Non-matrix-matched standardisation in LA-ICP-MS analysis: general approach, and application to allanite Th–U–Pb dating

Burn, Marco; Lanari, Pierre; Pettke, Thomas; Engi, Martin (2017). Non-matrix-matched standardisation in LA-ICP-MS analysis: general approach, and application to allanite Th–U–Pb dating. Journal of analytical atomic spectrometry, 32(7), pp. 1359-1377. Royal Society of Chemistry 10.1039/C7JA00095B

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To make use of the full geochronological potential of accessory minerals such as zircon, allanite, monazite,
and titanite, high spatial resolution isotopic analysis of the Th–U–Pb system is required. Laser ablation ICPMS
techniques are increasingly applied for this purpose, yet the matrix-dependence of analysis and the
paucity of high-quality standards for most of these minerals impose major limitations – as for all in situ
microbeam analytical techniques. A novel approach for LA-ICP-MS data reduction is presented here that
allows for non-matrix-matched external standardization while yielding highly accurate isotopic ratios and
age data. The matrix-dependent downhole fractionation during laser ablation is empirically quantified
and corrected; hence, well constrained reference materials (here: Plesovice zircon) can be employed as
primary standards for the analysis of rare yet petrologically essential minerals (here: SISS, CAP, and TARA
allanite). Using laser beam sizes of 32 and 24 mm, transient isotope ratio data show systematic
differences between zircon and allanite; these are attributed to matrix effects and are combined to
correct for the temporal evolution of the matrix-dependent downhole fractionation. The new data
reduction technique was tested on three allanite standard reference materials demonstrating analytical
accuracy at precisions equal to those achieved by ion probe and LA-ICP-MS rastering. The analytical
procedures presented here could be applied to any combination of two different matrices (calibration
standard and sample), thus greatly mitigating dependence on precisely characterized calibration
materials, and contribute to establishing universally applicable LA-ICP-MS dating protocols that can be
applied to a much broader range of minerals and chronometers.

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