have been influenced by second-phase pinning (Herwegh et al, 2011). …”

Section: Microstructural Analysismentioning

confidence: 99%

“…The development of localized highstrain zones requires some form of strain weakening as the deformation progresses. Many studies have highlighted the important role of progressive grain-size reduction in promoting weakening and strain localization in the lithosphere (Warren and Hirth, 2006;Herwegh et al, 2011;Kilian et al, 2011;Bercovici and Ricard, 2012;Montesi, 2013;Platt, 2015). Intense grain-size reduction can lead to the activation of grainsize-sensitive creep as the dominant deformation mechanism, with a consequent potential increase in strain rate of up to 2 orders of magnitude in localized high-strain zones (War-ren and Hirth, 2006;Platt, 2015).…”

Section: Introductionmentioning

confidence: 99%

“…Stünitz and Tullis, 2001;Goncalves et al, 2012) and (2) fracturing (Ree et al, 2005;Park et al, 2006;Negrini et al, 2014). In the polymineralic aggregates of reaction products grain growth is inhibited by second-phase pinning, so that the grain size is maintained sufficiently small to activate diffusion creep as the dominant deformation mechanism, thereby resulting in strain localization (Herwegh et al, 2011;Kilian et al, 2011;Menegon et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

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Brittle grain-size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

Viegas

Menegon

Archanjo³

2016

Solid Earth

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Abstract. The Pernambuco shear zone (northeastern Brazil)is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarsegrained (> 50 µm) feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (< 15 µm) localized in C' shear bands. Detailed microstructural observations and electron backscatter diffraction (EBSD) analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (∼ 1-2 mm) monomineralic veins transposed along the shear zone foliation or as thin ribbons (≤ 25 µm width) dispersed in the feldspathic mixture. The microstructure and c axis crystallographicpreferred orientation are similar in the thick monomineralic veins and in the thin ribbons, and they suggest dominant subgrain rotation recrystallization and activity of prism < a > and rhomb < a> slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the quartz monomineralic veins to the thin ribbons embedded in the feldspathic C' bands (14 µm vs. 5 µm respectively). The fine-grained feldspar mixture has a weak crystallographic-preferred orientation interpreted as the result of shear zone parallel-oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Once C' shear bands were generated and underwent viscous deformation at constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate 1 order of magnitude faster than the monophase quartz monomineralic veins, as evidenced by applying experimentally and theoretically calibrated flow laws for dislocation creep in quartz and diffusion creep in feldspar. Overall, our data set indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystal plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the polyphase feldspathic material localized much of the strain. Extensive grain-size reduction and weakening of feldspars is attained in the East Pernambuco mylonites mainly via fracturing which would trigger a switch to diffusion creep and strain localization without a prominent role of metamorphic reactions.

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Influence of mineral fraction on the rheological properties of forsterite + enstatite during grain size sensitive creep: 3. Application of grain growth and flow laws on peridotite ultramylonite

Tasaka

Hiraga

Michibayashi

2014

J. Geophys. Res. Solid Earth

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Microstructures of a layered peridotite ultramylonite from the Oman ophiolite are compared with that of experimentally deformed samples. Average grain sizes and grain size ratios of olivine and pyroxene from each layer are compared with respect to the fraction of pyroxene (f px ) in the layer. Grain size of the pyroxene is almost constant among different f px layers, whereas olivine grain size decreases significantly with increasing f px , both of which were characteristic features found in forsterite + enstatite aggregates after grain growth experiments . Furthermore, the Zener relationship (log d ol /d px versus log f px ) found in the ultramylonite is remarkably comparable to that observed in our experiments. These observations indicate effective pinning of olivine grain growth due to the presence of pyroxene grains during the deformation of the rocks. Olivine grains in layers with f px ≥ 0.03 do not exhibit lattice-preferred orientation (LPO), whereas the grains in layers with f px < 0.03 exhibit LPO, indicating that deformation proceeded via diffusion-and dislocation-accommodated creep in the former and the latter layers, respectively. We simulated the evolution of grain size and viscosity in the shear zone based on our grain growth and flow laws obtained for diffusion creep of forsterite + enstatite and successfully reproduced the observed grain sizes in the ultramylonite. We therefore conclude that the relative values of the kinetic parameters, some of which are functions of the f px , are applicable to nature.

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The role of second phases for controlling microstructural evolution in polymineralic rocks: A review

Cited by 150 publications

References 150 publications

Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing

Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing

Brittle grain-size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

Influence of mineral fraction on the rheological properties of forsterite + enstatite during grain size sensitive creep: 3. Application of grain growth and flow laws on peridotite ultramylonite

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