New Mexico Geological Society Annual Spring Meeting — Abstracts

The Multiple Domain Diffusion (MDD) method of ⁴⁰Ar/³⁹Ar thermochronology yields thermal histories from single K-feldspars by modeling natural and laboratory Ar release as a function of thermally activated diffusion from discrete subgrains of varying size. Tectonic studies rely on accurate and detailed thermal history data and thus are highly dependent on the fundamental assumptions from which thermal histories are derived. To this end, determination of diffusion boundaries within complex mineral systems remains a worthy goal. The boundaries of these subgrains are not physically defined and thus constitute a major criticism of the method and model. In lieu of a physical understanding of diffusion boundaries, the robustness of MDD-derived thermal histories comes from internally consistent data and corroboration by other methods. More complex and poorly consistent results provide new challenges that can be understood by conducting detailed age spectrum analysis on microtexturally characterized crystal fragments. A case study presented here utilizes the ca. 400 Ma Shap granite, England. The Shap contains orthoclase subgrains with well-defined film perthite, which yield nearly flat 400 Ma age spectra. In contrast, coarsened, microporous microcline subgrains yield more complex spectra with assigned ages between about 380 and 400 Ma. These experiments show that discrete structural and chemical textures common to alkali feldspars can be closely related to complicated Ar age spectra and provide insight into diffusion boundaries, dimensions, and Ar kinetic parameter variations between samples.