Use of multiple channeltrons for U-Pb laser ablation ICP-MS dating of accessory minerals with improved spatial resolution

Precision and accuracy of U-Pb laser ablation ICP-MS isotopic dating of accessory minerals is often limited by uncontrolled elemental fractionation of Pb and U during laser ablation, by variable mass discrimination of the ICP-MS instruments and also by imperfect correction for initial common Pb. While the early work was mostly restricted to determining the 207Pb/206Pb ages, the use of UV and DUV lasers, improvements in laser beam homogeneity, ablation in He, new sampling strategies, and lately also the implementation of femtosecond lasers and introduction of multiple-collector (MC) ICP mass spectrometers makes it now possible to achieve U-Pb age data that are comparable to those obtained from isotopic dating by secondary ion mass spectrometry. Fundamental studies of ablated aerosol and interactions of laser with the solid sample provided an insight into the mechanisms of Pb/U decoupling during the ablation and aerosol ionization and helped to focus on those aspects of the technique that affect the elemental fractionation. Improvements in the precision that can be as large as 10 fold have allowed for a more reliable correction for common (initial) Pb, increasing the range of accessory minerals that can now be dated by LA ICP-MS. Here we report results of laser ablation ICP-MS dating of accessory minerals that makes exclusive use of multiple channeltron detection (masses 202 - 238). The technique has been tested for analysis of zircon and monazite but is also well suited for analysis of minerals that typically require correction for common Pb, such as is titanite. Compared to previously reported methods of U-Pb multiple collector LA ICP-MS dating, no faraday-ion counter detector cross-calibration is required for multiple channeltron detection. The exclusive use of channeltrons results in improved spatial resolution of analysis (factor of 2 or better) and possibility to detect radiogenic Pb at low pg level. Ablation close to the sample surface results in low Pb/U fractionation rate that is statistically insignificant and does not have to be corrected for. Collectively, the improved precision and detection capability have both contributed to achieving spatial resolution of 1 - 2 * 103 μm3 in a typical zircon, making it possible to determine U-Pb ages from sub-grain domains and newly formed overgrowths that are only few μm wide. Although the laser ablation ICP-MS dating of minerals by U-Pb has became a widely accepted in-situ geochronological technique, consensus on data acquisition protocols, data reduction and reporting of results is needed in order to make the results truly comparable between individual laboratories.