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 c ommercial inductively coupled plasma-mass spectrometer (ICP-MS). The s ystem has been designed for micro sampling minerals in petrographic se ctions. Major advantages of this technique, compared to other in situ dating techniques, are the: (1) separation of the sampling process fro m 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) e ase of isolation and analysis of the isotopic data at all stages durin g an ablation sampling; (5) ability to date diverse materials; and (6) relatively low capital costs. Direct dating of uranium-rich phases su ch as pitchblende is rapid, precise and requires no chemical pretreatm ent. The ability to analyze small areas (diameters of 20-30 mum) facil itates studies on the timing of primary and secondary U mineralization . Micro sampling of an individual pitchblende grain from the complex C ollins Bay uranium deposit in Saskatchewan, Canada, illustrates this n ew technique's potential and the equivalence of conventional and LAM-I CP-MS age determinations. Analysis of 100-mum-diameter late Archean zi rcons further demonstrates that LAM-ICP-MS has the potential to become a rival of competing techniques for regional reconnaissance and sedim ent provenance age studies. In addition, this technique is uniquely ca pable of providing simultaneous monitoring of chemical and isotopic ho mogeneity during analysis of accessory minerals used in U-Pb geochrono logy.