MODELING OF THE TEMPERATURE SENSITIVITY OF THE APATITE (U-TH) HE THERMOCHRONOMETER/

Apatite (U-Th)/He apparent ages will generally reflect residence for e xtended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion. To characterize the response of apati te He ages to thermal histories involving partial He retention, we exp lored solutions to the He production-diffusion equation. Under thermal ly static conditions, the analytical solution to this equation, couple d with published diffusivity data, demonstrates that the zone of parti al He retention extends from about similar to 40 degrees C to similar to 85 degrees C. This zone lies at temperatures similar to 35 degrees C cooler than the analogous fission track partial annealing zone. He a ges within the partial retention zone ultimately achieve a balance bet ween He production and loss, yielding a steady state age. Both the ult imate age and the time it takes to achieve this age are temperature de pendent. For example, an apatite held at 75 degrees C equilibrates to an age of similar to 2 Ma after similar to 17 Myr, regardless of wheth er equilibrium is approached from a higher or a lower initial He age. Far representative dynamic thermal histories, we evaluated apatite He ages using a numerical solution to the ingrowth-diffusion equation. Th e results illustrate the sensitivity of He ages to various geologic hi stories and are useful for understanding He age-elevation relationship s and for testing time-temperature paths derived from apatite fission track length distributions. In addition, although He diffuses rapidly from apatite at shallow crustal temperatures, modeling of ambient temp erature fluctuations indicates that He ages are nearly unaffected by s urficial processes. (C) 1998 Elsevier Science B.V. All rights reserved .