The mechanism of fluid infiltration in peridotites at Almklovdalen, western Norway

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 (theta > 60degrees) 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 (theta > 60degrees) and emphasize the coupling between hydrofracturing and textural equilibration of the grain boundary networks and the fluid phase topology.