Oxygen diffusion in titanite: Lattice diffusion and fast-path diffusion in single crystals

Oxygen diffusion in natural and synthetic single-crystal titanite was characterized under both dry and water-present conditions. For the dry experiments, pre-polished titanite samples were packed in 18O-enriched quartz powder inside Ag-Pd capsules, along with a fayalite-magnetite-quartz (FMQ) buffer assemblage maintained physically separate by Ag-Pd strips. The sealed Ag-Pd capsules were themselves sealed inside evacuated silica glass tubes and run at 700--1050°C and atmospheric pressure for durations ranging from 1 hour to several weeks. The hydrothermal experiments were conducted by encapsulating polished titanite crystals with 18O enriched water and running at 700--900°C and 10--160MPa in standard cold-seal pressure vessels for durations of 1 day to several weeks. Diffusive uptake profiles of 18O were measured in all cases by nuclear reaction analysis (NRA) using the 18O (p,alpha) 15N reaction.; For the experiments on natural crystals, under both dry and hydrothermal conditions, two mechanisms could be recognized as responsible for oxygen diffusion. The diffusion profiles showed two segments: a steep one close to the initial surface attributed to self-diffusion in the titanite lattice; and a "tail" reaching deeper into the sample attributable to diffusion in a "fast path" such as planar defects or pipes. For the experiments on synthetic crystals, there is only lattice diffusion in crystals with euhedral morphology while there are also two mechanisms responsible in crystals without euhedral morphology.; For the dry experiments, the following Arrhenius relation was obtained for lattice diffusion: Ddrylattice =3.03x10-8exp- 276+/-16kJmol-1/RTm 2/s; Under wet conditions at PH2O = 100MPa, Oxygen lattice diffusion conforms to the following Arrhenius relation: Dwetlattice=2.05x10 -12exp-180+/- 39kJmol-1/RTm2 /s; Oxygen diffusivity shows no dependence on water pressure at the following conditions investigated: temperature 800°C, PH2O = 10--160MPa, and 880°C, PH2O = 10--100MPa.; Diffusive anisotropy was explored only at hydrothermal conditions, with little evidence of diffusive anisotropy observed.; Like many other silicates, titanite shows lower activation energy for oxygen diffusion in the presence of H2O than under dry conditions; the retentivity of oxygen isotope signatures will be quite different between dry and wet systems at geologically interesting conditions. For most cases, wet diffusion results may be the appropriate choice for modeling natural systems.