U-Th-Pb Dating of Monazite by LA-ICP-MS Using Ablation Spot Sizes of 16 μm and 10 μm

Monazite is a common geological chronometer and has complex textures and different chemical domains. Meaningful U-Th-Pb ages for different domains can be obtained only using a suitably small laser beam spot. Based on optimizing the parameters of Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and matching two channels of carrier gas flows (Ar and He), two groups of laser ablation parameters, 16 μm-3 J/cm²-5 Hz and 10 μm-3 J/cm²-4 Hz, can be obtained for dating monazite using small ablation spot sizes. A standard sample (44069) and an unknown sample (15SLS-01) were dated using the above laser ablation parameters. For 44069, the single-point precision of ²⁰⁸Pb/²³²Th and ²⁰⁶Pb/²³⁸U ratios were less than 3%, and U-Th-Pb age was found to be consistent with the previous SHRIMP and TIMS U-Pb ages. For 15SLS-01, the single-point precision of ²⁰⁸Pb/²³²Th is less than 3%, whereas that of ²⁰⁶Pb/²³⁸U ratios is 3%-8%. However, the weighted average ages of ²⁰⁸Pb-²³²Th and ²⁰⁶Pb-²³⁸U are consistent within the uncertainties, and the ²⁰⁸Pb-²³²Th age is more representative. The study demonstrated that monazite U-Th-Pb age can be successfully measured by LA-ICP-MS using laser beam spot sizes of 16 μm and 10 μm, which greatly improve the spatial resolution of this micro-dating method.