Grain Boundary Migration recrystallization
Grain Boundary Migration recrystallization, commonly abbreviated as GBM-recrystallization, is a high temperature recrystallization mechanism observed in quartz and other minerals from high grade metamorphic rocks. GBM recrystallization is characterized by the enhanced mobility of grain boundaries, which migrate from less deformed grains, characterized by lower dislocation density in their crystal lattice, towards more deformed grains, hence with higher dislocation density, that are progressively consumed in the process and replaced by less strained grains. This process produces highly lobate or interfingered grain boundaries, generally associated with coarse-grained crystal aggregates.
Microstructures associated with GBM recrystallization
The process of GBM recrystallization is dominated by the fast migration of grain boundaries, which can sweep entire grains producing highly irregular grain boundaries, characterized by strongly sutured and interfingered lobes and protrusions. The resulting microstructure is said to be interlobate up to amoeboid (i.e. grain boundaries so irregular to resemble amoebas). In general, grain boundary migration occurs at high temperatures and it is hence associated with relatively coarse grains (above 100 μm, generally around 500 μm). GBM-recrystallization is so efficient that strained grains are rapidly removed and replaced by nearly strain-free grains. Subgrain boundaries may nucleate during GBM-recrystallization and rapidly evolve to become independent grains from their parent grain with their own lobate grain boundaries. For these reasons, it is not straightforward to tell apart new grains from old grains in a grain aggregate produced by GBM-recrystallization, except when markers, like trails of inclusions that could outline the shape of the original grains, are preserved.
Island grains. In grain aggregates produced by GBM-recrystallization, it is common to find relatively small and isolated grains surrounded by larger grains and that are in optical continuity (extinguish at the same time) with respect to nearby grains. These ‘island grains’ are an effect of the strongly lobate/amoeboid shape of the grain boundaries and are actually often linked to larger grains outside of the 2-dimensional thin section plane. Indeed, this microstructure was also named by Urai et al. (1986) ‘dissection microstructure’, as island grains are not actual smaller grains included in larger grains, but an artifact produced by sectioning grains with highly irregular grain boundaries.
⇔ slider. Quartz aggregate with strongly lobate grain boundaries produced by GBM-recrystallization. The lambda plate highlights the presence of small ‘island grains’ in optical continuity with nearby grains (yellow grains in blue/blue grains in yellow). Most grain boundaries are arranged in a cross-hatched pattern (see below). Width: 1.2 mm. Mylonitic leucogranite, Calamita, Island of Elba, Italy.
Pinning microstructures. These structures form when GBM-recrystallization occurs in the presence of secondary phases, like platy micas, included in quartz. In this case, secondary phases may ‘pin’ the migrating grain boundaries producing arcuate lobes around them, known as ‘pinning microstructures’. Jessell (1987) identified many types of these ‘pinning microstructures’ (figure below). For example, in the case of oriented inclusions of platy micas in quartz, the micas may act as dams, obstructing the migration of grain boundaries. However, if a ‘hole’ in the ‘mica dam’ is present, grain boundaries can migrate across it, producing small pinned boundaries, referred to as ‘window microstructure’. Another common effect related to mica inclusions is that migrating grain boundaries may be dragged laterally along an inclusion (‘dragging microstructure’).
Left-over grains. The migration of grain boundaries may almost completely erase a parent grain leaving behind only small left-over grains in optical continuity. These differ from island grains as they are actual small grains that are not connected anymore. However, considering that grain boundaries are often lobate in aggregates deformed by GBM-recrystallization, it is rather hard to tell whether these grains are actually linked outside of the thin section plane or not.
⇔ slider. Small ‘windows’ between oriented biotite grains allow the migration of grain boundaries in quartz aggregates produced by grain boundary migration recrystallization. Slide to highlight the location of the window microstructures (yellow arrows). CPL. Width: 1.2 mm. Migmatitic schist, Calamita, Island of Elba, Italy.
Cross-hatched mosaic microstructures. Also known as reticular grains, it is a peculiar microstructure that commonly occurs in aggregates of quartz grains recrystallized by GBM at high temperatures. It is described as a tendency of grain boundaries to intersect at about 90° with respect to one another, resulting in a cross-hatched mosaic. The reason for this is currently not known. Lister and Dornsiepen (1982) hypothesized that grain boundaries migrate along orthogonal sets of micro shears, but this structure might be related to recrystallization in an aggregate deforming via two perpendicular slip systems, like basal slip along the a-axis and prism slip along the c-axis as in the case of chessboard subgrains.
Conditions of GBM recrystallization
GBM-recrystallization is a high temperature deformation mechanism. In quartz, the transition from subgrain rotation to grain boundary migration recrystallization was estimated by Stipp et al. (2002) to occur at temperatures above 500 °C in the synkinematic contact aureole of the Adamello intrusion (Italian Alps) and in general GBM-recrystallization is the typical recrystallization mechanism in quartz from rocks deformed at upper amphibolite- to granulite-facies metamorphic conditions. In feldspars, the activation of GBM-recrystallization requires even higher-temperature metamorphic conditions (T > 850 °C), which are seldom achieved in typical rocks. However, temperature is not the unique factor controlling recrystallization mechanisms: in quartz, high strain rates and low water content favor other deformation mechanisms that normally occur at lower temperatures. Rocks deformed at very high temperature showing quartz grains recrystallized by bulging and subgrain rotation recrystallization have, indeed, been reported from several granulite terranes (e.g. Menegon et al., 2011). Consequently, there is no absolute temperature at which quartz deforms by GBM-recrystallization and the physical conditions of GBM-recrystallization must be evaluated in natural rocks also based on T estimates from geothermometry or metamorphic assemblages.
Examples of GBM microstructures
Island grains and cross-hatched quartz microstructures
This pegmatite was deformed in a shear zone after its crystallization, likely at temperatures between 500 and 700 °C. Quartz is recrystallized by grain boundary migration and shows an abundance of lobate/inter-fingered to amoeboid grain boundaries that form chessboard-like patterns, intersecting at orthogonal angles (cross-hatched or reticular microstructure). Many smaller grains, surrounded by larger grains, are in optical continuity with larger grains (island grains).
Sample: mylonitic pegmatite
Assemblage: muscovite, quartz, alkali feldspar, tourmaline, andalusite
Locality: Fosso del Pontimento, Calamita, Island of Elba, Italy
Sample courtesy Giovanni Musumeci
Video. CPL. Width: 1.2 mm.
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