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%.

Automated summary

Apatite is a common accessory mineral in different types of rocks, but accurate U-Pb LA-ICP-MS dating is often affected by high levels of common Pb. A new Pb-204 correction method was used in this study to improve accuracy and precision by overcoming isobaric interference issues. Experimental results demonstrate that different reaction gases do not significantly affect the accuracy and precision of U-Pb dating.