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A relatively rare form of apatite in which most of the OH groups are absent and containing many carbonate and acid phosphate substitutions is a large component of bone material.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. Modeling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer. Helium diffusion and low-temperature thermochronometry of apatite. 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.
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. 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. 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. 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.
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.