Reduction of mercury background on ICP-mass spectrometry for in situ U–Pb age determinations of zircon samples

Abstract

Precise ²³⁸U–²⁰⁶Pb ages for three Phanerozoic zircons (<600 Ma) have been measured using a laser ablation-ICPMS technique. In order to obtain reliable age data from young zircons, precise measurement of ²⁰⁴Pb intensity is highly important. To achieve this, a newly developed Hg-trap device using an activated charcoal filter was applied to the Ar make-up gas before mixing with He carrier gas, and the resulting isobaric interference on ²⁰⁴Pb by the ²⁰⁴Hg signal was significantly reduced. The level of Hg reduction was almost linearly correlated with the volume (weight) of the activated charcoal, and higher reduction efficiency of Hg signals could be obtained with a larger volume of activated charcoal. However, too high a volume of charcoal filter (>300 mL) resulted in lowering the signal intensity of Pb and U. A series of measurements for Hg signal intensity and elemental sensitivity of the instrument revealed that the highest S/N ratio (minimal Hg signal with highest signal intensity of analytes) was obtained when 200 mL of activated charcoal was used. By filtering the Ar make-up gas using the charcoal filter, the signal intensity of ²⁰⁴Hg could be reduced down to <100 s⁻¹, which is almost an order of magnitude lower than that observed without the Hg-trap. With the charcoal filter device, the uncertainty on the measured ²⁰⁴Pb intensity could be reduced remarkably, and this resulted in improvements in reliability of the ²⁰⁶Pb and ²⁰⁷Pb intensity data after the correction of non-radiogenic Pb (common-Pb). This is especially important to improve the accuracy and precision of the U–Pb age data for young zircons. We have measured U–Pb ages of three Phanerozoic zircons (≲600 Ma) from 18–20 μm pit sizes. The resulting ²³⁸U–²⁰⁶Pb ages for zircons from Sri Lanka (SL13) and Fish Canyon Tuff (FCT) were 566 ± 26 Ma (2SD, n = 10) and 27.6 ± 1.8 Ma (2SD, n = 13), respectively, showing excellent agreement with the previously reported values. The ²³⁸U–²⁰⁶Pb age for Mount Dromedary Igneous Complex (108.8 ± 3.9 Ma) was significantly older than the widely accepted value (98.7 ± 0.6 Ma). The possible cause of this discrepancy will be discussed in the text. The data obtained here demonstrate that the dating of Phanerozoic zircons from 20 μm ablation pits using LA-ICPMS is a real possibility.