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 204 Pb signal (mainly from interference of 232 Th 144 Nd 16 O 2 ++ ) was observed in high-Th (>2 wt%) monazite, which hinders 204 Pb-based common Pb corrections. A 207 Pb-based common Pb correction method was used instead. By employing power law relationships between Pb + /U + versus UO 2 + /U + , Pb + /Th + versus ThO 2 + /Th + and suitable exponentials, monazites with ThO 2 contents in the range of ~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 ± 0.7, 158.0 ± 0.7, and 156.9 ± 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 ΣREE hereafter] contents. Suitable correction factors require end-member standards with extremely high or low U, Th, and Y (or ΣREE) contents. No excess 204 Pb was observed, indicating that the 204 Pb-based common Pb correction method is feasible. Independent 207 Pb/ 206 Pb 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 UO 2 + /U + and ThO 2 + /Th + with those in xenotime standards. Three xenotime samples from the first phase of Xihuashan granite yielded a weighted mean 207 Pb/ 206 Pb date of 159.5 ± 4.4 Ma (MSWD = 1.0) and a 206 Pb/ 238 U date of 159.4 ± 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.