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However, due to a number of factors, outlined in the following Sections, a (U-Th)/He age must be interpreted carefully before the true meaning of the measured age can be evaluated. m), a significant proportion of alpha particles produced with an apatite grain may be emitted from the grain, resulting in loss of radiogenic helium. (1996) showed how this effect can be corrected for, by calculation of a correction factor (known as F Calculations of Helium retention over geological timescales, based on laboratory diffusion measurements, suggest that Helium is progressively lost at temperatures between 40 and 90C (for timescales of tens of millions of years), with this temperature range constituting a Helium Partial Retention Zone or He PRZ. 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).
Fluorapatite (or fluoroapatite) is more resistant to acid attack than is hydroxyapatite; in the mid-20th century, it was discovered that communities whose water supply naturally contained fluorine had lower rates of dental caries.
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.
Modeling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer.
Helium diffusion and low-temperature thermochronometry of apatite.