Comment on "Zircon U-Th-Pb dating using LA-ICP-MS: Simultaneous U-Pb and U-Th dating on 0.1 Ma Toya Tephra, Japan" by Hisatoshi Ito

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of eight zircon reference materials and synthetic zircon-hafnon end-members indicate that corrections for abundance sensitivity and molecular zirconium sesquioxide ions (Zr2O3+) are critical for reliable determination of Th-230 abundances in zircon. Other polyatomic interferences in the mass range 223-233 amu are insignificant. When corrected for abundance sensitivity and interferences, activity ratios of (Th-230)/(U-238) for the zircon reference materials we used average 1.001 +/- 0.010 (1 sigma error; mean square of weighted deviates MSWD = 1.45; n = 8). This includes the 91500 and Plesovice zircons, which were deemed unsuitable for calibration of (Th-230)/(U-238) by Ito (2014). Uranium series zircon ages generated by LA-ICP-MS without mitigating (e.g., by high mass resolution) or correcting for abundance sensitivity and molecular interferences on Th-230 such as those presented by Ito (2014) are potentially unreliable. (C) 2015 Elsevier B.V. All rights reserved.