(U-Th)/He Dating of Phosphates: Apatite, Monazite, and Xenotime

The common phosphate minerals, apatite Ca₅ (PO₄) ₃ (F, OH, Cl), monazite (Ce, La, Th) PO₄, and xenotime YPO₄, have found widespread use in geochronology they incorporate U and Th into their structures. For example, apatite usually has few tens of ppm of both U and Th, while monazite and xenotime usually have hundreds ppm to weight percent levels of these elements. As a consequence, these phosphates be dated using several fundamentally different isotopic techniques. Elsewhere in this Harrison et al. describe phosphate dating using ingrowth of radiogenic Pb, the daughter of U and Th series decay, and Gleadow et al. describe dating based on tracks from the spontaneous fission of ^ (238) U. The most recently developed dating applied to phosphates, described in this chapter, uses the accumulation of α particles from U and Th series decay, (U-Th) /He dating. While phosphate U-Th-Pb is usually used to date high temperature events such as crystallization of igneous and the timing of prograde metamorphism, fission track and (U-Th) /He dating are commonly used to establish cooling histories through low temperatures, for, in the range ~110-40°C in the case of apatite. of minerals using radiogenic He was first explored shortly after the discovery radioactivity (Strutt 1908) and was investigated extensively in the 1950s and 1960s, on very U- and Th-rich minerals such as zircon and titanite (Hurley 1952, 1954; and Kulp 1957, Damon and Green 1963). Apatite He dating was first investigated Zeitler et al. (1987), who studied the diffusion rate of He from apatite and proposed apatite He dating might provide a useful thermochronometer, recording cooling about 100°C. Further studies both in the laboratory (Lippolt et al. 1994, Wolf et. 1996b, Warnock et al. 1997, Farley 2000) and in the natural setting (House et al. 1999, et al. 2000) have confirmed this idea, and the technique has now been applied to a of tectonic, geologic and geomorphologic problems (e. g. , House et al. 1997, 1998; et al. 1997, Farley et al. 2001, Stockli et al. 2000). Monazite and xenotime have recently come under scrutiny for He geochronology (e. g. , Pik and Marty 1999), and is yet known of their potential for routine geo- or thermochronometry.