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Posted by / 15-Aug-2016 23:27

Compared with TIMS and SHRIMP methods, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) dating is much more cost-effective and easy to apply (e.g.In regard to dating zircons by both U–Pb and U–Th methods, applied sensitive high-resolution ion microprobe (SHRIMP) to ~ 0.1 Ma granodiorite blocks ejected during a Holocene caldera-forming eruption of Mount Mazama (USA), in which U–Pb broadly constrained the timing of granodiorite intrusion and U–Th further refined its timing.applied SHRIMP U–Pb and U–Th dating to the 0.16 Ma Kos Plateau Tuff (Greece), in which zircon cores were dated by U–Pb to identify any inherited portion, whereas zircon rims were dated by U–Th to better determine the time scales of magma assembly and residence in the upper crust.Simultaneous U–Pb and U–Th zircon dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was performed on the ~ 0.1 Ma Toya Tephra, Hokkaido, Japan.The method employs the concurrent measurement of Th (U–Th) ages simultaneously.It was also revealed that the Toya Tephra magma crystallized zircons and experienced eruptions instantaneously at ~ 0.1 Ma, which is an important implication to decipher magmatic and eruptive processes for caldera-forming large volcanic systems.Few attempts have been performed on dating zircons using U–Pb and U–Th methods simultaneously, although there are essentially no technical barriers.

It was revealed that the simultaneous U–Pb and U–Th zircon dating technique using LA-ICP-MS is easy to apply and is useful for Quaternary tephras in that it can give age information on the tephra itself and xenocrystic zircons in a quick and cost-effective manner.The double dating technique employed here has the merit that zircons yielding Quaternary U–Pb ages can be immediately cross-checked by the U–Th method.This means that both methods can validate each other especially when dated simultaneously.With regard to dating young zircons by the U–Pb method, an initial Th disequilibrium correction is needed and common Pb contamination is problematic, while U–Th dating is free from these problems although its datable range is narrow.The U–Pb ages revealed that the Toya Tephra contains zircons with ages that cluster at ~ 0.1 Ma (0.11 ± 0.01 Ma: error shown as 95% confidence level) and xenocrysts with ages 2 Ma, the U–Th method also gave an isochron age of ~ 0.1 Ma (108 ± 19 ka or 0.11 ± 0.02 Ma; error shown as 2σ).

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Therefore, both U–Pb and U–Th methods result in similar ages of ~ 0.1 Ma for the Toya Tephra, in agreement with the quartz TL age (104 ± 30 to 118 ± 30 ka) of Ganzawa and Ike (2011).

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