Development of an <i>In Situ</i><scp>U‐Th</scp> Disequilibrium Dating Method Utilising <scp>Multiple‐Spot</scp> Femtosecond Laser <scp>Ablation‐CRC‐ICP‐MS</scp>
Sota Niki, Shuhei Kosugi, Hideki Iwano, Tohru Danhara, Takafumi Hirata
Abstract
In situ 238 U‐ 230 Th 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 230 Th, 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 Plešovice ) and the resulting 230 Th/ 238 U ratios for GJ‐1 and Plešovice 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.