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

The common phosphate minerals, apatite Ca_5(PO_4)_3(F,OH,Cl), monazite (Ce,La,Th)PO_4, and xenotime YPO_4, have found widespread use in geochronology because they incorporate U and Th into their structures. For example, apatite usually has a few tens of ppm of both U and Th, while monazite and xenotime usually have hundreds of ppm to weight percent levels of these elements. As a consequence, these phosphates can be dated using several fundamentally different isotopic techniques. Elsewhere in this volume Harrison et al. describe phosphate dating using ingrowth of radiogenic Pb, the final daughter of U and Th series decay, and Gleadow et al. describe dating based on damage tracks from the spontaneous fission of ^(238)U. The most recently developed dating technique 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 dating is usually used to date high temperature events such as crystallization of igneous rocks and the timing of prograde metamorphism, fission track and (U-Th)/He dating are more commonly used to establish cooling histories through low temperatures, for example, in the range ~110-40°C in the case of apatite. Dating of minerals using radiogenic He was first explored shortly after the discovery of radioactivity (Strutt 1908) and was investigated extensively in the 1950s and 1960s, mostly on very U- and Th-rich minerals such as zircon and titanite (Hurley 1952, 1954; Damon and Kulp 1957, Damon and Green 1963). Apatite He dating was first investigated by Zeitler et al. (1987), who studied the diffusion rate of He from apatite and proposed that apatite He dating might provide a useful thermochronometer, recording cooling through about 100°C. Further studies both in the laboratory (Lippolt et al. 1994, Wolf et al. 1996b, Warnock et al. 1997, Farley 2000) and in the natural setting (House et al. 1999, Stockli et al. 2000) have confirmed this idea, and the technique has now been applied to a range of tectonic, geologic and geomorphologic problems (e.g., House et al. 1997, 1998; Spotila et al. 1997, Farley et al. 2001, Stockli et al. 2000). Monazite and xenotime have only recently come under scrutiny for He geochronology (e.g., Pik and Marty 1999), and little is yet known of their potential for routine geo- or thermochronometry