DETRITAL U-TH/HE THERMOCHRONOLOGY: THE ALPHA EJECTION NON-PROBLEM

Detrital U-Th/He thermochronology has been limited by difficulties in determining an appropriate alpha ejection correction (AEC). Three issues with the AEC create problem for detrital U-Th/He ages. a) The irregular shapes of detrital grains are difficult to characterize, b) abrasion in the sedimentary environment can remove part or all of the He depleted rim, and c) the AEC procedure depends upon the post-depositional thermal history, with three different thermal histories leading to three possible AEC procedures. These three scenarios are: 1) heating and complete resetting of the He system following deposition, 2) no resetting or He loss following deposition, and 3) partial resetting and He loss following deposition.

The pattern of He ages recorded by individual samples should dictate the choice of the 3 AEC procedures. Scenario (1) samples should record ages that are younger than the strata in which they are found. Uncorrected ages should correlate with grain size but AEC corrected ages should be the same for all grains. Scenario (2) samples should record a spectrum of ages that are older than the strata in which they are found. Scenario (3) samples are expected to record a complex spectrum of ages that may be older or younger than their host strata or possibly both with a strong correlation between grain size and both raw and AEC corrected ages.

Numerical AEC modelling indicates that the AEC is strongly controlled by the surface area/volume ratio even for irregular detrital grain shapes. Existing AEC models (e.g., Farley et al. 1996) can be applied to detrital grains as long as the surface area/volume is approximated from the real grain shape. Numerical abrasion modelling indicates that grain morphology should be a strong predictive tool in assessing the effect of grain abrasion on the AEC. Taken together, these results suggest an AEC procedure that should not introduce errors for detrital ages that are much larger than those for igneous grains.