Oxygen diffusion in zircon

Oxygen diffusion in natural, non-metamict zircon was characterized under both dry and water-present conditions at temperatures ranging from 765 degrees C to 1500 degrees C. Dry experiments were performed at atmospheric pressure by encapsulating polished zircon samples with a fine powder of O-18-enriched quartz and annealing the sealed capsules in air. Hydrothermal runs were conducted in cold-seal pressure vessels (7-70 MPa) or a piston cylinder apparatus (400-1000 MPa) on zircon samples encapsulated with both O-18-enriched quartz and O-18 water. Diffusive-uptake profiles of O-18 were measured in all samples with a particle accelerator, using the O-18(p,alpha)N-15 reaction. For dry experimental conditions at 1100-1500 degrees C, the resulting oxygen diffusivities (24 in all) are well described by: D-dry (m(2)/s) = 1.33 x 10 - 4exp(- 53920/T) There is no suggestion of diffusive anisotropy. Under wet conditions at 925 degrees C, oxygen diffusion shows little or no dependence upon P-H2O in the range 7-1000 MPa, and is insensitive to total pressure as well. The results of 27 wet experiments at 767-1160 degrees C and 7-1000 MPa can be described a single Arrhenius relationship: D-wet (m(2)/s) = 5.5 X 10(-12)exp(- 25280/T) The insensitivity of oxygen diffusion to P-H2O means that applications to geologic problems can be pursued knowing only whether the system of interest was 'wet' (i.e., P-H2O > 7 MPa) or 'dry'. Under dry conditions (presumably rare in the crust), zircons are extremely retentive of their oxygen isotopic signatures, to the extent that delta(18)O would be perturbed at the center of a 200 mu m zircon only during an extraordinarily hot and protracted event (e.g., 65 Ma at 900 degrees C). Under wet conditions, delta(18)O may or may not be retained in the central regions of individual crystals, cores or overgrowth rims, depending upon the specific thermal history of the system.