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

Abstract

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.

Magma-driven hydraulic fracturing and infiltration of fluids into the damaged host rock, an example from Dronning Maud Land, Antarctica

“…This reveals a characteristic width of the alteration halo and a sharp boundary. Similar alteration zones around veins were described by Segall and Pollard (1983), Segall (1984a,b), Austrheim (1990) and Kostenko et al (2002), for example. In the present paper, the basic geometric properties of the alteration halos flanking the pegmatitic veins in central Dronning Maud Land are described.…”

Mineral Replacement Reactions

“…While tectonicallyinduced fracturing undoubtedly plays a major role in permeability development and introduction of fluid into rocks (Jamtveit and Yardley 1997), replacement reactions by dissolution-precipitation require that fluid infiltrates every part of a rock, moving through the minerals as they are replaced. Discussions about how fluids move through rocks are generally restricted to hydraulic fractures and grain boundaries (e.g., Kostenko et al 2002), but porosity generation by a reactive fluid greatly increases the number of possible fluid pathways and the permeability of a rock. A replacement reaction creates this permeability behind the reaction front and as long as there is sufficient fluid and mass transport through the created porosity, rocks can be reequilibrated on a large scale.…”

Hydrogen Incorporation in Natural Mantle Olivines

“…These crystals may have experienced peak pressures of 3.8 GPa [Medaris, 1999] or even higher [van Roermund et al, 2000]. However, they obviously had a more protracted history of exhumation to the surface than the xenoliths discussed above and have likely been affected by fluid infiltration events at lower pressures [Kostenko et al, 2002], reflected by the ubiquitous presence of serpentine in these samples. Because the diffusion of hydrogen in olivine is known to be rapid [Kohlstedt and Mackwell, 1998;Demouchy and Mackwell, 2003], at least on the several million-year timescale it must have taken for exhumation of these rocks, the proposed survival of a "high-pressure OH signature" is therefore surprising.…”