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 through geologic time.  With Ken Farley (CIT), I developed a method called: ⁴He/³He thermochronometry in which the natural spatial distribution of ⁴He is constrained by stepwise degassing ⁴He/³He analysis of a sample containing synthetic, proton-induced ³He. 

The particular attraction of the radiogenic ⁴He system is its sensitivity to uniquely low temperatures.  ⁴He/³He thermochronometry on apatite (Ca₅(PO₄)₃F) can constrain a sample’s geological t-T path down to very low temperatures (in some cases <30^oC!). 

The next decade should lead to exciting new avenues of research using ⁴He/³He thermochronometry.  The technique provides a highly sought after link between information obtained by cosmogenic techniques (e.g., ¹⁰Be, ²⁶Al, ³⁶Cl, etc.) which constrain erosion rates and surface exposure on <1 Myr timescales and low-temperature thermochronometric techniques (e.g., apatite fission track, (U-Th)/He, ⁴⁰Ar/³⁹Ar, etc.) which provide exhumation history over much longer timescales. 

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