FLUID FLUX GRADIENTS DURING METAMORPHISM: THE RESULT OF EXTREME CHANNELIZATION?

The question of whether or not large fluid fluxes are an integral part of regional metamorphism remains controversial. This question is critical to address, as fluids can play fundamental roles in driving mass transfer, heat transfer, and mineral reactions in orogenic belts. Numerical models indicate that large time-integrated fluid fluxes of ~10³ m³ m^-2 or more can develop during regional metamorphism of metapelites in collisional orogens, yet rocks which have undergone little discernable fluid infiltration are common in outcrop studies. This discrepancy suggests extreme channelization of fluid flow. Quartz veins are mineralized fractures that potentially channelize large fluid fluxes. This study investigates amphibolite facies quartz+kyanite veins in the Wepawaug Schist, Connecticut, USA; these rocks underwent peak thermal metamorphism at ~380 Ma during the Acadian orogeny.

The veins range in width from cm to dm scale and are surrounded by chemically-altered selvages. Kyanite crystal length increases toward veins, reaching several cm in veins and selvages. Al, Fe, Mn, Zn, Li, HREE, and Y were gained in the selvages. Al was taken up by growth of kyanite, staurolite, and garnet; Fe, Mn, HREE, and Y were taken up by garnet; and Fe, Zn, and Li were taken up by staurolite. Silica was lost to the veins. Destruction of mica resulted in losses of K, Ba, Pb, and volatiles. Na and Sr losses were more variable but correlate with feldspar destruction. The selvage metasomatism and the deposition of silica and kyanite in the veins indicate that the veins were conduits for large time-integrated fluid fluxes of 2–6x10⁴ m³ m^-2 at the outcrop scale.

Numerical models of flow in fractured porous media show that fluxes can vary by orders of magnitude over distances of a meter or less adjacent to veins. High-permeability vein conduits can carry the bulk of the regional fluid flow. The lowest fluxes are expected just outside selvage margins, as most of the fluid flow is diverted into the adjacent vein-selvage zones. Thus, different workers can come to vastly different conclusions about the magnitude of fluid fluxes depending on what part of a fractured outcrop they are working in. Determining “average” time-integrated fluid fluxes across outcrops remains as an important research challenge due to the extreme spatial variability of flow.