Disturbance to the ⁴⁰Ar/³⁹Ar system in feldspars by electron and ion beam irradiation

The extent to which intracrystalline microtextures influence the diffusion of radiogenic Ar within alkali feldspars from slowly cooled igneous rocks is a long standing question in thermochronology. By combining high-resolution electron microscopy with in-situ UV-laser ablation microprobe ⁴⁰Ar/³⁹Ar analysis the interplay of microtextures with isotope ages can be studied directly, enabling some of the assumptions underlying ⁴⁰Ar/³⁹Ar thermochronological techniques to be tested and allowing deduction of thermochronological and geological histories. However, there are numerous potential mechanisms by which a sample can be damaged and its Ar-isotope system disturbed by such microscopy techniques. To test this hypothesis, perthitic alkali feldspars from the 270-280Ma Dartmoor Granite, UK, and gem-quality orthoclase from Itrongay, Madagascar (~470Ma) were polished with colloidal silica or etched with hydrofluoric acid and irradiated with electrons and Ga⁺ ions. The accelerating voltages and currents used were typical of those for electron beam imaging by scanning electron microscopy, X-ray analysis and mapping by electron probe, and for extraction of foils using the focused ion beam technique. No disturbance to the Ar-isotope system was observed for Ga⁺ ion irradiation, or for low-resolution SEM imaging, but electron irradiation of small areas for long durations, as occurs during extended high-magnification SEM imaging, was found to disturb the Ar-isotope system over hundreds of micrometre sized areas by the addition of K and atmospheric Ar, producing anomalously young apparent ⁴⁰Ar/³⁹Ar ages. The best explanation for this age disturbance is electromigration of K and implantation of atmospheric Ar during sample charging.