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

Storey, C. D.; Jeffries, T. E. and Smith, M. (2006). Common lead-corrected laser ablation ICP-MS U-Pb systematics and geochronology of titanite. Chemical Geology, 227(1-2) pp. 37–52.

DOI: https://doi.org/10.1016/j.chemgeo.2005.09.003

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 sigma for Pb-206/U-238) 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 Pb-204 isotope is detailed. The isobaric Hg-204 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 Hg-204 is stripped from the Pb-204 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 sigma compared to reference age) and precise (2-4% at 2 sigma on Pb-206/U-238 ratios) U-Pb age data can be obtained from titanite, even as young as Oligocene. (c) 2005 Elsevier B.V. All rights reserved.