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

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 highresolution electron microscopy with in-situ UV-laser ablation microprobe 40 Ar/ 39 Ar analysis the interplay of microtextures with isotope ages can be studied directly, enabling some of the assumptions underlying 40 Ar/ 39 Ar 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 270-280 Ma Dartmoor Granite, UK, and gem-quality orthoclase from Itrongay, Madagascar (~470 Ma) 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 40 Ar/ 39 Ar ages. The best explanation for this age disturbance is electromigration of K and implantation of atmospheric Ar during sample charging.

Integrated single crystal laser ablation U/Pb and (U–Th)/He dating of detrital accessory minerals – Proof-of-concept studies of titanites and zircons from the Fish Canyon tuff

“…The puck was then polished to remove the outer 20-30µm of the crystals, which both eliminates the need for alpha ejection corrections and ensures a smooth surface for analysis. Next, both cathodoluminescence (CL) and backscatter electron (BSE) images were acquired before the puck was re-polished to eliminate the carbon coat and any (minimal) amount of the crystal surface that could have been damaged during electron beam imaging (e.g., Flude et al, 2013).…”

Application of benchtop micro-XRF to geological materials

“…This information may traditionally be acquired by a combination of petrographic study, with SEM imaging and EMPA analysis to characterise internal variation of mineral grains. One advantage of μ-XRF over electron beam techniques is that there is no need for charge neutralisation (carbon coating or charge neutralising gas in the sample chamber), as the excitation beam is of X-rays, rather than electrons, and there is no risk of sample damage due to charging, as can happen with electron-beam techniques (Flude et al 2013). However, the larger spot size of μ-XRF relative to electron beam techniques (~25 μm vs.…”

“…This feature can be exploited by employing multiple X-ray detectors in the instrument, located at different orientations to the sample, but even using a single detector can (Flude et al 2013). An example of this OC imaging is illustrated in Figure 7, which shows a combined element and diffraction peak map of the large alkali feldspar phenocryst on the left of the sample in Figure 3 A and B; simple twinning is visible in hand specimen for this crystal.…”

Exploring the variability of argon loss in Apollo 17 impact melt rock 77135 using high‐spatial resolution40Ar/39Ar geochronology

“…We did this to minimize the potential effects of K‐migration and 40 Ar* loss induced by the interaction of the sample with the electron beam (Flude et al. ).…”

“…Note that, for thick section 77135,209, we carbon coated the section to prevent sample charging, and we obtained BSE images and X-ray element maps at low magnification with standard operating conditions (i.e., we did not use a high accelerating potential or a high beam current). We did this to minimize the potential effects of K-migration and 40 Ar* loss induced by the interaction of the sample with the electron beam (Flude et al 2013). The details of our 40 Ar/ 39 Ar methods are nearly identical to those of Mercer et al (2015), but the basic aspects, pertinent changes, and parameter values relevant to our study of 77135 are described below.…”