In-situ SIMS U-Pb dating of phanerozoic apatite with low U and high common Pb

Apatite is a common accessory mineral in various igneous, metamorphic and sedimentary rocks. It has strong potential to provide important information for geochronology and petrogenesis. However, precise U-Th-Pb dating of apatite, especially young samples, is difficult due to low abundances of U and less radiogenic Pb, posing significant challenges for accurate common Pb correction. An additional issue is the lack of suitable apatite to serve as a standard for in-situ analyses. Here we present both analytical and data reduction protocols for SIMS in-situ U-Pb analyses on apatite with low U and high common lead. With NW-1 apatite as the U-Pb age standard, which was separated from similar to 1160 Ma carbonatite in the Prairie Lake complex in Canada, apatites from the Kovdor carbonatite in the Kola peninsula which contains low U (mostly similar to 2.5 ppm) and high common Pb (20-80%) yielded weighted average Pb-207-corrected U-Pb ages of 375 +/- 13 Ma (KV-8 and KV-18) and 377 +/- 11 Ma (KV-A), respectively. Apatites from the Quruqtagh ultramafic intrusion in NW China containing low U (similar to 2.3 ppm) and high common Pb (>70%) yield Pb-207-corrected U-Pb age of 805 +/- 21 Ma. The Cenozoic Durango apatite was dated at 31 +/- 2 Ma. All these new apatite U-Pb ages are indistinguishable from independently known age constraints to within 2-4%. Our results demonstrate that a U-Pb age can be accurately and precisely measured for apatite with low U (<3 ppm) and high common Pb (>50%) by SIMS with a suitable standard and a careful choice of common Pb composition. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.