He diffusivity from phosphates

The closure temperature computation assumes that He is uniformly distributed within the grain. However, this assumption is violated when the a-ejection-influenced grain edge and diffusion domain boundary correspond, as they do in apatite (there are no data for monazite or xenotime). This correspondence causes the diffusive loss rate to be lower than if the He concentration profile were uniform. Numerical modeling suggests that the effect is relatively small, equivalent to about a 2 or 3°C increase in the closure temperature (Farley 2000). 

in other cases it is hard to reconcile the two techniques (House et al. 1997). The most likely explanation is that the thermal sensitivity of one or the other method (or both) is not yet fully and accurately understood. This is an area of active research in both the (U-Th)/He and fission track communities. It may also be fruitful to combine phosphate (U-Th)/He ages with feldspar Ar-Ar dating to constrain cooling from 250 to 35°C. Apparently no such studies have yet been published. 

There is now substantial experience in applying the apatite (U-Th)/He method. At Caltech alone over a thousand samples have been dated from many different parts of the world. A variety of problems have been identified  

Some rocks yield only fractured and/or anhedral crystals. This seems especially common in gabbros, where apatites are often seen to be space-filling rather than euhedral. Such grains are difficult to use both because they make the a-ejection correction very difficult and because it is not obvious how to determine the original grain size. As a result the diffusion characteristics, which scale with grain size, are not well known. 

While apatites with these characteristics are problematic, it is readily apparent from simple visual inspection of the apatite separate. Of greater concern are problems that 

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