OXYGEN ISOTOPIC TRANSPORT AND EXCHANGE DURING FLUID-FLOW - ONE-DIMENSIONAL MODELS AND APPLICATIONS

Solutions of one-dimensional transport equations including the mechanisms of advection, diffusion, hydrodynamic dispersion, and non-equilibrium exchange between water and rock indicate that the time-space evolution of oxygen isotopic compositions of rock and infiltrating fluid is dependent on (1) the rate of fluid infiltration, (2) the diffusive and dispersive properties of the rock matrix, (3) the rate of isotopic exchange, and (4) the rock-water mass oxygen ratio in a unit volume of water-saturated, porous rock. Infiltration generally leads to the development of an isotopic exchange front within the flow path, which separates an upstream segment of the flow column in which the rock is essentially equilibrated with the infiltrating water (water-dominated region), from a lower portion of the how path in which the infiltrating water has become largely equilibrated with the original isotopic composition of the rock (rock-dominated region). The geometry of isotopic exchange fronts developed in an infiltrated rock sequence depends on the interplay between the advective, the dispersive/diffusive, and the isotopic exchange processes, represented in scaled solutions of the transport equation by the Peclet (N-Pe) and Damkohler (N-D) numbers. The exchange front migrates through the flow path with increasing cumulative fluid flux and over time; its distance from the inflow boundary of the infiltrating fluid can be a realistic measure of time-integrated fluid flux. If both the exchange front and a portion of the water-dominated segment of the how path are preserved, their relative positions are a guide to the direction of fluid flow, including flow direction with respect to temperature gradient. Conventional W/R ratios computed for rocks along a flow path will depend not only on cumulative fluid flux, but on the conditions of flow ana exchange characterized by N-Pe and N-D, and on the position of the rock in the flow system. Reflecting these limitations, the conventional W/R ratio generally is an inappropriate method of measuring the actual extent of fluid infiltration at specific sites within hydrothermal and metamorphic systems.