- © The Mineralogical Society Of America
Thermochronometry most often involves the determination of a cooling age from parent and daughter abundances within an entire crystal or population of crystals (Dodson 1973). Complementary information exists in the spatial concentration distribution of the daughter, C(x,y,z), within a single crystal. By combining a bulk cooling age with C(x,y,z) on the same sample, it is possible to place tight limits on the sample’s time-temperature (t-T) path. Techniques for this kind of analysis have been developed for several different parent/daughter systems including U-Th-Pb and K-Ar (Harrison et al. 2005). Here we describe how this approach is applied to the (U-Th)/He system. The particular attraction of the (U-Th)/He method is its sensitivity to uniquely low temperatures. For example, the nominal 4He closure temperatures (at 10 °C/Myr) for apatite, zircon and titanite are 70 °C, 180 °C, and 200 °C, respectively (Reiners and Farley 1999, 2001; Farley 2000; Reiners et al. 2002, 2004). In the case of apatite, we will show that significant diffusive mobility of 4He occurs at temperatures just slightly higher than those of the Earth’s surface. In this chapter, we present an overview of the 4He/3He thermochronometry technique in which the natural spatial distribution of 4He is constrained by stepwise degassing 4He/3He analysis of a sample containing synthetic, proton-induced 3He. We present the fundamental theory, assumptions, practical aspects of proton irradiation and stepwise 4He/3He analyses, as well as several example applications of 4He/3He thermochronometry.
In particular, we illustrate how the 4He/3He technique can be used to determine the helium diffusion kinetics and constrain the natural 4He distribution within an individual crystal or a small population of crystals, and how this information can be …