Splitting of dislocations in olivine,cross-slip-controlled creep and mantle rheology

The splitting of the 100] dislocations of forsterite Mg₂SiO₄ is investigated in a hard-sphere model. Glissile splittings exist in (001) and the most energetically favorable is the one that does not involve cutting of SiO₄ tetrahedra: $\text{100] →}\text{1}\text{6}\text{,}\text{1}\text{9}\text{,}\text{1}\text{6}\text{+}\text{2}\text{3}\text{, 0, 0] +}\text{1}\text{6}\text{,}\text{1}\text{9}\text{,}\text{1}\text{6}$ 100] screw dislocations are shown to be able to split simultaneously on (001) and (010) according to the reaction: $\text{100] =}\text{1}\text{6}\text{,}\text{1}\text{36}\text{,}\text{1}\text{4}\text{] +}\text{1}\text{6}\text{,}\text{1}\text{9}\text{,}\text{1}\text{6}\text{] +}\text{1}\text{3}\text{, 0, θ] +}\text{1}\text{6}\text{,}\text{1}\text{36}\text{,}\text{1}\text{4}\text{] +}\text{1}\text{6}\text{,}\text{1}\text{6}\text{,}\text{1}\text{6}\text{]}$ This sessile configuration is analogous to the one found for screw dislocations of body-centered cubic metals. It accounts forthe long rectilinear sessile screw segments commonly observed, in experimentally and naturally deformed olivine. A new creep model for the upper mantle is proposed, where recovery is controlled by cross-slip of screw dislocations instead of climb of edge dislocations. The creep law, fitted on the experimental results of the literature is: $\text{?}\text{?}\text{= 1.2 · 10}{}^4\text{σ}{}^2\text{exp}\text{? (125000 ? 11.7σ + PV}{}^1\text{/RT]>}$ (σ in bars) the activation volume for cross-slip is estimated and viscosity-depth curves are plotted. The proposed creep mechanism is grain-size independent and less non-Newtonian than the climb-controlled one; it is found to be dominant over the latter at stresses smaller than 100 bar.