Common lead-corrected laser ablation ICP–MS U–Pb systematics and geochronology of titanite

Abstract

U–Pb geochronology using laser ablation ICP–MS is a fairly recently introduced technique and thus far its development has centred on zircon dating. Using titanite as an example, the application of laser ablation ICP–MS to U–Pb dating of common Pb-bearing accessory phases is presented. A simple analytical set-up, comprising a 213 nm Nd:YAG laser coupled to a quadrupole-based ICP–MS instrument is used. It is demonstrated that accurate and precise Pb/U ratios (2–3% at 2 σ for 206Pb/ 238U) can be obtained on small volumes of analysed material, using the zircon geostandard 91500 as an external calibration standard. A robust common Pb correction, necessitating the measurement of the stable 204Pb isotope is detailed. The isobaric 204Hg background interference in ICP–MS is lowered to a level where it does not dominate the total 204 amu signal using in-line gold traps on the carrier gas lines. The remaining 204Hg is stripped from the 204Pb signal mathematically by applying a natural Hg isotope ratio. Common Pb correction using this method is demonstrated to work well in analyses with elevated U–Pb signal intensity. Where this correction fails, an alternative method is required and a novel approach, using three-dimensional U–Pb concordia is adopted. This approach is shown to be extremely powerful in assessing age, precision and common Pb composition/correction. The technique is applied to the well characterised Fish Canyon Tuff demonstrating that accurate (7.8% at 2 σ compared to reference age) and precise (2–4% at 2 σ on 206Pb/ 238U ratios) U–Pb age data can be obtained from titanite, even as young as Oligocene.