The effect of water on Si and O diffusion rates in olivine and implications for transport properties and processes in the upper mantle

TitleThe effect of water on Si and O diffusion rates in olivine and implications for transport properties and processes in the upper mantle
Publication TypeJournal Article
Year of Publication2008
AuthorsCosta F, Chakraborty S
JournalPhysics of the Earth and Planetary Interiors
Volume166
Pagination11-29
Date PublishedJan
ISBN Number0031-9201
Accession NumberWOS:000253617500002
Abstract

We performed piston cylinder experiments (1200-1350 degrees C, 2 GPa) to determine the diffusion rates of Si and O in mantle olivine under water undersaturated (brucite absent, 45 ppm H(2)O in olivine) as well as close to water-saturated (brucite present, similar to 370 ppm H(2)O in olivine) conditions. Diffusion couples consisted of oriented and polished San Carlos olivine cylinders coated with thin (similar to few 100 nm) films of the same composition enriched in (29)Si and (18)O, with a protective coating of ZrO(2) On top. Relationships between water solubility in olivine and water fugacity, combined with thermodynamic equilibrium calculations, indicate fH(2)O similar to 1 GPa,fO(2) similar to IW buffer for brucite absent and jH(2)O similar to 9 GPa,fO(2) similar to QFM buffer for brucite present experiments. We find that under hydrous conditions D(si) approximate to D(o) and diffusion anisotropy is weak to non-existent. Fitting the raw data at 2 GPa and fH(2)O similar to 0.93 GPa yields Arrhenius parameters [D(o) and E(p) in D = D(o) exp(-E(p)/RT)] of: 1.68 (+/- 3.52) x 10(-7) m(2) s(-1) and 358 28 kJ mol(-1) for Si, and 1.43 (+/- 1.80) x 10(-4) m(2) s(-1) and 437 +/- 17 kJ mol(-1) for O, respectively (1 sigma errors). D (2GPa, fH(2)O = 0.97 GPa, 1200 degrees C): D (1 atm., dry, 1200 degrees C) is 1000 for Si and 10 for O, respectively. Equations incorporating explicitly the effect of water are discussed in the text. Analysis of our data suggests that O diffuses by an interstitial mechanism whereas Si diffuses via vacancy complexes. The relation between the water fugacity and the Si diffusion rates seems to obey a power law with a water fugacity exponent of 0.2-1. The amount of H incorporated into olivine at the experimental conditions is orders of magnitude higher than the likely concentration of Si vacancies. Therefore, a small fraction (similar to 0.01%) of the total incorporated H in olivine suffices to considerably enhance the concentration of Si vacancies, and hence diffusion rates. Activation energies for O diffusion under dry and wet conditions are similar, indicating that the mechanism of this diffusion does not change in the presence of water. This inference is consistent with results of computer simulations. Dislocation creep in olivine under wet conditions appears to be controlled by both, Si as well as O diffusion. Absolute creep rates can be calculated from the diffusion data if it is assumed that climb and glide of dislocations contribute equally to creep. Finally, analysis of the various transport properties indicate that <10 ppm of water in olivine is sufficient to cause a transition from "dry" to "wet" laws for most processes. As these water contents are even lower than the observed water contents in most mantle olivines (i.e. minimum values measured at the surface), we conclude that results of water present but undersaturated kinetic experiments are directly applicable to the mantle. Indeed, "wet" kinetic laws should be used for modeling geodynamic processes in the upper mantle, even if the mantle is thought to be undersaturated with respect to water. (C) 2007 Elsevier B.V. All rights reserved.