APPLICATIONS AND LIMITATIONS OF (U-TH)/HE DATING FOR QUATERNARY GEOCHRONOMETRY

The recent resurgence of interest in dating by ingrowth of 4He from U and Th decay has been driven by thermochronometry applications, but a few studies have demonstrated its strengths and weaknesses for "absolute" dating, especially of very young materials. The main attraction of 4He for this application is that ingrowth in many minerals is rapid relative to blanks and detection limits; e.g., in just 1 Myr the 4He produced in 1 mg of material with 1 ppm U at secular equilibrium is >100x typical blank. Furthermore, it is commonly possible to trade-off lower U, Th contents against larger sample size, so even materials with ppb levels of parent nuclides may be datable. However the method suffers from two major limitations: 1) Some materials diffuse He at Earth surface temperatures, including volcanic glass and whole rock, quartz, feldspars, and Mn oxides, while apatite, zircon, titanite, garnet, fluorite, amphibole, pyroxene, and some Fe oxides are thought to be sufficiently retentive to be potential dating targets (though "excess He" may be a problem in some of these minerals). 2) Secular disequilibrium in the U series may cause He production to be time-varying as the intermediate nuclides decay into equilibrium. For volcanic samples the nuclide of concern is 230Th (t1/2=75 kyr), while in fluid precipitated samples 234U (t1/2=247 kyr) must also be considered. Secular equilibrium is achieved within ~5t1/2 of the longest lived daughter in initial disequilibrium, but He concentrations integrate disequilibrium ingrowth and so "remember" it for much longer periods. Methods for determining ages under secular disequilibrium are straightforward if the initial conditions are known or assumed; if they are not known it is possible to obtain precise but potentially inaccurate ages, with the inaccuracy decreasing with time since mineral formation. Examples of He ages of Quaternary apatite, zircon, garnet, fluorite and goethite obtained at Caltech and by other groups will be discussed. Over the range from ~1 kyr to ~ 2 Myr, He ages typically are precise to ±5-10%. Accuracy ranges from equally good to unacceptably poor depending on age and knowledge of secular equilibrium state.