Apatite U-Pb Dating with Common Pb Correction Using LA-ICP-MS/MS

Apatite is a ubiquitous accessory mineral in various rocks (e.g., igneous, metamorphic and clastic sedimentary rocks). However, precise and accurate U-Pb LA-ICP-MS dating of apatite is often compromised by high common Pb levels. Among the different common Pb correction methods, the main advantage of the Pb-204 correction method is that it does not assume U/radiogenic Pb concordance. However, Pb-204 is difficult to measure using ICP-MS instruments because of the isobaric interference of Hg-204 on Pb-204. We overcome this limitation by using a reaction cell sandwiched between two quadrupoles within an ICP-MS, which can allow the online chemical separation of two different elements. Ammonia reacts efficiently (> 98%) with Hg while isotopes of Pb are not affected. The approach was tested on eight apatite reference materials (McClure Mountain, NW-1, UWA-1, Otter Lake, Slyudyanka, MK-1, Durango and Fish Canyon Tuff) for which there are independent constraints on the U-Pb crystallisation age, by comparing U-Pb dating results employing different reaction gas mixes (NH3-N2O and NH3 only) in two laboratories. Based on the U-Pb data and SEM analyses on each sample, we can exclude apatite inter- and intra-grain U-Pb age heterogeneity, except for a c. 4% variability of ages in the Otter Lake sample. Our results show that accuracy and precision for U-Pb dating are not measurably affected by different reaction gases, and we accurately reproduce ages of numerous independently characterised apatites within 4% of the reference ages, and the age reproducibility is typically better than 2%.