ADVANCING THE 40Ar/39Ar MDD METHOD THROUGH MICROANALYSIS OF MICROTEXTURALLY CHARACTERIZED K-FELDSPAR CRYSTAL FRAGMENTS

The Multiple Domain Diffusion (MDD) method of ⁴⁰Ar/³⁹Ar thermochronology yields thermal histories from K-feldspars by modeling natural and laboratory Ar release as a function of thermally activated diffusion from discrete subgrains of varying size. In many cases 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 perhaps be understood by conducting detailed age spectrum analysis on microtexturally characterized crystal fragments. New 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 and Ar kinetic parameter variations between samples. Case studies presented here utilize the ca. 400 Ma Shap granite, England and the ca. 375 Ma Chain of Ponds pluton, Maine. For Shap, early formed orthoclase subgrains with well-defined film perthite yield nearly flat 400 Ma age spectra. In contrast, deuterically altered microcline subgrains yield overall more complex spectra with assigned ages between about 380 and 400 Ma. Additional work will evaluate if young age stems from low closure temperature or if deuteric alteration occurred at ca. 380 Ma. Published argon data (Heizler et al. 1988) for Chain of Ponds bulk samples vary widely in Ar diffusion behavior and apparent age. Correlation of young age to low closure temperature reconciled these results, however new work does not fully support published data as the young sample has subequal argon retentivity compared to older samples. Microprobe data reveal near end-member albite and K-feldspar compositions within large patch perthite subgrains and indicate low temperature recrystalizaiton as a possible explanation for the young ages.

These studies attest to the strength of combining argon age spectrum analyses to microtexturally characterized fragments; leads to more accurate understanding of simple and complex age spectra and in general supports the validity of the MDD method. Further advances will be made when new noble gas instrumentation allows high precision age spectrum analysis of significantly smaller subgrains.