Numerical modeling of fluid flow and oxygen isotope exchange in the Notch Peak contact-metamorphic aureole, Utah

Abstract

We investigated the macroscale fluid flow and oxygen isotope exchange in contact-metamorphic aureoles by using two-dimensional finite-element modeling. The model parameters are assigned according to geologic observations in the Notch Peak metamorphic aureole, Utah. The results show that fluid flow in contact aureoles depends strongly on time and space, unlike the commonly assumed uniform and unidirectional fluid flow in previous studies. In an aureole with homogeneous permeability, a simple convection cell develops in the inner aureole, causing δ 1 8 O values to decrease in the inner aureole and increase in the upper outer aureole. However, when the horizontally layered permeability structure of the Notch Peak aureole is represented in the model, the resulting fluid flow is concentrated within high-permeability aquifers, and local convection cells develop near the contact within each aquifer. Sub-horizontal down-temperature flow and up-temperature flow can coexist in the same aquifer. The up-temperature flow in the lower part of each aquifer mainly causes depletion of 1 8 O in the inner aureole. Release of magmatic fluid significantly enhances the depletion of 1 8 O in the inner aureole at early stages, although sub-horizontal up-temperature flow at later stages tends to reduce the effects of the early-stage exchange. The isotopic features of the Notch Peak aureole are best explained by (1) infiltration of magmatic fluid having a δ 1 8 O value of 8‰, (2) ∼10% of the wall rocks involved in the oxygen isotope exchange with fluids, (3) no isotope exchange below 250 °C and during retrograde cooling, and (4) enhanced permeability in the inner aureole in addition to the litho-logically layered permeability structure.