3he cosmogenic nuclide age dating
Reproducibility of typical samples is usually significantly worse than of typical analytical precision (2-3%, 2), 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.
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.
: 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 equation assumes secular equilibrium of U- and Th-series isotopes, though additional information can account for disequilibrium effects in young (He 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).
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.
(This also raises the possibility of devlopment of portable analytical apparatus).
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). alpha dating by some authors) stretches farther back than any other radioisotopic dating technique (Rutherford, 1905).