Non‐Matrix‐Matched Determination of Th‐Pb Ages in Zircon, Monazite and Xenotime by Laser Ablation‐Inductively Coupled Plasma‐Mass Spectrometry

Abstract

Similar to the U‐Pb system, the 232Th‐208Pb decay scheme is also widely utilised in geochronology of Th‐rich accessory minerals. The matrix‐dependent systemic deviations observed between different accessory minerals and the paucity of well‐characterised reference materials for some minerals of interest (e.g., xenotime and allanite) are the major obstacles to yield accurate and precise Th‐Pb ages with laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS). In this study, we investigated the effects of the addition of nitrogen, oxygen and water vapour before the ablation cell on the accuracy of Th‐Pb dating of accessory minerals (i.e., zircon, monazite and xenotime) using NIST SRM 610 glass as the calibrating reference material. Our results demonstrate that the measured 208Pb/232Th ages in zircon, monazite and xenotime were approximately 9–24% lower than their isotope dilution thermal ionisation mass spectrometry (ID‐TIMS) ages in normal ablation (without addition of additional gases). The measured 208Pb/232Th ages of zircon (Plešovice, M257 and Qinghu), monazite (44069 and Trebilcock) and xenotime (BS‐1 and MG‐1) showed excellent agreement with their respective reference values with the addition of small amounts of water vapour before the ablation cell. This may be due to the dramatically reduced biases for the down‐hole fractionation of Pb/Th intensity ratios in NIST SRM 610 and accessory minerals in the presence of water vapour. The Th‐Pb ages of zircon, monazite and xenotime were successfully analysed using NIST SRM 610 glass as the calibrating reference material with both 193 nm ArF excimer laser ablation‐quadrupole‐inductively coupled plasma‐mass spectrometry (LA‐Q‐ICP‐MS) and 213 nm Nd:YAG laser ablation‐sector field‐inductively coupled plasma‐mass spectrometry (LA‐SF‐ICP‐MS) in combination with the developed water vapour‐assisted method. This method is an effective and notably simple approach for non‐matrix‐matched analysis of Th‐Pb ages in zircon, monazite and xenotime by LA‐ICP‐MS.