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Detailed experimental measurements at Caltech have led to further refinements in understanding the diffusion systematics of Helium in apatite (Farley, 2000).

This work, focussed on the much-studied Durango apatite, has suggested that diffusion systematics are controlled by the physical grain size.

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.

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.

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 key observation implies that for any specified thermal history, modelled (U-Th)/He ages can be produced for a particular sample using the measured mean grain size together with single values of the key diffusion parameters E/s.

These values have been used in modelling (U-Th)/He ages for this report.

Because of the greater diffusive loss expected from smaller grains compared to larger grains, the helium closure temperatures in apatite will also vary with grain radius.

The overall variation in closure temperature for samples with grain radii of 50-150 microns is predicted to be only 5C (Farley, 2000).

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