Zeta equivalent fission-track dating using LA-ICPMS and examples with simultaneous U-Pb dating

The use of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in the measurement of uranium concentration for fission-track (FT) dating can provide additional geochronological information by simultaneously acquiring data for U-Pb dating. To circumvent the use of several calibration factors with uncertainties in the FT age equation (i.e. the decay constant, values related to neutron irradiation, and observation efficiencies), the zeta calibration, which is based on the analysis of age-known standard samples, is recommended internationally. In this report, procedures to acquire LA-ICP-MS data for simultaneous dating by FT and U-Pb methods, the age equation with zeta-equivalent calibration, and the statistical treatment of FT age data, are proposed. An excimer laser beam with 20m diameter was used for sampling. We empirically determined zeta calibration constants against NIST SRM610 for FT zircon and apatite dating by analyzing several age standards. The data agreed with the calculated values, which can be obtained using reasonable constants in the equation. The correction factor in U-238-Pb-206 age determination with NIST SRM610 as an external standard was defined as 0.920 +/- 0.034 by analyzing several standards (FCT, Pleovice, Duluth, GJ-1). Based on this calibration system, zircons from the Nisatai Dacite, Japan, which is a secondary age standard commonly used in the Japanese FT community, were simultaneously dated to be 22Ma by FT and U-Pb methods on the same grains.