Oxygen diffusion in San Carlos olivine

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Abstract
The self‐diffusion of oxygen in San Carlos olivine single crystals has been measured along the [001] direction. The diffusion experiments were performed using a gas mixture Ar + H2/Ar + H218O flowing around the olivine specimens. This gas mixture acts as the isotopic reservoir and is also used to establish and adjust the desired oxygen fugacity, allowing us to determine the oxygen diffusion coefficient D as a function of pO2. Thirty runs have been performed at 1 atm total pressure within the (T,pO2) olivine stability field, T ranging from 1363 to 1773 K and pO2 from 7×10−7 to 5×10−2 Pa. The annealing duration t was chosen such that the characteristic isotope penetration depth ranges between 600 and 11,000Å. The diffusion profiles of 18O were determined using the 18O(p, α)15N nuclear microanalysis technique. The diffusion coefficient can be written D* = D0* exp(−E/RT)(pO2/p0)m, where E is the activation energy, R is the gas constant, T is the temperature in K, pO2 is the oxygen partial pressure, p0 is the room pressure, m is a numerical coefficient, and D0* is the preexponential term. A data inversion method has been used to determine the three parameters and yields log(D0*) = −5.2(±3.6) with D0* in m2/s, E = 318(±17) kJ/mol, m = 0.34(±0.02); D* = 6.7 × 10−6exp(−38,000/T)(pO2/p0)0.34 m2/s; the number in parentheses is the standard deviation. D* is related to the self‐diffusion coefficient D by the relation D* = ƒD, with ƒ close to 1.0. An attempt to show pipe diffusion in predeformed samples, with a dislocation density of about 2×1011 m−2, along dislocation cores perpendicular to the specimen surface, showed no contribution to the main bulk diffusion. We conclude that in olivine, Fo≃90, within the temperature range investigated, DFe, DMg ≫ D0x ≫ DSi as has already been demonstrated in FO100. E is of the same order of magnitude as the Si ‐ O bond energy and as E in Fo100. We infer that the oxygen diffusion occurs via an interstitial mode and that these defects carry one negative charge. This interstitial mechanism is rather surprising in view of the compact oxygen sublattice of olivine. However, interstitial transport of oxygen has already been identified in other oxygen compact compounds.