High-Precision Pb Isotope Ratio Determination of Zircon By Multi-Ion Counter TIMS with Multi-Dynamic Collection Method

A high precision zircon Pb isotope ratio measurement method (using ²⁰⁵Pb as spike) was developed with a TRITON Plus thermal ionization mass spectrometer (TIMS) equipped with a multiple ion-counting (MIC) system. With the method, the Pb isotopes were measured with multi-ion counters using a multi-dynamic ion collection method. Compared to the traditional static multi-collection MIC method, the multi-dynamic MIC method can completely eliminate the influence of gain bias between different ion counters on the Pb isotope analytical results. Compared to the traditional zircon ID-TIMS Pb isotopic measurement method using a single secondary electron multiplier (SEM) with five peak jumps, the multi-dynamic MIC method can determine all the Pb isotope ratios of zircon in two peak jumps. Thus, the ion collection efficiency for the Pb isotopes were improved by 2.5 folds. In the meantime, the influences of ion beam stability on the analytical results of Pb isotopes was also reduced. To verify the reliability of the method, NIST981 Pb standards doped with ²⁰⁵Pb spike and Qinghu standard zircons were measured. The analytical precision of ²⁰⁷Pb/²⁰⁶Pb for 5×10^-11 g of ²⁰⁵Pb-NIST981 Pb mixture can achieve 0.079% (2RSD, n=20). The analytical ²⁰⁶Pb/²³⁸U age result for Qinghu standard zircon was 159.51±0.11 Ma(2SE, n=7; MSWD=1.1), which was consistent with the previously reported values within analytical error.