In situ micro-beam U–Pb dating of apatite using CAMECA 1300HR3 LG-SIMS

Abstract

Apatite is a U-bearing mineral commonly present in planetary materials and has been widely used for geochronological studies. However, precise in situ U–Pb dating of tiny apatite (grain size <15 μm) remains challenging due to its low abundance of uranium (U) and radiogenic lead (Pb). To address this issue, we have developed two micro-beam analytical methods (i.e., 5 nA and 500 pA primary beams) using three apatite reference materials (BR5, AFG2, and Otter lake) and a CAMECA IMS 1300HR³ large-geometry secondary ion mass spectrometry (LG-SIMS). The 5 nA method (mono) produces a larger sputter crater (12.5 × 12.5 × 4.1 μm) and achieves higher dating precision, with age errors (n = 20) of <0.5% (1σ) for BR5, <2.5% (1σ) for AFG2, and <0.7% (1σ) for Otter lake. In contrast, the 500 pA method (multi) produces a smaller sputter crater (5.3 × 5.3 × 1.4 μm) that results in lower dating precision, with age errors (n = 25) of <1.3% (1σ) for BR5, <4.1% (1σ) for AFG2, and <1.8% (1σ) for Otter lake. Both methods yield accurate ages within the uncertainties and are applicable for apatite U–Pb dating. For data reduction, we recommend the use of the Pb/U vs. UO₂/U calibration protocol, because it introduces less age bias compared to those of Pb/U vs. UO/U and Pb/UO vs. UO₂/UO protocols. The methodological approaches of this study enable high-precision in situ U–Pb dating of tiny apatite and they will expand the application of LG-SIMS in geochronological studies of (extra-) terrestrial apatite samples.