Laser depth profiling studies of helium diffusion in Durango fluorapatite

Ultraviolet lasers coupled with sensitive mass spectrometers provide a useful way to measure laboratory-induced noble gas diffusion profiles in minerals, thus enabling the calculation of diffusion parameters. We illustrate this laser ablation depth profiling (LADP) technique for a previously well-studied mineral-isotopic system: (4)He in Durango fluorapatite. LADP studies were conducted on oriented, polished slabs from a single crystal that were heated under vacuum to a variety of temperatures between 300 and 450 degrees C for variable times. The resolved (4)He profiles exhibited error-function loss as predicted by previous bulk (4)He diffusion studies. All of the slabs, regardless of crystallographic orientation, yielded modeled diffusivities that are statistically co-linear on an Arrhenius diagram, suggesting no diffusional anisotropy of (4)He in this material. The data indicate an activation energy of 142.2 +/- 5.0 (2 sigma) kJ/mol and diffusivity at infinite temperature-reported as ln(D(0)) - of -4.71 +/- 0.94 (2 sigma) m(2)/s. These values imply a bulk closure temperature for (4)He in Durango fluorapatite of 74 degrees C for a 50 mu m radius grain, infinite cylinder geometry, and a cooling rate of 10 degrees C/Myr. (C) 2011 Elsevier Ltd. All rights reserved.