The O diffusion in natural and synthetic single-crystal titanite was characterized under both dry and water-present conditions. In the dry experiments, pre-polished titanite samples were packed in 18O-enriched quartz powder inside Ag–Pd capsules; together with a fayalite–magnetite–quartz buffer, physically separated by Ag–Pd strips. The sealed Ag–Pd capsules were themselves sealed inside evacuated silica glass tubes and held at 700 to 1050C and atmospheric pressure for between 1h and several weeks. Hydrothermal experiments were performed by encapsulating polished titanite crystals with 18O-enriched water and holding at 700 to 900C and 10 to 160MPa in cold-sealed pressure vessels for between 24h and several weeks. The diffusive uptake profiles of 18O were measured by nuclear reaction analysis, using the 18O(p,α)15N reaction. In experiments on natural crystals under dry or hydrothermal conditions, 2 mechanisms were recognized as being responsible for O diffusion. The diffusion profiles exhibited 2 segments. One was steep, and close to the initial surface. It was attributed to self-diffusion in the titanite lattice. The other was a tail that stretched deeply into the sample. It was attributed to diffusion along so-called fast paths such as planar defects or pipes. In experiments on synthetic crystals, lattice diffusion alone was apparent in crystals having euhedral morphology. Both mechanisms operated in crystals lacking euhedral morphology. In the case of dry experiments, the Arrhenius relationship was:

D (m2/s) = 3.03 x 10-8 exp[-276(kJ/mol)/RT]

Under wet conditions, with PH2O = 100MPa, the O diffusion obeyed:

D (m2/s) = 2.05 x 10-12 exp[-180(kJ/mol)/RT]

Diffusive anisotropy was explored only under hydrothermal conditions, and little evidence for such anisotropy was found. The O diffusivity exhibited no dependence upon water pressure at 800C (PH2O = 10 to 160MPa) or 880C (PH2O = 10 to 100MPa). The titanite exhibited a lower activation energy for O diffusion in the presence of H2O than under dry conditions. Therefore, the difference between the dry and wet diffusivities increased as the temperature decreased below the range used here. Thus, at 500C, dry diffusion was almost 2.5 orders of magnitude slower than wet diffusion.

Oxygen Diffusion in Titanite - Lattice Diffusion and Fast-Path Diffusion in Single Crystals. X.Y.Zhang, D.J.Cherniak, E.B.Watson: Chemical Geology, 2006, 235[1-2], 105-23