TRANSPORT MECHANISMS BY FLUID-FLOW IN METAMORPHIC ROCKS - OXYGEN AND STRONTIUM DECOUPLING IN THE TROIS-SEIGNEURS MASSIF - A CONSEQUENCE OF KINETIC DISPERSION

The shape and displacement of isotopic or tracer concentration profiles perturbed by advective transport of a fluid phase reflect both the fluid volumes and the kinetics of solid-fluid exchange in addition to diffusion and hydrodynamic dispersion. Transport characteristics for flow with local solid-fluid equilibrium, linear kinetic exchange, and diffusive exchange are reviewed, and it is shown for most tracers in metamorphic environments that (1) characteristic transport distances are functions only of fluid:solid partition and the time-integrated fluid flux; (2) transport with diffusion-controlled exchange and transport with linear-kinetic exchange can be described by the same dimensionless parameter (the Damkohler-I number) allowing delineation of parameter spaces within which their transport characteristics are similar; and (3) transport by flow down parallel cracks with diffusion of tracer into the wall-rock can be approximated by linear-kinetic exchange at high values of the exchange parameter or by diffusion into a semi-infinite medium at low values of the exchange constant. These approximations admit relatively simple analytical solutions to the transport equations. Where diffusion through a pore fluid is the rate-limiting control on fluid-solid exchange, the very different concentrations of isotopic tracers in metamorphic fluids may lead to their apparent complete decoupling during advective transport. This is because the dimensionless constant, the Damkohler-I number, which describes the relative rates of fluid-solid exchange to the rate of advective transport, is a function of fluid-solid partition. A tracer more strongly partitioned into the fluid may show substantial alteration on a regional scale, whereas a tracer more strongly partitioned into the solid is only slightly perturbed over a much smaller length scale. Study of two or more tracers with differing fluid/solid partitions offers a powerful way of constraining fluid-flow regimes. This is illustrated by a study of oxygen and strontium isotope transport in the Hercynian Trois Seigneurs Massif, Pyrenees. The regional alteration of oxygen isotopic compositions may have been accomplished by flow along cracks spaced between 0.3 and 6 m if solid-fluid exchange was limited by diffusion through the wall-rock porosity. Such a fluid flow mechanism would have resulted in shifts of less than 15 percent in Sr-87/Sr-86 composition, apparent over almost-equal-to 10 m and possibly recorded as a small assymetric shift in the Sr-87/Sr-86 profile across a 15 m carbonate band.