Phase mixing in upper mantle shear zones: Olivine nucleation during dynamic recrystallization of orthopyroxene and clinopyroxene porphyroclasts

2020 ◽  
Author(s):  
Sören Tholen ◽  
Jolien Linckens
Keyword(s):  
Grain Size ◽  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Upper Mantle ◽  
Shear Zones ◽  
Grain Boundary Sliding ◽  
Diffusion Creep ◽  
Phase Assemblage ◽  
Secondary Phases ◽  
Phase Mixing

<p>Small grain size and a well-mixed phase assemblage are key features of upper mantle (ultra)mylonitic layers. In those layers, Zener pinning inhibits grain growth, which could lead to diffusion creep. This increases the strain rate for a given stress significantly. Prerequisite is phase mixing which can occur by dynamic recrystallization (dynRXS) plus grain boundary sliding (GBS), metamorphic or melt/fluid-rock reactions, creep cavitation plus nucleation, or by a combination of those processes. In order to get insights into the interplay of phase mixing and dynRXS we investigate microfabrics (EBSD, optical microscopy) displaying the transition from clasts to mixed assemblages. Samples are taken from the Lanzo peridotite shear zone (Italy).</p><p>Olivine dynamically recrystallizes from protomylonitic to ultramylonitic samples. Its grain size varies systematically between monomineralic (~20µm) and polymineralic layers (~10µm). Olivine is the dominant mixing phase for both, dynamically recrystallizing orthopyroxene (ol~55vol.%) and clinopyroxene clasts (ol~45vol.%). In contrast, recrystallizing olivine clasts show little evidence of phase mixing. In phase mixtures, olivine neoblasts show weak (J-index ~1.8) C-Type and weak (J-Index ~1.5) B-type CPO’s. Both types suggest the presence of water during deformation.</p><p>Isolated, equiaxial orthopyroxene clasts are present in all samples. DynRXS of opx starts in mylonites. Some clasts and tips of extensively elongated opx bands (max. axial ratios 1:50) are bordered by fine-grained (min. ECD~5µm) mixtures of olivine, opx ± anorthite/ cpx/ pargasite. Mixing intensities seem to depend on the connection to the olivine-rich matrix. Clast grain boundaries are highly lobate with indentations of secondary phases (mostly olivine). Opx neoblasts have no internal deformation and show large misorientations close to their host clast (misorientation angle >45° at ~20µm distance). Their grain shape is either flat and elongated or equiaxial. Both shapes have lobate boundaries. Their CPO depends on the host clast orientation. In ultramylonites, opx bands disappeared completely.</p><p>Clinopyroxene porphyroclasts dynamically recrystallize in protomylonite to ultramylonite samples. Olivine is the dominant mixing phase (~45vol.%). Cpx mixed area grain sizes tend to be coarser (~10µm) than in corresponding opx areas (~6µm). Ultramylonitic cpx-ol assemblages have a higher mixing percentage (phase boundaries/grain boundaries ~70%) than mylonitic assemblages (~40%). In the mylonitic layers, clusters of cpx neoblasts form ‘walls’ parallel to their host grain borders. Olivine neoblasts between these clusters show no CPO. The overall cpx CPO varies from [001] perpendicular and [010] parallel to the foliation with (J -Index ~2.5) to [100] perpendicular and [001] parallel to the foliation (J-Index ~1.2).</p><p>Beside few thoroughly mixed areas, bands of cpx+ol and of opx+ol are still distinguishable in ultramylonitic layers. This suggests their origin to be dynamically recrystallized opx and cpx clasts. Therefore, phase mixing is assumed to occur simultaneously to clast recrystallization. Beside a small gradient of opx/cpx abundance depending on the distance from their host clast there is little evidence for phase mixing by dynRXS+GBS only. High abundances of olivine neoblasts at grain boundaries of recrystallizing clasts and their instant mixed assemblage with host phase neoblasts suggest phase mixing being strongly dependent on olivine nucleation during dynRXS of opx and cpx.</p>

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

Solid Earth ◽  
2017 ◽  
Vol 8 (6) ◽  
pp. 1193-1209 ◽  
Author(s):  
James Gilgannon ◽  
Florian Fusseis ◽  
Luca Menegon ◽  
Klaus Regenauer-Lieb ◽  
Jim Buckman
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Finite Strain ◽  
Grain Boundary Sliding ◽  
Large Strains ◽  
Cavity Formation ◽  
Phase Mixing ◽  
Transient Phenomena ◽  
Boundary Sliding

Abstract. Establishing models for the formation of well-mixed polyphase domains in ultramylonites is difficult because the effects of large strains and thermo-hydro-chemo-mechanical 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 YZ 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 grain-boundary 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.

scholarly journals Myrmekite and strain weakening in granitoid mylonites

2018 ◽  
Author(s):  
Alberto Ceccato ◽  
Luca Menegon ◽  
Giorgio Pennacchioni ◽  
Luiz Fernando Grafulha Morales
Keyword(s):  
Grain Size ◽  
Heterogeneous Nucleation ◽  
Eastern Alps ◽  
Grain Boundary Sliding ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Phase Mixing ◽  
Fine Grained ◽  
Fine Grain ◽  
Creep Cavitation

Abstract. At mid-crustal conditions, deformation of feldspar is mainly accomplished by a combination of fracturing, dissolution/precipitation and reaction-weakening mechanisms. In particular, K-feldspar is reaction-weakened by formation of strain-induced myrmekite – a fine-grained symplectite of plagioclase and quartz. Here we investigate with EBSD the microstructure of a granodiorite mylonite, developed at 420–460 °C during cooling of the Rieserferner pluton (Eastern Alps), to assess the microstructural processes and the role of weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of the plagioclase of pristine myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain size reduction and ordered phase mixing by heterogeneous nucleation of quartz and plagioclase. The fine grain size of sheared myrmekite promoted grain size-sensitive creep mechanisms including fluid-assisted grain boundary sliding in plagioclase, coupled with heterogeneous nucleation of quartz within creep cavitation pores. Flow laws calculated for monomineralic quartz, feldspar, and quartz + plagioclase aggregates (sheared myrmekite), show that during mylonitization at 450 °C, grain-size-sensitive creep in sheared myrmekite accommodated strain rates several orders of magnitude higher than monomineralic quartz layers deforming by dislocation creep. Therefore, diffusion creep and grain size-sensitive processes contributed significantly to bulk rock weakening during mylonitization. Our results have implications for modelling the rheology of the mid-upper continental (felsic) crust.

scholarly journals Formation of Ultramylonites in an Upper Mantle Shear Zone, Erro-Tobbio, Italy

Minerals ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 1036
Author(s):  
Jolien Linckens ◽  
Sören Tholen
Keyword(s):  
Grain Size ◽  
Shear Zone ◽  
Upper Mantle ◽  
Microstructural Analysis ◽  
Shear Zones ◽  
Geodynamic Setting ◽  
Mixing Processes ◽  
Phase Mixing ◽  
Fine Grained ◽  
Order Of Magnitude

Deformation in the upper mantle is localized in shear zones. In order to localize strain, weakening has to occur, which can be achieved by a reduction in grain size. In order for grains to remain small and preserve shear zones, phases have to mix. Phase mixing leads to dragging or pinning of grain boundaries which slows down or halts grain growth. Multiple phase mixing processes have been suggested to be important during shear zone evolution. The importance of a phase mixing process depends on the geodynamic setting. This study presents detailed microstructural analysis of spinel bearing shear zones from the Erro-Tobbio peridotite (Italy) that formed during pre-alpine rifting. The first stage of deformation occurred under melt-free conditions, during which clinopyroxene and olivine porphyroclasts dynamically recrystallized. With ongoing extension, silica-undersaturated melt percolated through the shear zones and reacted with the clinopyroxene neoblasts, forming olivine–clinopyroxene layers. Furthermore, the melt reacted with orthopyroxene porphyroclasts, forming fine-grained polymineralic layers (ultramylonites) adjacent to the porphyroclasts. Strain rates in these layers are estimated to be about an order of magnitude faster than within the olivine-rich matrix. This study demonstrates the importance of melt-rock reactions for grain size reduction, phase mixing and strain localization in these shear zones.

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

2017 ◽  
Author(s):  
James Gilgannon ◽  
Florian Fusseis ◽  
Luca Menegon ◽  
Klaus Regenauer-Lieb ◽  
Jim Buckman
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Finite Strain ◽  
Grain Boundary Sliding ◽  
Cavity Formation ◽  
Phase Mixing ◽  
Creep Cavity ◽  
Boundary Sliding

Abstract. The dispersal of monomineralic quartz domains in a quartzofeldspathic ultramylonite is interpreted to be the result of the emergence of syn-kinematic 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 YZ 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 grain boundary 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.

scholarly journals Self-Consistent Grain Size Evolution controls Strain Localization during Rifting

2021 ◽  
Author(s):  
Jonas Ruh ◽  
Leif Tokle ◽  
Whitney Behr
Keyword(s):  
Grain Size ◽  
Upper Mantle ◽  
Numerical Models ◽  
Shear Zones ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Plate Boundaries ◽  
Mantle Dynamics ◽  
Size Evolution ◽  
Dominant Deformation Mechanism

Abstract Geodynamic numerical models often employ solely grain-size-independent dislocation creep to describe upper mantle dynamics. However, observations from nature and rock deformation experiments suggest that shear zones can transition to a grain-size-dependent creep mechanism due to dynamic grain size evolution, with important implications for the overall strength of plate boundaries. We apply a two-dimensional thermo-mechanical numerical model with a composite diffusion-dislocation creep rheology coupled to a dynamic grain size evolution model based on the paleowattmeter. Results indicate average olivine grain sizes of 3–12 cm for the upper mantle below the LAB, while in the lithosphere grain size ranges from 0.3–3 mm at the Moho to 6–15 cm at the LAB. Such a grain size distribution results in dislocation creep being the dominant deformation mechanism in the upper mantle. However, deformation-related grain size reduction below 100 μm activates diffusion creep along lithospheric-scale shear zones during rifting, affecting the overall strength of tectonic plate boundaries.

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

2018 ◽  
Vol 376 (2132) ◽  
pp. 20170417 ◽  
Author(s):  
Harison S. Wiesman ◽  
Mark E. Zimmerman ◽  
David L. Kohlstedt
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Plate Tectonics ◽  
Shear Zones ◽  
Grain Boundary Sliding ◽  
Outer Radius ◽  
Thin Layers ◽  
Two Phase ◽  
Phase Mixing ◽  
Grain Boundary Pinning

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'.

Self-consistent grain size evolution controls lithospheric shear zone formation during continental rifting

2021 ◽  
Author(s):  
Jonas B. Ruh ◽  
Leif Tokle ◽  
Whitney M. Behr
Keyword(s):  
Grain Size ◽  
Upper Mantle ◽  
Shear Zones ◽  
Size Reduction ◽  
Numerical Code ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Dominant Mechanism ◽  
Size Evolution ◽  
Growth Laws

&lt;p&gt;In geodynamic numerical models, grain-size-independent dislocation creep often solely defines the governing crystal-plastic flow law in the upper mantle. However, grain-size-dependent diffusion creep may become the dominant deformation mechanism if grain size is sufficiently small. Previous studies implying composite diffusion-dislocation creep rheologies and fixed grain size suggest that the upper mantle is stratified with the dominant mechanism being dislocation creep at shallow depths and diffusion creep further down. Studies with variable grain size in the upper mantle depending on common grain-size evolution models demonstrate that the contrary might be the case, where diffusion creep is acting within the mantle lithosphere and dislocation creep in the asthenosphere below. Diffusion creep as a dominant mechanism has important implications for the overall strength of the lithosphere and therefore for the dynamic evolution of lithospheric-scale extension and orogeny.&lt;/p&gt;&lt;p&gt;To investigate the importance of grain size and the effects of resulting crystal-plastic creep within the upper mantle, we developed a two-dimensional thermo-mechanical numerical code based on the finite difference method with a fully staggered Eularian grid and freely advecting Lagrangian markers. The model implies a composite diffusion-dislocation creep rheology and a dynamic grain-size evolution model based on the paleowattmeter including recently published olivine grain growth laws.&lt;/p&gt;&lt;p&gt;Results of upper mantle extension indicate olivine grain sizes of ~7 cm for large parts of the upper mantle below the LAB, while in the lithosphere grain size ranges from ~1 mm at the Moho to ~5 cm at the LAB. This grain size distribution indicates that dislocation creep dominates deformation in the entire upper mantle. However, diffusion creep activates along lithospheric-scale shear zones during rifting where intense grain size reduction occurs to local stress increase. We furthermore test the implications of wet and dry olivine rheology and respective crystal growth laws and interpret their effects on large-scale tectonic processes. Our results help explain strain localization during extension by strength loss related to grain size reduction and consequent diffusion creep activation.&lt;/p&gt;

scholarly journals Microstructural evolution during high-temperature tensile creep at 1,500°C of a MoSiBTiC alloy

2020 ◽  
Vol 39 (1) ◽  
pp. 136-145 ◽  
Author(s):  
Sojiro Uemura ◽  
Shiho Yamamoto Kamata ◽  
Kyosuke Yoshimi ◽  
Sadahiro Tsurekawa
Keyword(s):  
Grain Size ◽  
Grain Boundaries ◽  
Microstructural Evolution ◽  
Grain Boundary Sliding ◽  
Primary Creep ◽  
Tensile Creep ◽  
Tertiary Creep ◽  
Continuous Dynamic Recrystallization ◽  
Continuous Dynamic ◽  
Creep Regime

AbstractMicrostructural evolution in the TiC-reinforced Mo–Si–B-based alloy during tensile creep deformation at 1,500°C and 137 MPa was investigated via scanning electron microscope-backscattered electron diffraction (SEM-EBSD) observations. The creep curve of this alloy displayed no clear steady state but was dominated by the tertiary creep regime. The grain size of the Moss phase increased in the primary creep regime. However, the grain size of the Moss phase was found to remarkably decrease to <10 µm with increasing creep strain in the tertiary creep regime. The EBSD observations revealed that the refinement of the Moss phase occurred by continuous dynamic recrystallization including the transformation of low-angle grain boundaries to high-angle grain boundaries. Accordingly, the deformation of this alloy is most likely to be governed by the grain boundary sliding and the rearrangement of Moss grains such as superplasticity in the tertiary creep regime. In addition, the refinement of the Moss grains surrounding large plate-like T2 grains caused the rotation of their surfaces parallel to the loading axis and consequently the cavitation preferentially occurred at the interphases between the end of the rotated T2 grains and the Moss grains.

A Different View on Dynamic Recrystallization

2012 ◽  
Vol 715-716 ◽  
pp. 235-242 ◽  
Author(s):  
Günter Gottstein
Keyword(s):  
Grain Size ◽  
Steady State ◽  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Experimental Results ◽  
Critical Conditions ◽  
New Approach ◽  
Theoretical Predictions

A new approach to dynamic recrystallization (DRX) is introduced. It is based on the assumption that the critical conditions for DRX and the arrest of DRX grain boundaries are related to the development of mobile subboundaries. The theoretical predictions are compared to experimental results during incipient and steady-state DRX. The grain size sensitivity of the DRX grains establishes the desired link between deformation and DRX microstructure.

scholarly journals Effects of B on the Hot Ductility of Fe-36Ni Invar Alloy

2019 ◽  
Vol 38 (2019) ◽  
pp. 380-388 ◽  
Author(s):  
Yaxu Zheng ◽  
Fuming Wang ◽  
Changrong Li ◽  
Zhanbing Yang ◽  
Yutian He
Keyword(s):  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Secondary Phase ◽  
Grain Boundary Sliding ◽  
Hot Ductility ◽  
Temperature Range ◽  
Area Reduction ◽  
The Poor ◽  
Thermo Calc Software ◽  
Bearing Alloy

AbstractThis work conducted systematic studies on the effect of B on the hot ductility behavior of Fe-36Ni alloy over the temperature range of 900–1,200 °C by use of Gleeble-3500 thermal simulator, Thermo-Calc software, transmission electron microscopy and secondary ion mass spectroscopy. The influencing factors and mechanisms are also discussed in the present work. Results show that all the values of area reduction of the investigated alloy samples are below 60 % in the temperature range of 900–1,000 °C, indicating the poor hot ductility of the investigated alloys in this temperature range. When the grain boundary sliding occurs during the hot tensile processes, the fine secondary phase particles at grain boundaries prevent the occurrence of dynamic recrystallization and promote the nucleation and propagation of cracking simultaneously, resulting in the poor hot ductility of the investigated alloys in this temperature range. In the B bearing alloy, the segregation of B atoms around austenite grain boundaries promotes the solute dragging effects at grain boundaries and strongly inhibits the occurrence of dynamic recrystallization, which increases the brittle temperature to 1,000 °C. When the temperature exceeds 1,050 °C, the occurrence of dynamic recrystallization improves the hot ductility significantly. However, the coarsening of recrystallized grains and the formation of inter dendritic cracks decrease the hot ductility of the alloy gradually when the temperature increases from 1,100 °C to 1,200 °C.

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