THE APPLICATION OF LASER-ABLATION MICROPROBE-INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY (LAM-ICP-MS) TO IN-SITU (U)-PB GEOCHRONOLOGY

The direct dating of single pitchblende and zircon grains is reported, using a laser ablation microprobe (LAM) which has been coupled to a commercial inductively coupled plasma-mass spectrometer (ICP-MS). The system has been designed for micro sampling minerals in petrographic sections. Major advantages of this technique, compared to other in situ dating techniques, are the: (1) separation of the sampling process from the excitation (dissociation and ionization) processes; (2) minimal complex and variable ion species in the mass analyzer; (3) ability to monitor and adjust the analysis characteristics during ablation; (4) ease of isolation and analysis of the isotopic data at all stages during an ablation sampling; (5) ability to date diverse materials; and (6) relatively low capital costs. Direct dating of uranium-rich phases such as pitchblende is rapid, precise and requires no chemical pretreatment. The ability to analyze small areas (diameters of 20-30 mum) facilitates studies on the timing of primary and secondary U mineralization. Micro sampling of an individual pitchblende grain from the complex Collins Bay uranium deposit in Saskatchewan, Canada, illustrates this new technique's potential and the equivalence of conventional and LAM-ICP-MS age determinations. Analysis of 100-mum-diameter late Archean zircons further demonstrates that LAM-ICP-MS has the potential to become a rival of competing techniques for regional reconnaissance and sediment provenance age studies. In addition, this technique is uniquely capable of providing simultaneous monitoring of chemical and isotopic homogeneity during analysis of accessory minerals used in U-Pb geochronology.