SILICON DIFFUSION IN SAN-CARLOS OLIVINE

The diffusion of 30Si in single crystals of San Carlos olivine has been measured. To achieve diffusion experiments the olivine samples were coated with a thin Mg2SiO4 layer and then heated between 1130°C and 1530°C under controlled oxygen partial pressures in the range 10-5 to 10-1 Pa. The annealing durations ranged from 1 h 30 min to 150 h, yielding diffusion lengths of 150-425 10-10 m. Diffusion profiles were analysed using the α-RBS technics (Rutherford Backscattering Spectroscopy of α-particles). The results show that the silicon diffusion coefficient obeys the law D = D0 ( pO2 p0)m exp(-E/RT) with E = 291 ± 15 kJ mole-1, m ∼ -0.19 ± 0.1 and ln(D0) = - 29.3 ± 1 where D is in m2 s-1, pO2 is the oxygen partial pressure, p0 the room pressure and R the gas constant. We deduce from these results that DSi ≪ DOx ≪ DMg, DFc in the T-pO2 domain investigated, but extrapolation towards low pO2 and temperatures suggests that it is not impossible for DSi to be larger than DOx owing to opposite pO2 dependences. The negative value of m indicates a probable interstitial mechanism for silicon diffusion. Results show that silicon diffusion in San Carlos olivine is enhanced by ∼ 30 relative to pure forsterite in the T-pO2 range studied. © 1990.