SS14-28: An Age Reference Material for Zircon U-Th Disequilibrium Dating

U-Th disequilibrium dating uses the ratio of the intermediate Th-230 daughter isotope to the U-238 parent isotope to date zircon crystallisation for volcanic and plutonic rocks in Pleistocene-Holocene deposits. It is frequently used to constrain the duration and rate of magma recharge in volcanic systems. While less than or similar to 350 ka zircon is not in secular equilibrium in the Th-230 system, the current U-Th disequilibrium methodology uses reference materials that are > 350 ka. No reference material less than or similar to 350 ka has been available to validate the accuracy of the approach and ensure methods are repeatable across laboratories. This study presents zircon SS14-28 from Jeju Island (South Korea) as a suitable reference material for U-Th disequilibrium dating. Zircon SS14-28 was analysed using two analytical approaches (SIMS and LA-ICP-MS) and four instruments: CAMECA IMS 1280, ASI SHRIMP II, sector field high-resolution LA-ICP-MS and multi-collector LA-ICP-MS, in four laboratories. These methods each individually result in isochrons gradients within uncertainty (2s) of each other (CAMECA: 0.532 +/- 0.051 (MSWD = 0.64); SF-HR-LA-ICP-MS: 0.536 +/- 0.054 (MSWD = 1.3); MC-LA-ICP-MS: 0.533 +/- 0.041 (MSWD = 0.67); SHRIMP II: 0.68 +/- 0.22 (MSWD = 0.3)). The age proposed in this study is 82 +/- 6 ka calculated from a combined isochron gradient of 0.529 +/- 0.025 (MSWD = 0.87, n = 132).

Automated summary

U-Th disequilibrium dating method is commonly used to date zircon crystallization in volcanic and plutonic rocks, but the lack of suitable reference materials under 350 ka has been a challenge. This study introduces zircon SS14-28 from Jeju Island, South Korea as a potential reference material, with analysis showing consistent results across different laboratories. The proposed age of 82 +/- 6 ka is calculated based on a combined isochron gradient, demonstrating the accuracy and repeatability of the U-Th disequilibrium dating method.