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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. Dating schemes based on rates of radioactivity have been refined and scrutinized for several decades.The latest high-tech equipment permits reliable results to be obtained even with microscopic samples.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. The three isotopes represented in the equation represent the only significant contributors of helium in natural samples. By measurement of the amounts of each isotope, the time t can be evaluated by solving this equation iteratively. As with the case of fission track ages, in the absence of other factors, this would provide a measure of the time over which helium has accumulated in the apatite lattice. An empirical test of helium diffusion in apatite: borehole data from the Otway Basin, Australia.
While this has yet to be demonstrated in natural samples, this holds considerable promise for obtaining more precise thermal history control in sedimentary basins. Ken Farley of Caltech, based on the systematics presented in Farley (2000) and references therein, allows modelling of the (U-Th)/He age expected from any inputted thermal history, in grains of any specified radius. More recently, however, the realisation that the partial loss of radiogenic products could provide quantitative information on the thermal history of mineral grains led to a resurgence of interest in this topic (e.g. In particular, efforts at Caltech through the 1990s led to the development of (U-Th)/He dating of apatite as a rigorous, quantitative technique (Wolf et al., 1996). Studies of the diffusion systematics of Helium in apatite (Wolf et al., 1998; Farley, 2000) also revealed the unique temperature sensitivity of the technique, with all Helium being lost over geological timescales at temperatures as low as 90C or less, and a closure temperature as low as 75C. By modelling ages through a variety of different thermal history scenarios, it is possible to define the range of histories giving predictions which are consistent with measured ages. The thermal history framework provided by AFTA forms a solid basis for this procedure.