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

S. Flude, S.C. Sherlock, M.R. Lee, S.P. Kelley

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Abstract

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.

Original languageEnglish
Pages (from-to)1-12
Number of pages12
JournalChemical Geology
Volume355
Early online date13 Jul 2013
DOIs
Publication statusPublished - 26 Sept 2013

Keywords

  • Ar/Ar dating
  • electron irradiation
  • electron microscopy
  • thermochronology

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