Obtaining thermal history information from old geological settings: how useful are radiation damaged apatites?

International audienceApatite fission track thermochronology has been widely used to constrain the timing and rate of crustal exhumation at passive continental margins and cratonic settings by providing a record of rock cooling through c. 110 – 60 °C. Resolving more recent and smaller scale (c. < 2 km) exhumation requires a thermochronometer with a lower temperature sensitivity. Apatite (U-Th-Sm)/He (AHe) thermochronology can potentially provide information that fills the gap in the thermal history of these settings. However, at such ancient and slowly cooled settings AHe datasets are commonly over-dispersed and their geological significance is difficult to interpret.A major source of AHe age dispersion is attributed to the effect of radiation damage within apatite, which acts to lower He diffusivity and increase the closure temperature of an apatite grain. It is appreciated that the role of radiation damage is particularly significant for apatite enriched in U and Th with slow or complex cooling histories over c. 100 million year timescales but while models of radiation damage accumulation and annealing have been able to explain many AHe datasets and extract useful thermal history information, many uncertainties in these models remain. New developments in our understanding of how He diffusion changes depending on damage shape, size, and connectivity and how composition affects defect annealing may be able to explain the most complex AHe datasets.Using data from African passive continental margins and cratonic settings in South Africa and Fennoscandia we evaluate the factors causing AHe age dispersion (i.e. radiation damage, grain size, fragmentation) and jointly invert the data alongside independent AFT data. By allowing the energy required for He atoms to escape damage defects (i.e. trapping energy) to vary we show that the AHe data can be well reproduced and important thermal history information can be obtained