Abstract:

Monazite [(LREE,Th)PO₄], widespread in most geological samples, has high Th and low common lead contents, and can thus be used for U(Th)-Pb dating. In this paper, monazite LA-ICP-MS U(Th)-Pb age determination method with monazite (44069) as external standard was established, using laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) of the Element Geochemistry Laboratory at the China University of Geosciences (Beijing). Monazite from the Bohemian Massif granulite were selected to analyze, aiming to address the mass fractionation of monazite U(Th)-Pb isotope system under different conditions, such as the integration time, denudation conditions (point denudation and line scanning), beam spot diameter (25 μm and 12 μm) and parameters of data processing software. Isotopic ratio and age results show that the analytical accuracy is the best under 25 μm vertical and 25 μm line scanning mode, and that the mass fractionation of Th-Pb isotope system is slightly higher than that of U-Pb isotope system. The mass fractionation between Th-Pb and U-Pb system of monazite, however, is significantly smaller than that of zircon. We suggest that the monazite ²⁰⁸Pb/²³²Th age is consistent with the ²⁰⁶Pb/²³⁸U age within error, which can be used as a new way to date geological bodies with high-U zircon. Based on the age results of monazite and zircon from metamorphic rocks of the Qingshuiquan granulite belt in East Kunlun, it is revealed that monazite geochronology can effectively constrain the metamorphic age, especially in sedimentary-metamorphic rocks that lack zircon. LA-ICP-MS monazite U(Th)-Pb dating method will be widely used in determining regional metamorphism, sedimentary diagenesis and S-type granite intrusion.

Key words: monazite, LA-ICP-MS, U(Th)-Pb isotope system, fractionation effect, geochronology