The O diffusion in natural and synthetic single-crystal titanite was characterized under both dry and water-present conditions. 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 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. 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]

X.Y.Zhang, D.J.Cherniak, E.B.Watson: Chemical Geology, 2006, 235[1-2], 105-23