LASER ABLATION MC-ICP-MS AND MULTIPLE ION COUNTING- THE PERFECT COMBINATION FOR GEOCHRONOLOGY

The coupling of a laser ablation system to a multi collector ICP-MS provides the spatial resolution required for rapid “in-situ” analyses of individual crystals down to 10 microns without the lengthy sample preparation procedures required for TIMS. For some minerals the Pb concentration is so low that the ion beams are insufficient for precise Faraday/Faraday measurement. In these cases it is possible to use single ion counting mode of operation where one isotope is of low intensity. For many minerals, ion beams from laser ablation are too small for single ion counting coupled with multiple faraday analysis. In this case the use of multiple ion counters allows precise analysis.

The VG Axiom Plus Multi Collector is fitted with three ion counting detectors along with the normal nine-Faraday array and wide Faraday collector. This allows simultaneous analysis of ²⁰²Hg, ²⁰⁴Pb(+Hg) and ²⁰⁷Pb measured on the ion counters, with ²⁰³Tl & ²⁰⁵Tl (for mass bias correction) and ²⁰⁸Pb measured on the normal Faradays and either ²³⁸U or ²³²Th on the wide Faraday.

Data will be presented on accessory minerals such as zircons, monazites and titanites, commonly used for geochronology. Typically 1% (2s) reproducibility is achieved for ²⁰⁷Pb/²⁰⁶Pb ratios after correction for common-Pb and Hg, when analysed in a multi-static procedure including a single axial ion counter. Acquisition in multiple ion counting mode should allow precision and reproducibility up to an order of magnitude better. This level of data quality would place LA-MC-ICP-MS techniques on an equal footing with the ion microprobe, the only other available in-situ method of high-precision geochronology.