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More recently, measurements of (U-Th)/He ages in samples from hydrocarbon exploration boreholes in the Otway Basin of S. Australia (House et al., 1999) have confirmed this general pattern of behaviour. Their results also suggest that, in general, helium diffusion systematics derived from laboratory measurements can be extrapolated to geological conditions with confidence, although the exact details details remain to be quantitatively assessed.
This work, focussed on the much-studied Durango apatite, has suggested that diffusion systematics are controlled by the physical grain size. Further studies of possible variation in diffusion rates between different apatite species are currently being acrried out at Caltech. An evaluation of low-temperature apatite U-Th/He thermochronometry. Again analogous to the case of fission track ages in apatite, the progressive reduction of (U-Th)/He ages with increasing temperature means that a measured (U-Th)/He age from a sample of detrital apatite from a sediiment cannot be interpreted as representing the timing of a specific cooling episode (with the exception of the situation where a sample cools very rapidly from above 90C to less than 40C). Instead, the measured age must be interpreted in terms of the interplay between production of Helium by alpha decay and loss due to thermally controlled diffusion (as described below). Cenozoic thermal evolution of the central Sierra Nevada, California, from (U-Th)/He thermochronometry.