OXYGEN DIFFUSION IN ZIRCON

Oxygen diffusion in natural, non-metamict zircon was characterized und er both dry and water-present conditions at temperatures ranging from 765 degrees C to 1500 degrees C. Dry experiments were performed at atm ospheric pressure by encapsulating polished zircon samples with a fine powder of O-18-enriched quartz and annealing the sealed capsules in a ir. Hydrothermal runs were conducted in cold-seal pressure vessels (7- 70 MPa) or a piston cylinder apparatus (400-1000 MPa) on zircon sample s encapsulated with both O-18-enriched quartz and O-18 water. Diffusiv e-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 experiment al conditions at 1100-1500 degrees C, the resulting oxygen diffusiviti es (24 in all) are well described by: D-dry (m(2)/s) = 1.33 x 10 - 4ex p(- 53920/T) There is no suggestion of diffusive anisotropy. Under wet conditions at 925 degrees C, oxygen diffusion shows little or no depe ndence 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 degre es 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 oxyg en 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 t he crust), zircons are extremely retentive of their oxygen isotopic si gnatures, to the extent that delta(18)O would be perturbed at the cent er of a 200 mu m zircon only during an extraordinarily hot and protrac ted 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 c rystals, cores or overgrowth rims, depending upon the specific thermal history of the system.