Apatite (U-Th-Sm)/He thermochronology on the wrong side of the tracks

Despite the increasing application of apatite (U-Th-Sm)/He thermochronology in a range of settings, the technique suffers from two major unresolved problems which preclude reliable thermal history interpretations on a routine basis. One problem is the common but unexplained over-dispersion of single grain ages compared to predictions based on accepted models of diffusion systematics (including enhanced He retentivity due to the buildup of radiation damage in the apatite crystal lattice). A second related problem is that the widely adopted RDAAM model does not provide an accurate representation of the degree to which helium retentivity is enhanced as a result of the accumulation of radiation damage. Until these problems are resolved and eliminated, thermal history interpretations derived from this method cannot be regarded as reliable. Routine application of the technique requires a more rigorous quantitative understanding of these factors. Further experimental studies are required to identify all significant sources of variation in measured ages and to develop more accurate models of the thermal response of the system, combined with rigorous calibration and validation against independent methods in well-constrained natural settings. Only once this is achieved will the technique be capable of producing robust and reliable interpretations.