Strategies for joint inversion of apatitefission track and (U-Th) data:application of radiation damage andfragment models.

International audienceApatite fission track (AFT) and (U-Th-Sm)/He (AHe)thermochronology has the potential to constrain rock coolingpaths through a temperature range of c. 120 – 30°C. The annealingbehaviour of fission tracks in apatite has been well calibrated andit has been shown that the annealing rate of tracks decreases withtemperature through a temperature range of 110 – 60°C. Retentionof He within apatite, however, occurs over a wide range oftemperatures with the closure temperature of an individual grainbeing dependant on the grain size and, significantly, the amount ofradiation damage accumulated within the crystal.The influence of radiation damage is particularly important forapatite crystals that are enriched in U and Th and haveexperienced protracted cooling histories over 10 – 100 millionyear timescales. At present, current models of radiation damageaccumulation and annealing are not ideally parameterised fornatural apatites with high eU concentrations or complexcompositions. For this reason, single-grain AHe ages from higheU and/or slowly cooled samples are typically highly dispersedand poorly reproduced during thermal history inversionmodelling.Here, we present new AHe data from samples collected fromFennoscandia, which have resided at temperatures < 110ºC sincethe Precambrian, and from South Africa, which have anomalouslyhigh eU concentrations (e.g. up to 430 ppm). These data presentan opportunity to investigate the extreme cases of radiationdamage accumulation in natural samples. Using these data weemploy the latest radiation damage annealing model and vary keyparameters (e.g. trapping energy and Rmr0) to explore thestructural and compositional factors influencing helium diffusionand explain the observed data. By integrating these AHe data withindependent AFT data and accounting for the different phenomenacausing single-grain age dispersion (e.g. radiation damage, grainsize, fragmentation) we can obtain robust thermal historyinformation from complex thermochronology data and betterunderstand single-grain age dispersion in geologically old setting