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

Abstract

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.

Extensive fluid–rock interaction and pressure solution in a UHP fluid pathway recorded by garnetite, Lago di Cignana, Western Alps

“…In host‐rock garnet, the mineral inclusion assemblages are: (a) quartz, rutile, dolomite, magnesite, apatite, zircon in the core, (b) coesite, kyanite, clinozoisite, phengite, zircon in the mantle, (c) quartz in the rim (Table 1). The distribution of quartz and coesite in this garnet (Figure 7a,d) indicates a primary origin of such inclusions, despite the fractures in the host garnet which could have re‐equilibrated the inclusions (Griffiths et al., 2014; Whitney, 1996). The inclusions thus correspond to preserved remnants of the evolving rock matrix at different stages of garnet growth.…”

Crystallographic orientation relationships in host–inclusion systems: New insights from large EBSD data sets

“…The original (Permian) pegmatite microstructure of the samples has been affected by Cretaceous deformation and recrystallization (Bestmann et al 2008;Griffiths et al, 2014). Polycrystalline quartz ribbons (grain size 40 µm -1 mm), recrystallized feldspar (K-feldspar and albite, grain This is a preprint, the final version is subject to change, of the American Mineralogist (MSA) Cite as Authors (Year) Title.…”

“…During Cretaceous eclogite facies metamorphism garnet underwent both crystal plastic and brittle deformation. These processes locally promoted microstructural and compositional reequilibration of inclusions and garnet, resulting in two microstructures crosscutting the inclusion zoning: recrystallization zones (Bestmann et al 2008) and inclusion trails (Griffiths et al 2014).…”

“…Sufficiently large numbers of measurements of inclusion orientations provide information on many different parameters: which -and how many -CORs each inclusion phase assumes, the relative frequencies of these CORs, whether these CORs are specific or statistical, and the abundances, grain sizes and habits. Inclusion trails with bleaching zones (Griffiths et al 2014) crosscut the zoning. Oscillatory zoning visible at bottom right of (a).…”

Direct observation of dislocation nucleation in pyrite using combined electron channelling contrast imaging and electron backscatter diffraction

“…We argue that continuous deformation and local stress concentration at the ct facilitate the nucleation of dislocations, possibly supporting the onset of crystal plasticity at a later stage. A similar introduction of dislocations into the crystal lattice is likely to occur in other ‘strong’ minerals (Griffiths et al., ) but has so far been mainly reported from material sciences (Anderson, ; Ohr, ; Ugˇuz & Martin, ). The combination of fluid‐assisted fracturing and dislocation activity is of special interest for pyrite as dislocations and substructures such as micro‐cracks and low‐angle grain boundaries might serve as high‐diffusivity pathways (Piazolo et al., ; Fougerouse et al., ) helping to incorporate Au from hydrothermal fluids into the crystal lattice or acting as Au‐traps in pyrite (Dubosq et al., ).…”