Allanite U-Th-Pb geochronology by laser ablation inductively coupled plasma mass spectrometry: Evaluation and development of reference materials with low common Pb

Abstract

Allanite, a member of the epidote supergroup, is widely distributed across various rock types. The U-Th-Pb geochronology of allanite can provide critical time constraints for investigating igneous rock formation, metamorphic events, and hydrothermal mineralization processes. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) offers high spatial resolution for in-situ U-Th-Pb dating of allanite with the advantage of avoiding compositional zoning and inclusions. However, current allanite reference materials for LA-ICP-MS U-Th-Pb dating often contain high and variable common Pb contents, limiting their effectiveness as primary calibration standards. This study introduces two new allanite reference materials (AMK and ASP) with low common Pb content. Allanite AMK and ASP exhibit f206 values (the proportion of common 206Pb in the total measured 206Pb) below 0.8 % and 5.5 %, respectively. AMK contains 140 ± 41.9 μg g−1 (1σ) of U and 306 ± 145 μg g−1 (1σ) of Th, while ASP contains 261 ± 143 μg g−1 (1σ) of U and 130 ± 102 μg g−1 (1σ) of Th. ID-TIMS analysis yielded a weighted mean 207Pb/206Pb age of 1533.6 ± 1.9 Ma (2σ, MSWD = 5.1, n = 6) for AMK and a weighted mean 206Pb/238U age of 335.86 ± 0.52 Ma (2σ, MSWD = 5.9, n = 8) for ASP. To mitigate the impact of common Pb in primary reference materials, two 207Pb method-based calibrating procedures (VizualAge_UcomPbine software correction and a ‘two-step’ calibration strategy) were performed to the commonly used Tara allanite standard. While accurate allanite UPb ages can be obtained with both calibration strategies, analytical precision was reduced due to uncertainty propagated from the common Pb correction. Furthermore, a simple and robust allanite LA-ICP-MS U-Th-Pb dating method was established by calibrating against allanite AMK without prior common Pb correction. We propose allanite AMK as an effective primary reference material for LA-ICP-MS U-Th-Pb dating, while ASP serves as a suitable secondary reference material due to its very low common Pb content. Our results confirm no significant matrix effects during LA-ICP-MS analyses of allanites across a wide range of Th contents (from 0.03 to 21,900 μg g−1) and FeO content variations (from 12.8 to 14.6 wt%).