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

• Published in: Geostandards and geoanalytical research 2022-12, Vol.46 (4), p.589-602
• Main Authors: Niki, Sota, Kosugi, Shuhei, Iwano, Hideki, Danhara, Tohru, Hirata, Takafumi
• Format: Article
• Language: English
• Subjects: Ablation; collision/reaction cell; geochronology; Isotopes; Kinetic energy; laser ablation‐inductively coupled plasma‐mass spectrometry; Lasers; Lava; LA‐ICP‐MS; Mass; Monazite; Pumice; Quaternary; Spectrometers; Tephra; U‐Th disequilibrium dating; Zircon
• ISSN: 1639-4488; 1751-908X
• DOI: 10.1111/ggr.12458

In situ 238U‐230Th 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 230Th, 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 230Th/238U 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. Key Points An in situ 238U‐230Th dating method using a collision/reaction cell ICP‐MS (CRC‐ICP‐MS) system coupled with a multiple‐spot laser ablation (msLA) system was developed. A kinetic energy discrimination technique using the CRC was employed to minimise interferences on 230Th+. Analytical accuracy of the method using the msLA‐CRC‐ICP‐MS system was demonstrated by analyses of three zircon reference materials; the method was applied to date four Quaternary samples from Japan.