Oxygen diffusion in monazite

Abstract

We report measurements of oxygen 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 1-atm furnaces. Hydrothermal runs were conducted in cold-seal pressure vessels, where monazite grains were encapsulated with 18O-enriched water. Following the diffusion anneals, oxygen concentration profiles were measured with Nuclear Reaction Analysis (NRA) using the reaction 18O(p,α)15N. Over the temperature range 850–1100 °C, the Arrhenius relation determined for dry diffusion experiments on monazite is given by:Ddry=1.9×10−6exp(−356±26kJmol−1/R⁢T)m2s−1There is no evidence of diffusional anisotropy.Under wet conditions at 100 MPa water pressure, over the temperature range 700–880 °C, oxygen diffusion can be described by the Arrhenius relationship:Dwet=3.1×10−17exp(−100±20kJmol−1/R⁢T)m2s−1Under hydrothermal conditions at 800 °C, oxygen diffusion shows little dependence upon PH2O over the range 10–160 MPa.Oxygen diffusion under hydrothermal conditions has a significantly lower activation energy for diffusion than under dry conditions, as has been found the case for many other minerals, both silicate and nonsilicate. Given these differences in activation energies, the differences between dry and wet diffusion rates increase with lower temperatures; for example, at 600 °C, dry diffusion will be more than 4 orders of magnitude slower than diffusion under hydrothermal conditions. These disparate diffusivities will result in pronounced differences in the degree of retentivity of oxygen isotope signatures. For instance, under dry conditions (presumably rare in the crust) and high lower-crustal temperatures (∼800 °C), monazite cores of 70-μm radii will preserve O isotope ratios for about 500,000 years; by comparison, they would be retained at this temperature under wet conditions for about 15,000 years.