Allanite U[sbnd]Pb geochronology by 193 nm LA ICP-MS using NIST610 glass for external calibration.

Abstract

The viability of using NIST610 glass as an external standard for allanite UThPb geochronology by 193nm laser ablation inductively-coupled plasma mass spectrometry was investigated. Laser ablation conditions were optimized to minimize time-dependent fractionation of Pb/U and Pb/Th. A data reduction scheme (VizualAge_UMod) was additionally used to apply Pb/U fractionation corrections when the standard contains an enriched 238U/235U ratio (NIST610=420) compared to natural materials. Optimization of ablation conditions was carried out on natural allanite material separated from a ~415Ma tonalite intrusion (Hartfield intrusion, New Brunswick Canada). Conditions of 25s of ablations at 1J/cm2, 3Hz, and a 45μm crater diameter were established. This produced 3μm deep craters, and a near-steady-state, low mass-load, material transfer to the ICP-MS producing negligible time-dependent Pb/U and Pb/Th fractionation. Increasing fluence only modestly up to 3J/cm2 induced time-dependent Pb/U fractionation. To test for any residual matrix dependencies, the long-term reproducibility, accuracy, and absolute precision of the method was verified using ~31.5Ma Siss3 allanite material (Bergell Intrusion, Switzerland). The method yielded accurate and precise Pb/U ad Pb/Th ages that compare favourably to alternative allanite dating methods. As a further test, allanite separated from the Hartfield tonalite was analyzed and yielded an age that overlaps within error of the TIMS titanite UPb age for the intrusion. This is despite a partially amorphous microstructure as documented by Raman spectroscopy and optical microscopy. The results of this study emphasize the need to empirically calibrate laser fluence conditions that yield minimal element fractionation and to establish robust plasma settings. These conditions are unlikely to yield highest sensitivity, but should minimize the effects of the numerous laser-induced and plasma ionization processes that lead to Pb/U fractionation.