Microstructure and crystallographic preferred orientation of polycrystalline microgarnet aggregates developed during progressive creep, recovery, and grain boundary sliding

Massey, Matthew A.; Prior, David J.; Moecher, David P.

doi:10.1016/j.jsg.2010.12.009

Cited by 22 publications

(12 citation statements)

References 63 publications

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“…The presence of microstructures and textures consistent with dislocation climb and recovery, as well as subgrain rotation, in garnet at around 600 °C is in agreement with previous studies (Bestmann et al, 2008;Massey et al,2011). No evidence for grain boundary sliding is observed, since subgrains show rotation around a specific crystallographic axis.…”

Section: Tem Investigationssupporting

confidence: 91%

Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Hawemann¹,

Mancktelow²,

Camacho³

et al. 2019

Preprint

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Garnet is a high strength mineral compared to other common minerals, such as quartz and feldspar, in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade intracrystalline deformation, except under relatively high temperature conditions. The microstructure of garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal-plastic deformation. Granulite facies metamorphism at ~ 1200 Ma generally dehydrated the rocks and produced mm-sized garnets in peraluminous gneisses. A later ~ 550 Ma overprint under sub-eclogitic conditions (600-700 °C, 1.1-1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite-facies garnet porphyroclasts have high calcium content along rims and fractures. However, in certain cases, these rims are narrower than equivalent rims along original grain boundaries, indicating contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal-plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. Fracturing of garnet under dry lower crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte.

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Section: Tem Investigationssupporting

confidence: 91%

Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Hawemann¹,

Mancktelow²,

Camacho³

et al. 2019

Preprint

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“…In nature, garnet represents an extremely versatile recorder of metamorphism (Baxter & Scherer, ) and in particular garnet coronas capture metamorphic processes ‘in flagranti’ (Carlson & Johnson, ; Carlson, ; Ague & Carlson, ). Consequently, garnet coronas and their metamorphic significance have been intensely studied over the past decades (Mørk, ; Johnson & Carlson, ; Johnson, ; Spiess et al ., ; Prior et al ., ; Konrad‐Schmolke et al ., ; Massey et al ., ; Goergen & Whitney, ).…”

Section: Introductionmentioning

confidence: 99%

“…In nature, garnet represents an extremely versatile recorder of metamorphism (Baxter & Scherer, 2013) and in particular garnet coronas capture metamorphic processes 'in flagranti' (Carlson & Johnson, [Correction added on 25 September 2018 after first print and online publication: The name of the fourth author was previously incorrect and has been corrected in this version] 1991; Carlson, 2011;Ague & Carlson, 2013). Consequently, garnet coronas and their metamorphic significance have been intensely studied over the past decades (Mørk, 1985;Johnson & Carlson, 1990;Johnson, 1993;Spiess et al, 2001;Prior et al, 2002;Konrad-Schmolke et al, 2005;Massey et al, 2011;Goergen & Whitney, 2012).…”

Section: Introductionmentioning

confidence: 99%

“…Garnet also readily partakes in mylonitic deformation: crystal plastic deformation of garnet at lower crustal conditions was documented by, for example, Ji & Martignole (1994, Prior et al (2002), Storey & Prior (2005), Bestmann et al (2008), Massey et al(2011) and Martelat et al (2012). Garnet in mylonitic eclogites from SW Norway was shown to have been deformed by grain-boundary diffusion creep and by pressure solution (Smit et al, 2011).…”

Section: Introductionmentioning

confidence: 99%

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The strain‐dependent spatial evolution of garnet in a high‐P ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X‐ray microtomography study

Macente

Fußeis

Menegon

et al. 2017

Journal Metamorphic Geology

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Reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono‐metamorphic processes, however, they are usually analysed in two dimensions, omitting important information regarding the third spatial dimension. We applied synchrotron‐based X‐ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite–garnet eclogite in four dimensions, where the 4th dimension is represented by the degree of strain. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway), we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain. The microtomographic data allowed quantification of garnet volume, shape and spatial arrangement evolution with increasing strain. The microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe (EMPA) and electron backscatter diffraction (EBSD) analysis to correlate mineral composition and orientation data with the X‐ray absorption signal of the same mineral grains. With increasing deformation, the garnet volume almost triples. In the low‐strain domain, garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample. We also observed that garnet coronas in the gabbros never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics. We interpret the aligned garnet aggregates to direct synkinematic fluid flow, and consequently influence the transport of dissolved chemical components. EBSD analyses reveal that garnet shows a near‐random crystal preferred orientation that testifies no evidence for crystal plasticity. There is, however evidence for minor fracturing, neo‐nucleation and overgrowth. Microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies, becoming more almandine‐rich. We interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization, and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming.

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“…Following TEM observations in deformed garnets (Ji and Martignole 1994;Voegelé et al 1998a;Voegelé et al 1998b), evidence for crystal plastic deformation in garnet has grown steadily (Prior et al 2002;Storey and Prior 2005;Vollbrecht et al 2006;Massey et al 2011;Martelat et al 2012), but has so far only been linked to inclusion re-equilibration in one case (Bestmann et al 2008).…”

Section: Introductionmentioning

confidence: 95%

Localization of submicron inclusion re-equilibration at healed fractures in host garnet

Griffiths

Habler

Rhede³

et al. 2014

Contrib Mineral Petrol

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Microstructures in Permian inclusion-bearing meta-pegmatite garnets from the Koralpe (Eastern Alps, Austria) reveal re-equilibration by coarsening of abundant submicron-sized inclusions (1 µm-2 nm diameter) at the site of healed brittle cracks. The microstructures developed during Cretaceous eclogite facies deformation and the related overprinting of the host-inclusion system. Trails of coarsened inclusions (1-10 µm diameter) crosscut the garnet, defining traces of former fractures with occasional en-echelon overlaps. Trails are flanked by 10-100 µm wide 'bleaching zones' characterized by the absence of ≤1 µm sized inclusions in optical and SE images. FEG-microprobe data show that trails and bleaching zones can form isochemically, although some trails exhibit nonisochemical coarsening. Cross correlation EBSD reveals subtle garnet lattice rotation of up to 0.45° around consistent misorientation axes, spatially correlated with bleaching zones. Elevated dislocation density within these zones is confirmed by TEM observations. Brittle fracture enhanced diffusion rates in the lattice adjacent to crack planes, priming these areas to behave differently to the bulk of the garnet during Cretaceous metamorphism and facilitating localized coarsening of inclusions. The preferred mechanism for this is increased dislocation density near former cracks, with the dislocations interpreted as a plastic wake originating from crystal plastic deformation at the crack tip. This partially closed host-inclusion system clearly records the influence of deformation mechanisms on re-equilibration and contributes to a wider understanding of the interaction between deformation and chemical reaction during metamorphism.

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Microstructure and crystallographic preferred orientation of polycrystalline microgarnet aggregates developed during progressive creep, recovery, and grain boundary sliding

Cited by 22 publications

References 63 publications

Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

The strain‐dependent spatial evolution of garnet in a high‐P ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X‐ray microtomography study

Localization of submicron inclusion re-equilibration at healed fractures in host garnet

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