HELIUM DIFFUSION IN NATURAL MONAZITE AND SYNTHETIC REE AND Y PHOSPHATES

As a continuation of studies of helium diffusion in accessory minerals, which have included measurements of He diffusion in apatite and zircon (Cherniak et al., 2009) and rutile and titanite, we are investigating diffusion of helium in natural monazite and synthetic REE and Y phosphates (grown by a flux technique). Samples were implanted with 100 keV ³He at a dose of 5x10¹⁵ /cm², and the implanted materials annealed in 1-atm furnaces at temperatures from 252-600°C. ³He distributions were measured with Nuclear Reaction Analysis using the reaction ³He(d,p)⁴He.

For diffusion normal to (100) we obtain the following Arrhenius relation for He diffusion in natural monazite:

D = 1.6x10^-7 exp(-150 ± 11 kJ mol^-1/RT) m²sec^-1.

Diffusivities of He normal to (001) and (010) appear similar, indicating little anisotropy for helium diffusion in natural monazite. Measurements on monazites from two different localities yield comparable results. Over the investigated temperature range, diffusivities are similar to those measured by Boyce et al. (2005), and fall between the values measured by Farley (2007) for diffusion in synthetic monazite-structure REE phosphates.

Our initial results for He diffusion in synthetic REE phosphates of monazite structure suggest systematic behavior, with He diffusivities in the lighter REE phosphates faster than diffusion in heavier REE phosphates. Activation energies for He diffusion are similar to those for natural monazite The trend of faster diffusivities for lighter REE phosphates of the monazite structure is broadly consistent with the observation of Farley (2007), but differences among diffusivities appear less pronounced.

He diffusion in natural monazite is similar to He diffusion in zircon normal to c, and about 4 orders of magnitude slower than He diffusion in apatite (Cherniak et al., 2009). Monazite lacks the pronounced anisotropy measured for He diffusion in zircon and rutile, for which diffusivities parallel to c are nearly 2 orders of magnitude faster than diffusion normal to c.

Boyce, J.W., Hodges, K.V., Olszewski, W.J., Jercinovic, M.J.(2005) G3, 6, Q12004; Cherniak, D.J., Watson, E.B., Thomas, J.B. (2009) Chem. Geol.; Farley, K.A. (2007) GCA 71, 4015-4024, Shuster, D.L., Farley, K.A., Sisterson, J.M., Burnett, D.S.(2003) EPSL 217, 19-32.