U-PB ISOTOPE GEOCHRONOLOGY OF ZIRCON - EVALUATION OF THE LASER PROBE-INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY TECHNIQUE

Using a laser ablation microprobe-inductively coupled plasma mass spec trometer( LP-ICPMS) we have determined U-238, Pb-207, Pb-206, and Pb-2 04 abundances of several zircon populations whose ages have previously been measured by other techniques (principally ion microprobe). Ages of the samples range from 360-2800 Ma. A frequency quadrupled Nd-YAG U V laser (266 nm) which produces pit sizes of 10-15 mu m was used to ab late the zircon samples. Prolonged measurements of the Pb-206/U-238 is otope ratio using ablation techniques show a serious fractionation eff ect because of nonconstant focusing of the laser. This ablation fracti onation dramatically increases the Pb/U isotopic ratio because of the greater volatility of Pb. If the beam is constantly refocussed during ablation (active focusing), the effect of fractionation is minimised a nd the duration of the signal intensity increased. Pb-207/Pb-206 isoto pic ratio measurements on zircons with ages greater than 2400 Ma previ ously measured by SHRIMP give an excellent agreement. Typical precisio n of the measurement for Pb-207/Pb-206 isotopic and Pb/U elemental rat ios determined on single zircon grains is about 0.6-5% and 3-20%, resp ectively. In the case of Phanerozoic zircons (c. < 500 Ma), the LP-ICP MS allows the use of Pb-206/U-238 ratios and this provides more accura te data than the comparable Pb-207/Pb-206 system. The data presented h ere demonstrate clearly that the LP-ICPMS has a potential to become a significant and cost-effective tool for use in U-Pb zircon geochronolo gy.