Determination of U–Pb Ages for Young Zircons using Laser Ablation‐<scp>ICP</scp>‐Mass Spectrometry Coupled with an Ion Detection Attenuator Device

Abstract

We have developed new analytical procedures to measure precise and accurate 238 U– 206 Pb and 235 U– 207 Pb ages for young (~ 1 Ma) zircons using laser ablation‐ ICP ‐mass spectrometry. For young zircons, both careful correction for the background counts and analysis of very small Pb/U ratios (i.e., 206 Pb/ 238 U &lt; 0.00016 and 207 Pb/ 235 U &lt; 0.0001 for 1 Ma zircons) are highly desired. For the correction of the background, the contribution of the background signal intensities for the analytes, especially for the residual signal intensities for 206 Pb and 207 Pb, was defined through laser ablation of synthesised zircons (ablation blank) containing negligible Pb. The measured signal intensities for 202 Hg, 206 Pb and 207 Pb signals obtained by the ablation blank were slightly higher than those obtained by data acquisition without laser ablation (gas blank). For the wider dynamic range measurements on Pb/U ratios, an attenuator device for the ion detection system was employed to extend the capability to monitor high‐intensity signals (i.e., &gt; 3 Mcps). Through the attenuator device, the ion currents were reduced to 1/450 of the signal intensity without the attenuator. Because the switching time for the attenuator was shorter than 1 ms, signal intensities for only specific isotopes could be reduced. With attenuation of the 238 U signal, counting statistics on 206 Pb and 207 Pb isotopes could be improved and counting loss on the 238 U signal could be minimised. To demonstrate the reliability of this new analytical technique, 238 U– 206 Pb and 235 U– 207 Pb ages for three young zircon samples (collected from Osaka Group Pink Volcanic Ash, Kirigamine and Bishop Tuff) were measured. The data presented here demonstrate clearly that the present technique could become a major analytical tool for in situ U–Pb age determination of young zircons (~ 1 Ma).