In principle, therefore, this technique provides a useful supplement to the information provided by AFTA.
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.
By incorporating both AFTA and (U-Th)/He ages into the modelling, a more restricted range of thermal history solutions can be extracted. Helium diffusion from apatite: general behaviour as illustrated by Durango fluorapatite.
Several studies suggest that the composition of the apatite does not appear to affect the sensitivity of the He closure temperature (Wolf et al., 1996; House et al., 1999), in contrast to the effect of Cl contents on AFTA annealing kinetics. Journal of Geophysical Research, 105 (B2), 2903-2914.
Helium is produced within apatite grains as a result of alpha decay from uranium and thorium isotopes, present as impurities at ppm levels. (1994), this process formed the basis of the first attempts at geochronology (Rutherford, 1907a). Rutherford, 1907b) that at least a fraction of radiogenic Helium was lost from the host crystal lattice, and with the advent of apparently more reliable methods of geochronology (e.g.
K-Ar, Rb-Sr, U-Pb), interest in the Helium systematics of minerals waned.
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.
However, effects related to grain size may be significant in the interpretation of apatites from sediments which have been heated to paleotemperatures within the He PRZ, as grains of different radii will give different ages for a particular thermal history. The effects of long alpha-stopping distances on (U-Th)/He ages.