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: ⁴He 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°C for variable times. The resolved ⁴He profiles exhibited error-function loss as predicted by previous bulk ⁴He 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 ⁴He in this material. The data indicate an activation energy of 142.2±5.0 (2σ)kJ/mol and diffusivity at infinite temperature - reported as ln(D₀) - of -4.71±0.94 (2σ)m²/s. These values imply a bulk closure temperature for ⁴He in Durango fluorapatite of 74°C for a 50μm radius grain, infinite cylinder geometry, and a cooling rate of 10°C/Myr.