He diffusion systematics in minerals: Evidence from synthetic monazite and zircon structure phosphates

Abstract

Helium diffusivity was measured in synthetic rare-earth-element orthophosphates with systematically varying properties to evaluate potential controls on He transport in minerals. In the zircon structure phosphates (in this study, the phosphates of Tb, Dy, Ho, Er, Tm, Yb, and Lu as well as synthetic xenotime, YPO 4), He diffusion is strongly anisotropic. Transport apparently proceeds preferentially through channels aligned with the c-axis. The activation energy for diffusion is almost the same (122 ± 6 kJ/mol) in all members of this family, but there is a monotonic decrease in D o with atomic number from TbPO 4 (∼10 5 cm 2/s) to LuPO 4 (∼10 cm 2/s). The c-parallel channels become increasingly constricted in the same sequence, likely accounting for the systematically decreasing diffusivity. The He closure temperature ( r = 1 cm, d T/d t = 10 °C/Myr) increases with atomic number from 44 °C for TbPO 4 to 88 °C for LuPO 4. Diffusion of radiogenic helium from natural zircon and xenotime is much slower than these synthetic analogs predict, suggesting that coupled substitution of REE and P for Zr and Si and/or radiation damage profoundly modify the energetics of interstitial He diffusion. In particular, α-recoil may play a key role by damaging the continuity and integrity of the channels. Monazite structure phosphates (here La, Ce, Pr, Nd, Sm, and Gd phosphate) are far more He retentive than those of the zircon structure. Activation energies increase smoothly with atomic number from LaPO 4 (183 kJ/mol) to NdPO 4 (224 kJ/mol) then decrease again to GdPO 4 (198 kJ/mol). D o values mimic this pattern, spanning a range from ∼10 −1 cm 2/s (GdPO 4) to 10 4 cm 2/s (NdPO 4). Nevertheless, He closure temperatures increase monotonically with atomic number, from 300 °C in LaPO 4 to 410 °C in GdPO 4. No evidence was obtained bearing on diffusion anisotropy, but the monazite structure lacks through-going channels so it is not expected. Diffusion parameters for radiogenic helium in natural monazite are similar to those obtained on the synthetic analogs. Ionic porosity is not the primary control on He diffusion in the orthophosphates. Within a given structure and with limited elemental substitution, ionic porosity and He closure temperature are negatively correlated, as predicted. However, differences between crystal structures are far more important than ion packing density: at comparable ionic porosity the monazite structure phosphates have He closure temperatures ∼300 °C higher than the xenotime structure phosphates. Modifications to the structures by radiation damage likely play a similarly significant role in controlling He diffusion.