A study was made of measurements of O diffusion in natural monazites under both dry, 1-atm conditions and hydrothermal conditions. For dry experiments, 18O-enriched CePO4 powder and monazite crystals were sealed in Ag–Pd capsules with a solid buffer (to buffer at NNO) and annealed in a 1atm furnace. Hydrothermal runs were conducted in cold-seal pressure vessels, where monazite grains were encapsulated with 18O-enriched water. Following the diffusion anneals, O concentration profiles were measured by nuclear reaction analysis, using the reaction: 18O(p,α)15N. At 850 to 1100C, the Arrhenius relationship determined for dry diffusion experiments on monazite was:

D (m2/s) = 1.9 x 10-6 exp[-356(kJ/mol)/RT]

There was no evidence of diffusional anisotropy. Under wet conditions at 100MPa water pressure, at 700 to 880C, the O diffusion could be described by:

D (m2/s) = 3.1 x 10-17 exp[-100(kJ/mol)/RT]

Under hydrothermal conditions at 800C, the O diffusion exhibited little dependence upon PH2O over the range of 10 to 160MPa. The O diffusion under hydrothermal conditions had a significantly lower activation energy for diffusion than under dry conditions, as was found the case for many other minerals, both silicate and non-silicate. Given these differences in activation energies, the differences between dry and wet diffusion rates increased at lower temperatures: at 600C, dry diffusion would be more than 4 orders of magnitude slower than diffusion under hydrothermal conditions. These disparate diffusivities then resulted in marked differences in the degree of retentivity of O isotope signatures.

Oxygen Diffusion in Monazite. D.J.Cherniak, X.Y.Zhang, M.Nakamura, E.B.Watson: Earth and Planetary Science Letters, 2004, 226[1-2], 161-74