Oxygen and silicon self-diffusion in natural olivine at T = 1300° C

Abstract

Oxygen and silicon self-diffusion in natural olivine single crystals (Mg 0.89 Fe 0.11) 2 SiO 4 from San Carlos, Arizona, have been measured for the first time at T = 1300°C in controlled oxygen partial pressure pO 2 ranging from 10 −4 to 10 Pa. Polished and chemically-etched specimens were heated for several hours in a gas mixture H 2/CO 2 or H 2/H 2 18O. The latter gas mixture provides both the controlled pO 2 and the isotopic reservoir for the 18O diffusion across the gas-solid interface. For 30Si diffusion, a thin layer of isotopic forsterite Mg 2 30SiO 4 (few hundreds of angstroms) was deposited onto the olivine surface. Diffusion profiles for oxygen and silicon were determined using the nuclear reaction 18O(p, α) 15N and Rutherford back-scattering of 2 MeV α particles, respectively. The diffusion coefficients are D ox = 10 −18 ± 0.6m 2s −1 for oxygen and D Si = 10 −20m 2s −1 for silicon. We conclude from these measurements on San Carlos olivine at T = 1300°C that: (1) Si is the slowest diffusing species in natural olivine, as it is in forsterite, and (2) 30Si and 18O diffuse 30 to 200 times faster in olivine than in forsterite.