Development of an In Situ U-Th Disequilibrium Dating Method Utilising Multiple-Spot Femtosecond Laser Ablation-CRC-ICP-MS

In situ U-238-Th-230 dating was conducted using an ICP-mass spectrometer coupled with a laser ablation sampling system (LA-ICP-MS). To minimise the contribution of mass spectrometric interferences on Th-230, a kinetic energy discrimination technique using a collision/reaction cell (CRC) system was employed. The major problem associated with the CRC-ICP-MS system is the lower transmission efficiencies of ions than sector field mass spectrometers. To improve the signal-to-background ratio for analytes, a combination of a multiple-spot laser ablation (msLA) system together with a sensitivity-enhanced dry plasma cone was employed. Analytical accuracy was evaluated through analyses of three zircon reference materials (GJ-1, OD-3 and Plesovice) and the resulting Th-230/U-238 ratios for GJ-1 and Plesovice zircons agreed with the secular equilibrium value. The developed technique was applied to Quaternary volcanic zircon and monazite samples from Japan. The resulting ages are 113.5 (+5.4)(-5.2) ka for the Toya tephra, 105.1 (+6.4)(-6.1) ka for the Sambe Kisuki tephra, 55.2 (+7.7)(-7.2) ka for the Hikageyama lava and 16.2 (+4.2)(-4.1) ka for the Susaki pumice (95% confidence levels). The resulting ages are consistent with previous chronological constraints including zircon fission track ages. The data obtained here demonstrate clearly that the combination of the CRC-ICP-MS system and the msLA protocol can become a powerful analytical tool for in situ U-Th disequilibrium dating.