U-Pb and Th-Pb dating of apatite by LA-ICPMS

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 mu m spot over a 40 x 40 mu m square to a depth of 10 mu m 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 TI-U-Bi-Np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias. External standards (Plesovice 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 Pb-207 method, and also a Pb-208 correction method for samples with low Th/U. The Pb-207 and Pb-208 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 Pb-204 correction was undertaken because of low Pb-204 counts on single collector instruments and Pb-204 interference by Hg-204 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 2 sigma 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 Pb-207-corrected, Th-232-Pb-208, and common Pb-anchored Tera-Wasserburg Concordia intercept ages. (C) 2010 Elsevier B.V. All rights reserved.