Evaluation of time-resolved mean-of-ratios reduction for laser ablation zircon U-Pb dating using quadrupole ICPMS

Abstract

We examined quality of zircon U-Pb dating results by mean-of-ratios (MOR) reduction of time-resolved analysis acquired with quadrupole inductively coupled plasma mass spectrometer (ICPMS) and 213 nm YAG laser ablation system. Time-resolved MOR reduction has an advantage to enable statistic evaluation of stability of isotopic ratios and detect accidents in each measurement. We practically applied internal mean square weighted deviation (iMSWD) and correlation coefficient of linear regression to dating of five reference zircons. The MOR reduction provided reasonably accurate U-Pb ages with 2-3% errors of precision. However, their iMSWD suggested dispersion of isotopic ratios in excess of counting errors. The dispersion is inferably attributed to non-simultaneous measurements by quadrupole ICPMS, in addition to potential heterogeneity of samples. We also tested quality of age results from manually trimmed time profiles. The test suggested that ages from clipped profiles as short as 5 sec. are still useful if reduction of precision is acceptable. The method enables to extract meaningful ages from measurements containing accidents, when statistically stable parts out of accidents are properly clipped from the time profiles.