U-Pb LA-ICPMS dating using accessory mineral standards with variable common Pb

• Published in: Chemical Geology
• Main Authors: Chew, David, Petrus, Joseph, Kamber, Balz
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2014
• Subjects: Apatite; Common Pb; Geochemistry and Geophysics; LA-ICPMS; Rutile; Titanite; U-Pb dating; akilia; geochronological evidence; laser-ablation icpms; lead diffusion; ms; southwest greenland; th-pb; trace-element; zircon ages; Greenland
• Online Access: https://eprints.qut.edu.au/220837/

Precise and accurate U-Pb LA-ICPMS dating of many U-bearing accessory minerals (e.g. apatite, allanite, titanite and rutile) is often compromised by common Pb. LA-ICPMS dating of these U-bearing accessory phases typically requires a matrix-matched standard, and data reduction is often complicated by variable incorporation of common Pb not only into the unknowns but also particularly into the reference material. We present here a general approach to common Pb correction in U-Pb LA-ICP-MS dating using a modified version of the VizualAge U-Pb data reduction package for Iolite (VizualAge_UcomPbine). The key feature of the method is that it can correct for variable amounts of common Pb in any U-Pb accessory mineral standard as long as the standard is concordant in the U/Pb (and Th/Pb) systems following common Pb correction. Common Pb correction of the age standard can be undertaken using either the Pb-204, Pb-207 or Pb-208((no Th)) methods, and the approach can be applied to raw data files from all widely used modern multi-collector and single-collector ICPMS instruments. VizualAge_UcomPbine first applies a common Pb correction to the user-selected age standard integrations and then fits session-wide "model" U-Pb fractionation curves to the time-resolved U-Pb standard data. This downhole fractionation model is applied to the unknowns and sample-standard bracketing (using a user-specified interpolation method) is used to calculate final isotopic ratios and ages. Pb-204- and Pb-208((no) (Th)) corrected concordia diagrams and Pb-204-, Pb-207- and Pb-208((no Th))-corrected age channels can be calculated for user-specified initial Pb ratio(s). All other conventional common Pb correction methods (e.g. intercept or isochron methods on co-genetic analyses) can be performed offline. The approach was tested on apatite and titanite age standards (for which there are independent constraints on the U-Pb crystallization age) using a Thermo Scientific iCAP-Qc (Q-ICP-MS) coupled to a Photon Machines Analyte Excite 193 nm ArF Excimer ...