Helium diffusion from apatite: General behavior as illustrated by Durango fluorapatite

High-precision stepped-heating experiments were:performed to better charact erize helium diffusion from apatite using Durango fluorapatite as a model s ystem. At temperatures below 265 degrees C, helium diffusion from this apat ite is a simple, thermally activated process that is independent of the cum ulative fraction of helium released and also of the heating schedule used. Across a factor of similar to 4 in grain size, helium diffusivity scales wi th the inverse square of grain radius, implying that the physical grain is the diffusion domain 1. Measurements on crystallographically: oriented thic k sections indicate that helium diffusivity in Durango apatite is nearly is otropic. The,best estimate of the activation energy for He diffusion from t his apatite is E-a = 33 +/- 0.5 kcal/mol, with log(D-0) = 1.5 +/- 0.6 cm(2) /s. The implied He closure temperature for a grain of 100 mu m radius is 68 degrees C assuming a 10 degrees C/Myr cooling rate; this figure varies by, +/-5 degrees C for grains ranging from 50 to 150 mu m radius. When this ap atite is heated to temperatures from 265 to 400 degrees C, a progressive an d irreversible change in He diffusion:behavior occurs: Both the activation: energy and frequency factor are reduced. This transition in behavior coinci des closely with progressive annealing of radiation damage in Durango apati te, suggesting that defects and defect annealing play a role in the diffusi vity of helium through apatite.