Monazite and xenotime U-Th-Pb geochronology by ion microprobe: dating highly fractionated granites at Xihuashan tungsten mine, SE China

Zircon, monazite, and xenotime have proven to be valuable chronometers for various geological processes due to their commonly high-U-Th and low common Pb contents. However, zircons that have crystallized in highly fractionated granites often have such high-U contents that radiation damage can lead to scattered U-Pb ages when measured with secondary ion mass spectrometry (SIMS). In this study, monazite and xenotime were separated from a number of highly fractionated granites at the Xihuashan tungsten mine, Southeast China, for alternative dating methods by SIMS. For monazite analysis, obvious excess Pb-204 signal (mainly from interference of (ThNdO2)-Th-232-Nd-144-O-16 (++)) was observed in high-Th (> 2 wt%) monazite, which hinders Pb-204-based common Pb corrections. A Pb-207-based common Pb correction method was used instead. By employing power law relationships between Pb+/U+ versus UO2 (+)/U+, Pb+/Th+ versus ThO2 (+)/Th+ and suitable exponentials, monazites with ThO2 contents in the range of similar to 3-19 % do not exhibit this matrix effect. Independent SIMS U-Pb ages and Th-Pb ages of three phases of Xihuashan granite samples were consistent with each other and yielded dates of 158.7 +/- A 0.7, 158.0 +/- A 0.7, and 156.9 +/- A 0.7 Ma, respectively. Xenotime does show marked matrix effects due to variations of U, Th, and Y [or total rare earth element (REE), referred as I REE pound hereafter] contents. Suitable correction factors require end-member standards with extremely high or low U, Th, and Y (or I REE) pound contents. No excess Pb-204 was observed, indicating that the Pb-204-based common Pb correction method is feasible. Independent Pb-207/Pb-206 ages can be obtained, although multi-collector mode is necessary to improve precision. The main difficulties with dating xenotime are when high-Th (U) mineral inclusions are ablated. We can identify when this occurs, however, by comparing the measured UO2 (+)/U+ and ThO2 (+)/Th+ with those in xenotime standards. Three xenotime samples from the first phase of Xihuashan granite yielded a weighted mean Pb-207/Pb-206 date of 159.5 +/- A 4.4 Ma (MSWD = 1.0) and a Pb-206/U-238 date of 159.4 +/- A 0.9 Ma (MSWD = 1.6), which are consistent with monazite U-Pb and Th-Pb ages from the same granites. This study demonstrates that monazite and xenotime are better SIMS chronometers for highly fractionated granites than zircon, which can yield doubtful ages due to high-U contents.