(U-Th)/He Dating: Techniques, Calibrations, and Applications

The possibility of dating minerals by the accumulation of ^4He from U and Th decay
\nhas been recognized for many years (e.g., Strutt 1905), but in the century since the idea
\nwas first conceived, the method has rarely been applied successfully. After several
\ninvestigations of (U-Th)/He dating of various minerals (e.g., Damon and Kulp 1957;
\nFanale and Kulp 1962; Damon and Green 1963; Turekian et al. 1970; Bender 1973;
\nLeventhal 1975; Ferreira et al. 1975) the technique was essentially abandoned as yielding
\nunreliable and usually low ages, presumably as a result of diffusive He loss possibly
\nassociated with radiation damage. In 1987, Zeitler and coworkers rekindled interest in the
\nmethod by proposing that in the case of apatite, He ages might be meaningfully
\ninterpreted as ages of cooling through very low temperatures. Laboratory diffusion data
\npresented by these authors indicated a closure temperature of about 100ºC, a value
\nsupported by more recent studies (Lippolt et al. 1994; Wolf et al. 1996b; Warnock et al.
\n1997). Consistent with this interpretation Wolf et al. (1996a) found that apatite He ages
\nincrease systematically with sample elevation in a mountain range, as expected for
\nexhumation-induced cooling through a low closure temperature. Based on the strength of
\nthese results and additional laboratory (Farley 2000) and natural (Warnock et al. 1997;
\nHouse et al. 1999; Stockli et al. 2000) constraints on He diffusivity, recent attention has
\nfocused on applications of apatite He thermochronometry. There is also renewed interest
\nin He dating of other U- and Th-bearing minerals both for dating mineral formation and
\nfor thermochronometry. For example, Lippolt and coworkers have undertaken detailed
\nstudies of He diffusion and dating of various phases, most notably hematite formed in
\nhydrothermal systems (Lippolt and Weigel 1988; Wernicke and Lippolt 1992; Lippolt et
\nal. 1993; Wernicke and Lippolt 1994a,b).
\nHere I present an overview of recent techniques, calibrations, and applications of the
\n(U-Th)/He dating method; Hurley (1954) provides an excellent summary of earlier work in
\nthis field. Much of this paper focuses on apatite, because the He behavior and requisite
\nanalytical techniques are better established for this phase than for other target minerals,
\nsuch as zircon and titanite. Similarly, much of this paper concerns He diffusivity behavior
\nrequired for thermochronometric applications, yet recent work is also considering applications
\nto direct dating, for example, of young tephras (Farley et al. 2001).
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