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⁴He/³He Thermochronometry: Theory, Practice, and Potential Complications

Division of Geological and Planetary Sciences California Institute of Technology Pasadena, California, 91125, U.S.A., dshuster@caltech.edu, farley@gps.caltech.edu

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
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 used to constrain . . . [Full Text of this Article]

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