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Grain growth simulations using the microstructure simulation system Elle have been performed in materials with a pre-existing grain shape foliation. As might be expected, the foliation is destroyed by the end of the experiment, and grain areas have increased by a factor of seven. The area of material swept by the migrating grain boundaries was monitored, and it was found that at every stage, virtually all of the grains which survived the grain growth process contain one and only one core of ÔunsweptÕ material. Remarkably these remnant unswept cores preserve a useable record of the initial grain size and the orientation of the grain shape foliation. This work suggests that, even for samples where no equivalent protolith can be found, it may be possible to see past a grain growth episode to estimate the original grain shape and grain size of the rock, and perhaps even reconstruct the grain boundary kinematics. In addition the identification of unswept cores has the potential to help unravel the evolution of grain boundary chemistry in rocks during metamorphism.As an example of a natural system showing these microstructures, we describe a peridotite from Almklovdalen, Norway. This peridotite was infiltrated by aqueous fluids at several stages during late Caledonian exhumation and retrogressive metamorphism. Grain boundary migration associated with the last of these infiltration events swept off abundant intragranular fluid inclusions in the original chlorite-peridotite. At the grain scale, microstructural mapping of the fluid inclusion rich areas shows that, as with the numerical simulations, many of the grains retain exactly one core of unswept material. Examples of other natural systems discussed include dislocation density distributions and trace element zoning.

A major Alpine-type peridotite located at Almklovdalen in the Western Gneiss Region of Norway was infiltrated by aqueous fluids at several stages during late Caledonian uplift and retrogressive metamorphism. Following peak metamorphic conditions in the garnet-peridotite stability field, the peridotite experienced pervasive fluid infiltration and retrogression in the chlorite-peridotite stability field. Subsequently, the peridotite was infiltrated locally by nonreactive fluids along fracture networks forming pipe-like structures, typically on the order of 10 m wide. Fluid migration away from the fractures into the initially impermeable peridotite matrix was facilitated by pervasive dilation of grain boundaries and the formation of intragranular hydrofractures. Microstructural observations of serpentine occupying the originally fluid-filled inclusion space indicate that the pervasively infiltrating fluid was characterized by a high dihedral angle (y > 608) and 'curled up' into discontinuous channels and fluid inclusion arrays following the infiltration event.Re-equilibration of the fluid phase topology took place by growth and dissolution processes driven by the excess surface energy represented by the 'forcefully' introduced external fluid. Pervasive fluid introduction into the peridotite reduced local effective stresses, increased the effective grain boundary diffusion rates and caused extensive recrystallization and some grain coarsening of the infiltrated volumes. Grain boundary migration associated with this recrystallization swept off abundant intragranular fluid inclusions in the original chlorite peridotite, leading to a significant colour change of the rock. This colour change defines a relatively sharp front typically located 1-20 cm away from the fractures where the nonreactive fluids originally entered the peridotite. Our observations demonstrate how crustal rocks may be pervasively infiltrated by fluids with high dihedral angles (y > 608) and emphasize the coupling between hydrofracturing and textural equilibration of the grain boundary networks and the fluid phase topology.

Deformation-band networks at Buckskin Gulch, Utah, and the Big Hole fault, Utah, both formed in the Navajo Sandstone with similar initial porosity and permeability, at similar burial depths, and result in similar reductions in effective permeability. However, the band networks at Buckskin Gulch, which formed in a contractional tectonic setting, appear to be much more areally extensive and are not associated with any discrete faults having displacements greater than at most a few meters and more likely only a few tens of centimeters. In contrast, the bands at Big Hole fault are generally limited to the damage zone of a about 25-m (82-ft) displacement normal fault formed in a locally extensional environment. These results suggest that deformation bands in well core from extensional settings may be indicative of discrete damage zones associated with normal faults, whereas deformation bands in well core from contractional settings may be indicative of much more areally extensive deformation-band networks. The band networks in both cases will affect similar reductions in reservoir effective permeability, but only in the latter case will the affected area be sufficiently large to affect well performance.

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