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

摘要

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 correc tions. 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~+ andsuitable exponentials, monazites with ThO2 contents in the range of -3-19 % do not exhibit this matrix effect. Inde pendent 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 EREE hereafter] contents. Suitable correction factors require end-member standards with extremely high or low U, Th, and Y (or ZREE) con tents. 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 xeno time 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 mona zite 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.