U-Th-He dating of apatite: A potential thermochronometer

We found a gem quality crystal of Durango fluorapatite to have a ⁴He content consistent with complete retention of radiogenic helium since its formation at about 31 Ma. Isothermal heating and step-heating analysis reveal ⁴He loss to occur systematically by volume diffusion at low temperatures. The linear, low-temperature portion of the diffusion data yields an activation energy of 38.5 ± 8.1 kcal/mol and a frequency factor of In ( D₀ a²) = 16.4 ± 2.8 sec^-1, corresponding to a closure temperature of 105°C ± 30°C (cooling rate 10°C/m.y.). It appears that U-Th-He dating of apatite might represent a useful new thermochronometer with a range similar to that of fission-track dating of apatite. From these results, we infer that a number of the too-young U-Th-He dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low-temperature thermal history of their host rocks.