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

Abstract

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 h to several weeks. The hydrothermal experiments were conducted by encapsulating polished titanite crystals with 18O enriched water and running at 700–900 °C and 10–160 MPa 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,α) 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 deep into the sample attributable to diffusion in a “fast path” such as planar defects or pipes. For the experiments on synthetic crystals, lattice diffusion only is apparent in crystals with euhedral morphology, while both mechanisms operate in crystals lacking euhedral morphology. For the dry experiments, the following Arrhenius relation was obtained: D dry lattice = 3.03 × 10 − 8 exp (− 276 ± 16 kJmol − 1 /RT) m 2/s Under wet conditions at P H 2O = 100 MPa, Oxygen diffusion conforms to the following Arrehenius relation: D wet lattice = 2.05 × 10 − 12 exp (− 180 ± 39 kJmol − 1 /RT) m 2/s Diffusive anisotropy was explored only at hydrothermal conditions, with little evidence of diffusive anisotropy observed. Oxygen diffusivity shows no dependence on water pressure 800 °C ( P H 2O = 10–160 MPa) or 880 °C ( P H 2O = 10–100 MPa). However, like many other silicates, titanite shows a lower activation energy for oxygen diffusion in the presence of H 2O than under dry conditions; therefore the difference between the “dry” and “wet” diffusivities increases as temperature decreases below the range of this study. For example, at 500 °C, dry diffusion is almost 2.5 orders of magnitude slower than wet diffusion. Accordingly, 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.