MERITS OF PB-PB AND U-PB DATING OF TITANITE WITH MC-ICP-MS

Titanite has generated increasing interest as a relatively high temperature thermochronologic alternative to the Ar-Ar dating of amphiboles because of its common occurrence in a wide range of rocks. Because titanite has sites suitable for the incorporation of initial Pb, it is important to accurately assess the common Pb content of this mineral if accurate ages are to be determined. While the determination of a suitable common Pb composition is feasible for young igneous rocks, this is a more difficult proposition for older, metamorphic titanite. In order to evaluate the best method for accurately assessing the common Pb isotopic composition of titanites in general, we analyzed samples of titanite from two Cretaceous plutons and a Grenville-age pegmatite for U-Pb and Pb-Pb systematics to investigate the relative merit of these two methods. Analyses were conducted using a Nu-Plasma Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) in static collector mode with Pb isotopes (206, 207, and 204) acquired on ion counters and the U isotopes (235 and 238) on Faraday detectors. Direct introduction of dissolved single titanite grains in solutions (wet plasma) without chromatographic purification was used to acquire U and Pb isotopic compositions in Time Resolved Analysis mode. This approach essentially simulates laser ablation analyses, but eliminates the fractionation effects caused by ablation. Using this technique, all three samples yield relatively precise 206Pb/238U ratios indicating that accurate U-Pb ages can be determined provided initial Pb is well constrained. In contrast, only the older (Grenvillian-aged) titanite provides a realistic 207Pb/206Pb isochron age (983 +/- 8Ma), indicating that Pb-Pb dating of titanite is likely to have its principal application in Precambrian rocks. Our results indicate that MC-ICP-MS can be used to obtain reliable 206Pb/238U ages from young igneous titanite when accurate initial Pb isotopic compositions can be determined and Pb-Pb ages from older titanite. Current studies are focused on applying the Pb-Pb method to metamorphic titanite using MC-ICP-MS.