Use of Laser-Ablation ICP-MS for U-Pb Zircon Geochronology

Abstract

Zircon is the most frequently used mineral for age determination with the U-Pb dating technique, despite its disadvantage of highly complex growth structures, inheritance of older pre-crystallization lead and its tendency to lose some of the radiogenic lead through the further P-T-evolution. Geological processes in deep crustal rocks are known to be very complex and polyphased and in part can be of short duration. The unravelling of such complex evolutionary histories demands for both high precision dating and high spatial resolution of different growth stages that have been visualized by imaging methods such as cathodoluminescence or backscatter electron imaging proir to isotopic dating. Isotope dilution thermal mass-spectrometry (IDTIMS) allows the assessment of highly precise U-Pb ages within a few permil uncertainty. Complex growth structures and/or system behaviour are, however, only resolved by in-situ analyses, e.g. using an ionprobe. Laserablation ICP-mass spectrometry now offers a third possibility for accurate, fast and cheap determination of U-Pb ages. The introduction of UV lasers lead to improved ablation behaviour (Gtinther et al., 1997), reduced inter-elemental fractionation effects and improved accuracy when compared to Nd:YAG lasers (e.g. Hirata and Nesbitt, 1995).