Ablation behaviour of allanites during U–Th–Pb dating using a quadrupole ICP-MS coupled to a 193 nm excimer laser

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been employed for U–Th–Pb geochronology of allanite, using various analytical parameters. Opinions diverge on analytical protocols and data reduction, and particularly, on the possibility of using non-matrix-matched standards and samples to obtain accurate and precise ages. In this study, ablation experiments were carried out in spot mode using an ArF 193 nm excimer laser coupled to a quadrupole ICP-MS, in order to characterise the ablation behaviour of allanite under various fluence (4, 7.8 and 12 J/cm2) and ablation rate conditions (4, 8 and 12 Hz). The U–Th–Pb data were compared to the Plešovice zircon (337.13 ± 0.37 Ma) in order to investigate the effects of the different matrices. The ratio-of-the-mean intensities method was used for data reduction. Analyses in spot mode at low frequency and fluence provide accurate and precise allanite 206Pb/238U and 208Pb/232Th ages, using a matrix-matched external standard. Matrix effects between allanites with different ages (30.1 to 417 Ma), common 208Pb (0.5–33%) and 206Pb (5.8–90%) proportions, and FeOtotal (11.5–16.1%) and REE + Th (0.52–0.90 apfu) compositions do not affect the age accuracy. The difference of mass bias correction coefficients between allanites and zircon shows evidence of matrix effects, which are often responsible for systematic errors on ages. Matrix-matched standardisation with an allanite standard is required for precise U–Th–Pb allanite ages. Even though laser-induced elemental fractionation can be minimised for non-matrix-matched standards and samples, other sources of matrix effects still affect age accuracy and precision.