AL AND SI DIFFUSION IN RUTILE

Minor and trace elements incorporated into rutile can provide information about ages, timing of geologic events, oxygen fugacity, past temperatures, pressures and chemical environments. We have measured Al and Si diffusion in rutile, exploring the effects of oxygen fugacity and crystallographic orientation on diffusion. Experiments were conducted on both synthetic and natural rutile using sources consisting of TiO₂-Al₂O₃ powder mixtures for Al diffusion, and TiO₂-SiO₂ powder mixtures or quartz-rutile diffusion couples for Si diffusion. Experiments were run in air in 1-atm furnaces, or under buffered conditions using solid buffers with samples contained in evacuated silica glass ampoules. Si and Al diffusion profiles were measured with Rutherford Backscattering Spectrometry (RBS), with complementary measurements of Al profiles using Nuclear Reaction Analysis (NRA). The following Arrhenius relation is obtained for Al diffusion normal to (001), under NNO-buffered conditions:

D_Al = 1.2x10^-2 exp(-531 ± 27 kJ mol^-1/RT) m²/s

The Arrhenius relation for Si diffusion is:

D_Si = 8.5x10^-13 exp(-254 ± 31 kJ mol^-1/RT) m²/s

For both Al and Si, there is little evidence of diffusional anisotropy.

Al and Si are among the slowest-diffusing species measured thus far in rutile. Al concentrations measured in rutile have recently been shown to have potential as a thermobarometer (Hoff et al., 2018). These data indicate that Al concentrations in rutile will be highly resistant to diffusional alteration subsequent to crystallization. For example, rutile grains of 50 μm radius would retain Al signatures in grain centers over times greater than 1 billion years at 850°C.

C. Hoff, E.B. Watson, F.S. Spear (2018) V.M. Goldschmidt Conference