Laser ablation microprobe (LAM)-ICP-MS is established as a very powerful tool for moderate resolution in situ analysis of minerals at very low concentrations. It also has a demonstrated capability of performing useful reconnaissance 207Pb/206Pb age determinations (Feng et al., 1993; Fryer et al., 1993), but this application is limited largely to older (>1,000 Ma) zircons due to the small change of the 207Pb/206Pb isotope ratio with age for younger materials. Attempts to make age determinations using 235U/207Pb and 238U/206Pb isotope ratios with geologically useful precision have been hampered by very large elemental fractionation of U with respect to Pb during the ablation and transport processes. Procedures such as "active focusing" have been shown to reduce the magnitude of the fractionation and have allowed Pb/U elemental ratios to be determined on single zircon grains with typical precision of 3-20% (Hirata and Nesbitt, 1995). However, for many geological studies, significantly higher precision is required. This study examines new procedures to minimise elemental fractionation and their application to U/Pb zircon geochronology using
The instrumentation used is an in-house built laser ablation microprobe based on a frequency quadrupled Nd:YAG laser operating at a wavelength of 266 nm in the UV. A petrographic microscope incorporated into the system provides the specialised optics required for viewing and readily locating sample sites in petrographic mounts. The laser ablation cell is coupled to an enhanced sensitivity ICP-MS, the VG PQII+"S". This instrumentation has low ppb detection limits for U and Pb at 30 micron sampling resolution. Using modified laser beam delivery optics, which image a mask on to the sample to reduce thermal "edge effects", and a novel ablation spot cooling jet to further reduce the thermal processes that result in elemental fractionation, negligible fractionation of U with respect to Pb can be achieved routinely. Furthermore, greatly enhanced sensitivity is obtained compared to standard operating conditions. Data is acquired in a time-resolved mode, which allows the user to examine depth-resolved signals, to detect zones of Pb loss or high common Pb and inherited cores, and to select and integrate the most appropriate signal intervals. Laser "abrasion" of the sample can also be performed. Instrumental Hg background, which compromises accurate measurement of common Pb using 204Pb, is minimised by filtering the argon carrier gas through activated charcoal. The Pb background is minimised largely by careful attention to system cleanliness.
The strategies described allow U/Pb ratios to be measured with precision (1 r.s.d.) down to ca. 1%, providing accurate zircon ages with comparable, or better, precision. These figures of merit will be demonstrated using data for a suite of Palaeozoic zircons analysed by thermal ionisation mass spectrometry.
Feng, R., Machado, N., & Ludden, J.N., Geochim. Cosmochim. Acta 57, 3479-3486 (1993).
Fryer, B.J., Jackson, S.E., and Longerich, H.P., Chem. Geol. 109, 1-8 (1993).
Hirata, T. & Nesbitt, R.W., Geochim. Cosmochim. Acta 59, 2491-2500 (1995).