U–Pb and Th–Pb dating of apatite by LA-ICPMS David M. Chew a, ⁎, Paul J. Sylvester b , Mike N. Tubrett b a Department of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland b Department of Earth Sciences and Inco Innovation Centre, Memorial University, St. John's, Newfoundland, A1B 3X5 Canada abstract article info Article history: Received 6 July 2010 Received in revised form 4 November 2010 Accepted 5 November 2010 Available online 10 November 2010 Editor: R.L. Rudnick Keywords: U–Pb Th–Pb Geochronology Apatite LA-ICPMS Common Pb Provenance Apatite is a common U- and Th-bearing accessory mineral in igneous and metamorphic rocks, and a minor but widespread detrital component in clastic sedimentary rocks. U–Pb and Th–Pb dating of apatite has potential application in sedimentary provenance studies, as it likely represents first cycle detritus compared to the polycyclic behavior of zircon. However, low U, Th and radiogenic Pb concentrations, elevated common Pb and the lack of a U–Th–Pb apatite standard remain significant challenges in dating apatite by LA-ICPMS, and consequently in developing the chronometer as a provenance tool. This study has determined U–Pb and Th–Pb ages for seven well known apatite occurrences (Durango, Emerald Lake, Kovdor, Mineville, Mud Tank, Otter Lake and Slyudyanka) by LA-ICPMS. Analytical procedures involved rastering a 10 μm spot over a 40 × 40 μm square to a depth of 10 μm using a Geolas 193 nm ArF excimer laser coupled to a Thermo ElementXR single-collector ICPMS. These raster conditions minimized laser-induced inter-element fractionation, which was corrected for using the back-calculated intercept of the time-resolved signal. A Tl–U–Bi–Np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias. External standards (Plešovice and 91500 zircon, NIST SRM 610 and 612 silicate glasses and STDP5 phosphate glass) along with Kovdor apatite were analyzed to monitor U–Pb, Th–Pb, U–Th and Pb–Pb ratios Common Pb correction employed the 207 Pb method, and also a 208 Pb correction method for samples with low Th/U. The 207 Pb and 208 Pb corrections employed either the initial Pb isotopic composition or the Stacey and Kramers model and propagated conservative uncertainties in the initial Pb isotopic composition. Common Pb correction using the Stacey and Kramers (1975) model employed an initial Pb isotopic composition calculated from either the estimated U–Pb age of the sample or an iterative approach. The age difference between these two methods is typically less than 2%, suggesting that the iterative approach works well for samples where there are no constraints on the initial Pb composition, such as a detrital sample. No 204 Pb correction was undertaken because of low 204 Pb counts on single collector instruments and 204 Pb interference by 204 Hg in the argon gas supply. Age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common Pb-corrected ages, a Tera–Wasserburg Concordia intercept age and a Tera–Wasserburg Concordia intercept age anchored through common Pb. The samples in general yield ages consistent (at the 2σ level) with independent estimates of the U–Pb apatite age, which demonstrates the suitability of the analytical protocol employed. Weighted mean age uncertainties are as low as 1–2% for U- and/or Th-rich Palaeozoic– Neoproterozoic samples; the uncertainty on the youngest sample, the Cenozoic (31.44 Ma) Durango apatite, ranges from 3.7–7.6% according to the common Pb correction method employed. The accurate and relatively precise common Pb-corrected ages demonstrate the U–Pb and Th–Pb apatite chronometers are suitable as sedimentary provenance tools. The Kovdor carbonatite apatite is recommended as a potential U–Pb and Th–Pb apatite standard as it yields precise and reproducible 207 Pb-corrected, 232 Th– 208 Pb, and common Pb-anchored Tera–Wasserburg Concordia intercept ages. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Apatite is a common accessory mineral in igneous, metamorphic and clastic sedimentary rocks. It is a nearly ubiquitous accessory phase in igneous rocks, due in part to the low solubility of P 2 O 5 in silicate melts and the limited amount of phosphorus incorporated into the crystal lattices of the major rock-forming minerals (Piccoli and Candela, 2002). Apatite is common in metamorphic rocks of pelitic, carbonate, basaltic, and ultramafic composition and is found at all metamorphic grades from transitional diagenetic environments to migmatites (Spear and Pyle, 2002). Apatite is also virtually ubiquitous in clastic sedimentary rocks (Morton and Hallsworth, 1999). Chemical Geology 280 (2011) 200–216 ⁎ Corresponding author. Tel.: +353 1 8963481; fax: +353 1 6711199. E-mail address: chewd@tcd.ie (D.M. Chew). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.11.010 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo