EXHUMATION OF APATITE HE PARTIAL RETENTION ZONES: AN EXAMPLE FROM THE TRANSANTARCTIC MOUNTAINS AND IMPLICATIONS FOR (U-TH)/HE DATING OF APATITES

As with the concept of the exhumed partial annealing zone (PAZ) in fission track analysis, an understanding of the He partial retention zone (PRZ) is fundamental for interpretation of (U-Th)/He ages. Samples within an exhumed PAZ have a greater spread of single grain ages compared to rapidly cooled samples resulting from magnification of slight differences in annealing characteristics between grains of variable chemistry. Until recently all (U-Th)/He ages were determined by furnace heating of multiple grain aliquots – in essence producing a pooled age. Laser heating, now routinely used by some labs to produce single grain ages, can provide critical information about the characteristics of exhumed (U-Th)/He PRZs.

(U-Th)/He dating of apatite, in conjunction with AFT analysis, has been applied to the Kukri Hills, Transantarctic Mountains. Vertical sampling profiles across the range indicate very slow Cretaceous exhumation was followed by an acceleration in exhumation rate in the Eocene. Replicate (U-Th)/He multiple grain ages often were not reproducible within error. Similarly, (U-Th)/He ages determined on individual grains using a laser yielded a wide spread in age. To test whether lack of reproducibility arises from undetected monazite micro-inclusions, grains from some samples were dissolved in HCl rather than HNO3 before being analyzed for U and Th by ICP-MS. No systematic difference was detected. Rather than an analytical problem, our observations suggest the single grain age scatter arises from long residence in a PRZ, where small differences in He retention characteristics can yield large variations in He age. This is supported by the fact that scatter is most pronounced for samples undergoing very slow exhumation, whereas samples that cooled more rapidly showed less scatter. The most likely sources of variation in He retention are grain size and chemistry, however we found only weak correlations between age and minimum crystal dimension and between He apparent age and apatite cation chemistry (e.g., REE content). Further work is necessary to identify the cause of age variations in these samples. However, prolonged residence time in the PRZ resulting in large variations in apparent age in samples such as these provide a potential new way to identify the controls on He diffusion from apatite.