Microsc. Microanal. 23, 404–413, 2017 doi:10.1017/S1431927616012745
© MICROSCOPY SOCIETY OF AMERICA 2017
Laser-Assisted Atom Probe Tomography of Deformed Minerals: A Zircon Case Study
Alexandre La Fontaine,1,2,* Sandra Piazolo,3 Patrick Trimby,2 Limei Yang,2 and Julie M. Cairney1,2
1School of Aerospace, Mechanical, Mechatronic Engineering, The University of Sydney, NSW 2006, Australia 2Australian Centre for Microscopy and Microanalysis, The University of Sydney, NSW 2006, Australia 3Department of Earth and Planetary Science, Macquarie University, NSW 2109, Australia
Abstract: The application of atom probe tomography to the study of minerals is a rapidly growing area. Picosecond-pulsed, ultraviolet laser (UV-355 nm) assisted atom probe tomography has been used to analyze trace element mobility within dislocations and low-angle boundaries in plastically deformed specimens of the nonconductive mineral zircon (ZrSiO4), a key material to date the earth’s geological events. Here we discuss important experimental aspects inherent in the atom probe tomography investigation of this important mineral, providing insights into the challenges in atom probe tomography characterization of minerals as a whole. We studied the influence of atom probe tomography analysis parameters on features of the mass spectra, such as the thermal tail, as well as the overall data quality. Three zircon samples with different uranium and lead content were analyzed, and particular attention was paid to ion identification in the mass spectra and detection limits of the key trace elements, lead and uranium. We also discuss the correlative use of electron backscattered diffraction in a scanning electron microscope to map the deformation in the zircon grains, and the combined use of transmission Kikuchi diffraction and focused ion beam sample preparation to assist preparation of the final atom probe tip.
Key words: atom probe tomography, mineral zircons, lead detection, deformation, dislocations, TKD
INTRODUCTION The non-conductive mineral zircon (ZrSiO4) is ideally suited for radiogenic dating of rocks. Not only does it contain trace amounts of uranium (U) and thorium (Th), enabling radiogenic dating (Williams et al., 1984; Hay & Dempster, 2009), it is also physically and chemically stable, surviving a range of geological processes such as erosion, deformation, and high-grade metamorphism (up to 900°C) (Harley et al., 2007). However, several recent studies have suggested that zircons may not be as chemically robust as once believed, especially at the micron and sub-micron scale, in regions of the sample that have been affected by deformation (Reddy et al., 2006). Measuring such chemical variations is a chal- lenge as the spatial resolution of conventional geochemical analysis techniques such as sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) is in the range of 5–15 μm and 15–50 μm, respectively. Atom probe tomography (APT) is a powerful micro-
scopic technique that can provide three-dimensional (3D) maps showing the position and atomic mass of individual atoms with sub-nanometer resolution. In the last 10 years, laser-assisted APT has been increasingly used to analyze oxide-based materials (Gault et al., 2012; Oberdorfer et al., 2007; Larson et al., 2008; Hudson et al., 2009; Marquis et al., 2010; Hono et al., 2011; Mazumder et al., 2011; Bachhav et al., 2011b; Sundell et al., 2012; Baik et al., 2013; Dong et al.,
*Corresponding author.
alex.lafontaine@
sydney.edu.au Received June 2, 2016; accepted December 19, 2016
2013; Stiller et al., 2013; Kirchhofer et al., 2014; La Fontaine et al., 2015a, 2015b; Stiller et al., 2016). APT is ideal for studying the spatial distribution of atoms across small volumes. The isotopic sensitivity of APT analysis is also an advantage for its application in the study of geological materials, where isotope ratios are commonly used to derive characteristics of the geological history of a specimen (Davis et al., 2003). For example, APT has recently been applied for the first time to U/Pb isotope dating in zircon (Valley et al., 2014, 2015), where the results were found to agree well with secondary ion mass spectrometry (SIMS)-based results. APT was also used to reveal the diffusion of trace elements at dislocations and low-angle boundaries in deformed zircons (Piazolo et al., 2016). However, unanswered questions remain about the distribution of trace elements within zir- cons, and the unique capabilities of APT applied to the fundamental study of zircons at the atomic level, mean that the technique is likely to become a common approach to investigate this important geological mineral. However, the study of minerals with laser-assisted APT
remains challenging due to decreased yield and lower data quality, which arises from both their intrinsic properties, such as low electrical conductivity and thermal diffusivity and high fractions of defects (Kirchhofer et al., 2014). There is a need for more detailed information about the influence of APT analysis parameters on data quality from zircons, and the limitations in the quantification and detectability of sig- nificant trace elements such as Pb and U. In addition, in order to investigate potential trace element diffusion within dislocations in deformed zircons, it is important to be able to identify the nature of local deformation within the region of
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