The history of (U-Th)/He chronometry (a.k.a. alpha dating by some authors) stretches farther back than any other radioisotopic dating technique (Rutherford, 1905). It can arguably be interpreted as an allegory for the history of thermochronology in general, in which early, presumably nonsensical or inconsistent ages are later interpreted as geologically meaningful cooling ages in the context of additional kinetic and geologic constraints (e.g., see review in Reiners et al., 2005). The basic foundation of the technique is production of 4He from alpha decay of U and Th and intermediate daughter isotopes; in some cases 147Sm may also produce a significant fraction of 4He in a sample. The decay equation is:
where the lambdas are the parental decay constants. This equation assumes secular equilibrium of U- and Th-series isotopes, though additional information can account for disequilibrium effects in young (<~1 Myr) samples (Farley et al., 2002). The 4He concentration of a sample is a function of both production (as above) and diffusive loss, and can be represented and modeled as a function of time and temperature (e.g., Wolf et al., 1998). The properties governing diffusive loss are described by Arrhenius laws for thermally activated volume diffusion, with parameters specific to each mineral species, crystal or diffusion domain size, and, in some cases, composition or radiation dosage (e.g., Shuster et al., 2006).
An additional complication to (U-Th)/He ages arises from the fact that He nuclei travel, on average, ~15-20 micrometers away from parent nuclides upon production. Because this distance is similar (within factor of ~10) as the size of typically dated crystals, an upward correction to the measured He (or downward correction to measured U, Th, and Sm) is required to account for He ejected from them (Farley et al., 1996; Hourigan et al., 2005). This alpha-ejection correction ranges from about 20-40% of the measured age for zircon or apatite crystals with typical sizes. Uncertainty associated with the correction depends on uncertainties in measurements of crystal dimensions, the correspondence of analyzed and original crystal morphologies, and the distribution of parent nuclides both within the crystal and within the ~20- mm external environment during He accumulation in the rock. The latter two factors are difficult to quantify a priori, and may be largely responsible for the fact that the observed reproducibility of He ages for most crystals (typically >6-10%, 2s ) is significantly worse than that of analytical precision (~2-3%, 2s ). Nonetheless, even some samples requiring alpha-ejection corrections of ~30% show reproducibility approaching analytical precision, and accurate ages, as compared with 40Ar/39Ar ages on the same samples (e.g., Blondes et al., 2007). This indicates that alpha-ejection corrections can be quite accurate and precise if appropriate microscopic measurements are made and assumptions about dated crystals hold up.
Modern He dating, including 4He/3He thermochronology, has contributed much to a wide range of geologic and some extraterrestrial studies. Although complications to simple diffusion kinetic models and other aspects that complicate interpretations are beginning to be recognized, several factors make He dating a powerful technique that is uniquely suited to many geologic problems. Some of the most distinctive properties of the (U-Th)/He dating system follow.
• High diffusivity to low temperatures : Relatively high diffusivity of He in apatite (and some other phases) to near-surface temperatures at geologic timescales provides low bulk or intragrain closure temperatures (Dodson, 1973, 1979, 1986). This makes apatite He dating uniquely suited to providing low-temperature cooling ages and constraining low temperature thermal histories in the uppermost few kilometers of crust. This allows estimates of average erosion rates over timescales of 10^5 -10^7 yr, and constrains spatial patterns of erosion over lengthscales that address questions of paleotopography (e.g., House et al., 1998; 2001; Braun, 2002; Shuster et al., 2005, Ehlers et al., 2006), and possible climate-tectonic feedbacks.
• High daughter production rate : Alpha decay of U, Th, and intermediate daughter products is highly productive relative to many other extant natural radioisotopic decay systems, and the 4 He daughter is stable. This allows measurement of (U-Th)/He ages over a wide range of geologic ages. He ages as old as 4.5 Ga (Min et al., 2003) to as young as 2 ka (Aciego et al., 2003) have been measured. Our lab has routinely measured meteoritic phosphate He ages as old as 4.3-4.5 Ga, and clinker and volcanic zircon ages as young as ~10 ka and ~100 ka, respectively (Heffern et al., in press; Blondes et al., 2007).
• Relatively simple analysis . Although certain analytical aspects, such as alpha ejection corrections, can be tricky to deal with, most parts of the analytical procedures for conventional He dating are relatively straightforward, and can be performed with gas-source quadrupole mass spectrometry and standard ICP-MS approaches. Reproducibility of typical samples is usually significantly worse than of typical analytical precision (2-3%, 2s ), so extremely high precision measurements are not required for many He dating applications. Many important geologic questions can be easily addressed with percent-level precision. (This also raises the possibility of devlopment of portable analytical apparatus).
• Sensitivity to a range of surface and near-surface processes . Sensitivity of He diffusion to relatively low temperatures in many phases makes He dating useful for examining a wide range of processes surface or near-surface processes besides exhumation, such as wildfire (Reiners et al., submitted), meteorite impacts (Min et al., 2003, in press), or hydrothermal fluid flow (Whipp and Ehlers, submitted).
• Combination with 3He . Measurement of radiogenic 4He can be combined with measurements of either cosmogenic or articially-produced 3He in minerals. Recent developments in 4He/3He thermochronology have led to robust modeling of finite time-temperature histories of the shallowest crust with great power to resolve topographic evolution (Shuster et al., 2005). Also, many of the same phases that can be dated by 4He/3He and (U-Th)/He methods also have the potential to provide cosmogenic nuclide exposure ages or erosion rates (Farley et al., 2006). These approaches can be combined with conventional (U-Th)/He methods relatively easily, with the addition of a gas source sector mass spectrometer capable of resolving 3He from isobaric interferences.