An investigation of the laser-induced zircon ‘matrix effect’
Introduction
The uptake of laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) as an essential tool for rapid, cost-effective and high spatial resolution dating of zircon has been unprecedented (e.g., Schoene, 2014). Unfortunately, however, the matrix dependency of the ablation process remains a major drawback of the current method, limiting its accuracy and precision. Currently, LA-ICP-MS is routinely used to generate zircon U–Pb ages with an accuracy of ~ 2–3% (2σ) relative to the long-established benchmark thermal ionisation mass spectrometry (TIMS) U–Pb analysis of bulk zircon (e.g., individual grains or parts of grains; Schaltegger et al., 2015). The potential for any improvement in these figures appears to be influenced by systematic biases, which are associated with matrix differences between unknowns and the reference zircon materials used for standardization (e.g. Klötzli et al., 2009, Košler et al., 2013). These effects are further aggravated by the fact that it is unusual for ‘unknown’ natural zircon samples to share the traits that characterise a zircon reference material (i.e., well-ordered crystalline materials without many inclusions). It is now clear that the mitigation of these laser-induced ‘matrix effects’ will only be possible if the underlying causes are identified and understood.
Recent studies have linked the analytical bias between LA-ICP-MS dates and those determined by TIMS to the differential response of unknown and standard zircon materials to laser radiation (Marillo-Sialer et al., 2014, Steely et al., 2014). Although the underlying causes of this variable response have not yet been fully elucidated, a variety of factors are thought to influence the ablation efficiency (i.e., coupling of the laser to the material being ablated), such as crystal chemistry (Black et al., 2004), crystal colour (Kooijman et al., 2012), amount of accumulated radiation damage (Allen and Campbell, 2012, Steely et al., 2014) and crystal orientation (Mikova et al., 2009). The present study takes a systematic approach to investigating the relative importance of each of these factors.
In general, when using a single wavelength of pulsed laser light at constant laser fluence, differences in ablation rates arise from differences in absorptivity between samples (Horn et al., 2001). The absorption of radiation by accessory minerals, such as zircon, is strong in the ultraviolet (UV). For this reason, either excimer lasers operating at 193 nm (ArF) or solid state lasers operating at 213 nm (Nd:YAG) are now the norm for the analysis of these materials. However, slight changes in the degree of absorptivity between different zircon samples due, for example, to the presence of an increasing concentration of trace elements absorbing in the UV range of the laser, could lead to differences in the optical penetration depth of the laser beam and thus result in significant variations in rates of material removal. Horn et al. (2001) reported such variations in ablation behaviour among the NIST SRM 61 × series glasses and correlated this with absorptivity of the sample, i.e., sample colour. This effect was noted when using a 266 nm Nd-YAG laser, but no apparent variation in ablation efficiency was observed when using a 193 nm excimer laser.
Structural damage produced by the process of alpha-decay also imparts a pronounced effect on the crystal structure and properties of natural zircon which could, in principle, alter the response to laser radiation. It is known that properties such as density, birefringence, hardness, compressibility and thermal conductivity, among others, change as a function of increasing radiation dose (Holland and Gottfried, 1955, Özkan, 1976, Chakoumakos et al., 1991, Oliver and McCallum, 1994, Ewing et al., 2003, Salje, 2006). Thus, for example, Steely et al. (2014) reported a close relationship between the degree of crystallinity of zircon samples and their laser penetration depth. However, an exclusive dependence of the ablation behaviour on the degree of structural distortion caused by alpha radiation could not be confirmed since the variations in ablation rates could also have been caused by other factors including the change in crystal colour, and thus be triggered by variations in the degree of optical absorption of the laser light. Furthermore, zircon shows different optical and mechanical properties on different crystallographic planes (Finch and Hanchar, 2003) that might also affect the rate at which the zircon ablates.
It is clear from the above that there are a multiplicity of factors potentially influencing the extent of laser coupling to the zircon target. It is therefore highly probable that the observed variability in ablation behaviour between zircon matrices is due to a combination of some or all the factors listed above and that no simple generalization is possible. This study represents a first attempt to systematically investigate the multiple variables affecting the rate of material removal as a function of the zircon properties. Our approach was designed to simplify the highly complex physical phenomena of laser-material interaction to a one-dimensional case. This was achieved by careful consideration of experimental design, as well as appropriate zircon sample selection, as detailed below.
Section snippets
Analytical approach, instrumentation and methods
Our study is composed of three parts. In the first part, the amount of accumulated radiation damage is used to evaluate the extent to which zircon matrices with different properties respond to laser radiation. In the second part, we investigate the role played by trace element composition on the different optical absorption properties of zircon crystals, and thus on their ablation behaviour. Finally, we evaluate the role that crystallographic orientation and crystal colour play on the ablation
Part I: Accumulated radiation damage
Fig. 2 shows examples of Raman spectra obtained for the zircon samples investigated in this study. Two spectra are shown for each zircon sample, which correspond to the minimum (solid line) and maximum (dashed line) FWHM values measured for the ν3 (SiO4) band of spectra acquired at different spatial locations. It can be seen that the Mud Tank and 91500 zircons have highly crystalline structures, with ν3 (SiO4) FWHM and frequency close to those obtained for synthetic zircon: 1.8 cm− 1 and 1008 cm− 1
Role of radiation-induced structural defects on the ablation behaviour of natural zircon
Based on the overall positive linear correlation between zircon ablation rate and radiation damage, it is reasonable to assume that quantification of the total accumulated radiation-induced damage (i.e., using Raman spectroscopy) can be used as a proxy for laser drill rate into natural zircon. However, there are practical constraints to this approach, which are imposed by the limited accessibility to the Raman spectroscopy technique among the U–Pb geochronology community, and the potential time
Conclusions and recommendations
Accurate identification and refinement of the multiple variables affecting the rate of material removal by the laser is a first step towards the adequate correction for the systematic LA-ICP-MS U–Pb age bias observed between zircon matrices, and could potentially lead to a significant decrease of measurement uncertainty in LA-ICP-MS age determinations. Differences in the degree of metamictisation between natural zircon samples seem to play a dominant role in defining the observed variation in
Acknowledgments
The authors would like to acknowledge B. Johnson and S. Rubanov for technical assistance with Raman spectrometry and HR-TEM analyses, respectively. X-H. Li is thanked for providing the Qinghu zircon. M. Horstwood and an anonymous reviewer are thanked for providing comments that helped improve the quality of this manuscript. This work was supported by an Albert Shimmins Postgraduate Award 5-152-1 from the University of Melbourne to EMS, and NSERC Discovery grant to JMH.
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