The mechanisms of creep in olivine

Abstract

We summarize the progress made in providing experimental verification for the deformation map of polycrystalline olivine published by Stocker & Ashby in 1973 (Rev. Geophys. 11, 391). Porosity-free polycrystalline deformation data, applicable to the mantle, were found to be obtainable only from high-pressure deformation studies. Combination of the results of such studies with hardness measurements and single crystal deformation studies on olivine provides narrow constraints on the flow of olivine resulting from dislocation mechanisms from room temperature to the melting point along a band of experimentally accessible strain rates. A good fit is obtained combining a Dorn law above 2 kbar differential stress, $\dot{\epsilon}$/s$^{-1}$ = 5.7 $\times $ 10$^{11}$ exp $\left\{- \frac{128\,\text{kcal/mol}}{RT}\left(1-\frac{\sigma _{1}-\sigma _{3}}{85000}\right)^{2}\right\}$, with a power law below 2 kbar, $\dot{\epsilon}$ = 70($\sigma _{1}-\sigma _{3}$) exp {-122(kcal/mol)/RT}, where stress is measured in bars (1 bar = 10$^{5}$ Pa). Indirect data on a mechanism phenomenologically resembling the Coble creep regime are now available from two sources. The observed strain rates are only slightly faster than those predicted by Stocker & Ashby (1973). The 'wet' data, previously believed to show hydrolytic weakening, are found to fall within this Coble field. The asthenosphere is still expected to deform by the dislocation mechanism summarized by the two formulae given above, but higher stress deformation within the lithosphere is almost certainly dominated by this Coble creep regime once dynamic recrystallization sets in.