Diffusion of stable isotopes in metamorphic rocks: Theoretical and applied studies.

摘要

Diffusional exchange is a principle means of mass transport in high-temperature geologic environments, and potentially controls the record of geothermometers and geochronologic ages in slowly cooled rocks. This thesis describes the development of a new theoretical model for diffusional exchange between coexisting minerals in rocks, and tests of this model using ion microprobe and laser-based stable isotope analysis in field-based studies.;Numerical modeling of interdiffusion during cooling demonstrates that conventional models of diffusional closure are insufficient for describing exchange of oxygen isotopes, and that apparent temperatures and intracrystalline zonation recorded in slowly cooled rocks can be a strong function of modal abundances. Calculations with this model for a variety of common rock types investigates the importance of diffusional exchange in applications of stable isotopes to thermometry, "speedometry" and identification of fluid-rock interaction.;Ion microprobe analysis of oxygen isotope zonation in magnetite from quartzo-feldspathic gneiss in the Adirondack Mts, NY provides the first evidence that minerals are zoned in a way consistent with diffusional exchange during cooling. Comparison of zoning profiles from rocks with different bulk isotopic fractionations between constituent minerals illustrates the ability of numerical modeling to discriminate between closed- system, diffusion controlled exchange and open-system alteration.;Ion microprobe and laser-probe analysis of oxygen isotope zonation at both the micron-and millimeter-scales in gneiss from the Adirondack Mts., NY. shows the importance of grain boundary diffusion and texture on stable isotope distributions in slowly cooled rocks. Grain boundaries are pathways allowing isotopic exchange between non-touching grains, and substantially enhance bulk diffusion coefficients over volume diffusion coefficients within minerals.;Similar micron- and millimeter-scale zonation across a lithologic contact between magnetite and quarto-feldspathic gneiss allows calibration of the rate of oxygen self-diffusion on grain boundary diffusion in regional metamorphic rocks. The resulting grain boundary diffusion coefficient is in agreement with recent diffusive-penetration experiments, and confirms the ability of grain boundaries to enhance the scale of diffusive exchange in rocks.