09:00 〜 09:15
[SCG46-01] Low stress power-law creep of olivine
★Invited Papers
キーワード：オリビン多結晶体、べき乗クリープ
Strain localization plays a critical role in mantle shear zone. Some studies attributed it to grain-size-sensitive creep (i.e., diffusion creep) through grain-size reduction by dislocation creep (e.g., Warren & Hirth, 2006). Some friction experiments suggest a superplastic deformation mechanism accommodated by diffusion to play an important role even during seismic faulting (De Paola et al., 2015). Conventionally, it has widely assumed that Newtonian diffusion creep overwhelms dislocation creep at the condition of the lower stress and/or the smaller grain size. However, it is well known in material science community that power-law interface creep dominates at the lower stress and the smaller grain size than diffusion creep, which could potentially be important for the rheology of the Earth interior.
We conducted one-atmosphere uniaxial compression experiments on fine-grained (~1 μm) Fe-bearing olivine (Mg1.8Fe0.2SiO4) aggregates that were variably doped with CaO ± Al2O3. We identified power-law interface-controlled creep at low stresses and grain-boundary diffusion creep at high stresses, which operate as mutually coupled, i.e., sequential processes. We established constitutive equations for interface-controlled creep and diffusion creep of undoped olivine and used the combined rate equation as a reference to examine the effect of doping on creep rates. Ca and Al were found to enhance rates of both interface-controlled creep and diffusion creep above certain temperatures, and this effect becomes significant with increasing temperature. We attribute the rate enhancements to grain-boundary-disordering promoted by grain boundary segregation of the dopants at near-solidus conditions. The enhancements are well described in relation to the sample solidus temperature and an additional activation energy relative to that of the reference creep state. Based on our established flow laws of diffusion and interface-controlled creep, we constructed deformation mechanism map and found that grain-boundary diffusion creep dominates in most of the upper mantle, while interface-controlled creep can be effective during seismic faulting in the mantle at a shallow depth.
We conducted one-atmosphere uniaxial compression experiments on fine-grained (~1 μm) Fe-bearing olivine (Mg1.8Fe0.2SiO4) aggregates that were variably doped with CaO ± Al2O3. We identified power-law interface-controlled creep at low stresses and grain-boundary diffusion creep at high stresses, which operate as mutually coupled, i.e., sequential processes. We established constitutive equations for interface-controlled creep and diffusion creep of undoped olivine and used the combined rate equation as a reference to examine the effect of doping on creep rates. Ca and Al were found to enhance rates of both interface-controlled creep and diffusion creep above certain temperatures, and this effect becomes significant with increasing temperature. We attribute the rate enhancements to grain-boundary-disordering promoted by grain boundary segregation of the dopants at near-solidus conditions. The enhancements are well described in relation to the sample solidus temperature and an additional activation energy relative to that of the reference creep state. Based on our established flow laws of diffusion and interface-controlled creep, we constructed deformation mechanism map and found that grain-boundary diffusion creep dominates in most of the upper mantle, while interface-controlled creep can be effective during seismic faulting in the mantle at a shallow depth.