Field measurement and implications of reaction rates and chemical diffusivities in regional metamorphic systems.

摘要

Understanding the rates and mechanisms by which chemicals are transported and exchanged within the Earth's crust is crucial to our ability to interpret rock and mineral chemistry. This thesis focuses on field analysis and interpretation of reaction rates, and bulk diffusivities of Sr, Nd, and Ar isotopes as well as major elements, in an effort to better constrain these parameters and to explore the implications thereof to the endeavors of geochronology, geothermobarometry and geochemistry in general.;Chapter 1 presents a new field based technique for measuring reaction rates. Such analysis is needed to compare to laboratory estimates that may not extrapolate well to nature. The technique involves isotopic analysis of whole rock and garnet samples about a lithologic contact. Chapters 2 and 3 summarize the implementation of this technique at two field sites: Simplon Pass, Switzerland, and Townshend Dam, Vermont. The results from Simplon Pass indicate that bulk metamorphic reaction rates are ∼10-7 g/g/yr, several orders of magnitude slower than laboratory based estimates. This result suggests that mineral chemistry may lag the evolving conditions of metamorphism and that the assumption of local equilibrium, so often invoked in geochemical applications, will not always hold. The Vermont data proved less useful due to unsatisfactory leaching of those garnets. Nevertheless, the whole rock data and an unleached garnet analysis do provide some constraint on reaction rates and diffusivities.;Chapter 4 discusses the successful geochronological application of garnet Sm/Nd measurements in the Barrovian type-locality, Scotland. Whereas previous chapters dealt specifically with situations where garnets were in disequilibrium, this chapter shows how useful and precise geochronological data may be obtained, when the appropriate steps are taken.;Chapter 5 presents Ar isotopic data from biotites at the Simplon Pass field site. Systematic variations in the measured 40Ar/ 39Ar age are used to explain the origin of excess Ar in this system and to derive information about the bulk diffusivity of Ar in the system. The results suggest that "excess Ar" is controlled by the bulk diffusivity of Ar in the rock and the apparent variations in rock "transmissivity" with respect to Ar in heterogeneous systems.