The application of laser ablation microprobe (LAM)-ICP-MS to in-situ U-Pb zircon geochronology

S. Jackson, GEMOC, Macquarie

LAM-ICP-MS has a demonstrated capability of performing reconnaissance 207Pb/206Pb zircon age determinations, but useful application is limited largely to older (>1,000 Ma) samples. Attempts to make age determinations using 207Pb/235U and 206Pb/238U isotope ratios with useful precision have been hampered by insufficient sensitivity and severe elemental fractionation of Pb with respect to U during the ablation/transport processes. This study examines procedures to minimise elemental fractionation and their application to Pb/U geochronology using modern LAM-ICP-MS instrumentation.

The instrumentation consists of an in-house built LAM incorporating a frequency quadrupled Nd:YAG laser (λ= 266 nm in the UV). Ablated material is analysed by an enhanced sensitivity ICP-MS fitted with a flared torch bonnet attachment to provide improved stability. This instrumentation has low ppb detection limits for U and Pb at 30 micron sampling resolution. Using an imaged, defocussed laser beam to reduce thermal edge effects, and a novel ablation spot cooling Ar jet to further reduce the thermal processes that result in elemental fractionation, minimal fractionation of Pb with respect to U is achieved. Calibration against a concordant mineral standard under constant ablation conditions is used to correct residual fractionation. High instrumental Hg background prohibits accurate measurement of common Pb using 204Pb. However, acquiring data in a time-resolved mode allows the use of depth-resolved signal and isotopic ratios profiles to detect zones enriched in common Pb, together with zones of Pb loss, inherited cores and other artifacts. Appropriate signal intervals are then integrated accordingly. Pb background is minimised by careful attention to system cleanliness.

The strategies described allow Pb/U ratios to be measured with precision (1 r.s.d.) down to ca. 1.5%, providing ages with comparable, or better, precision. Accuracy has been tested by analysing zircons ranging from ca. 290 to 2700 Ma. In each case, zircons have revealed concordant ages or Pb-loss discordia which provide concordia intercept ages that are in agreement with TIMS ages for the same samples. Preliminary attempts to date monazite and baddeleyite using LAM-ICP-MS will also be presented. UV-LAM-ICP-MS is a very fast (2 min/spot analysis) and competitive tool for moderate precision U-Pb geochronology.

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