独居石矿物学特征及其在Th-U-Pb同位素定年的应用
Mineralogical Characteristics of Monazite and Its Application to Th-U-Pb Isotope Dating

独居石是U-Pb和Th-Pb同位素测年的理想选择，因为它们在富含Th-U的同时一般Pb含量较低。本文比较和分析了火成岩、热液岩和沉积岩独居石中稀土和微量元素的内部结构、矿物成分和地球化学性质，以确定区别不同类型独居石的标志。岩浆型独居石具有宽而直的岩浆环，通常与磷质方解石同时存在，重稀土含量相对丰富；而热液型独居石可能具有与流体冲击有关的扇形环，通常与斜长石同时存在。沉积型独居石通常在形态上不规则，并与自生矿物有关。它们具有典型的四分组稀土分布模式，稀土总量较高，相对富集轻稀土(如La、Ce)，贫Y、Sc、Th、U、Sm、Gd等，Th/U比值较低。运用LA-MC-ICP-MS技术，可以得到较好的年龄数据。
Monazite is ideal for U-Pb and Th-Pb isotope dating because of its low initial common Pb content when it is enriched in Th-U. In this paper, the internal structure, mineral assemblage, rare earth and trace element geochemical characteristics of magmatic, hydrothermal and sedimentary mo-nazite are compared and analyzed to determine the distinguishing marks of different genetic types of monazite; magmatic monazite has a wide and straight magmatic annulus, often coeval with phosphorichalcite and relatively enriched in heavy rare earths, while hydrothermal monazite may have a fan-shaped annulus related to fluid action, often coeval with plagioclase thorite. Sedimentary monazite is mostly irregular in morphology and associated with authigenic minerals. They have a typical four-group rare earth distribution pattern, with high total rare earths, relatively enriched in light rare earths (e.g. La, Ce), poor in Y, Sc, Th, U, Sm, Gd, etc., and low Th/U ratio. Using LA-MC-ICP-MS technique, better age data can be obtained.