11:45 AM - 12:00 PM
[SCG50-11] Grain-boundary diffusion creep of olivine and the rheology of the oceanic upper mantle
★Invited Papers
Keywords:olivine aggregates, Grain-boundary diffusion creep, The rheology of the oceanic upper mantle
The mantle is known to have anisotropic elasticity, which is primarily explained by crystallographic preferred orientation (CPO) of olivine that forms during mantle flow. Because the CPO has long been considered to result uniquely from dislocation processes, diffusion creep has received less attention than dislocation processes. Recently, olivine CPO has been found to develop during diffusion creep under a condition that seems to correspond to the depth of seismically anisotropic mantle (Miyazaki et al., 2013). This led us to examine mantle viscosity in terms of olivine diffusion creep.
Diffusion creep of olivine is, however, a subject of some debate. Differences of up to two orders of magnitude in viscosity at the same grain sizes and temperatures have been reported from experimental diffusion creep studies. Impurity content can range significantly among samples synthesized by different methods and is known to affect diffusional properties through grain-boundary segregation.
Motivated by our previous findings on the segregation of Ca and Al at olivine grain boundaries, we synthesized and reported experimental creep results for fine-grained Fe-bearing olivine (Mg1.8Fe0.2SiO4; so-called Fo90 olivine) aggregates that were variably doped with CaO±Al2O3 to explore the effect of Ca and Al on creep rate. By comparing creep properties of undoped and doped olivine aggregates, we identified the roles of Ca and Al in enhancing grain-boundary diffusion creep at >0.92Ts (Ts: solidus in K), an effect which became significant with increasing temperature. We considered the enhancement to result from grain-boundary disordering promoted by grain-boundary segregation. Based on the Arrhenius-like behavior of the disordering effect, we established an olivine diffusion creep law that suitably describes the creep rates in both temperature ranges of <0.92Ts and >0.92Ts.
We estimated solidus temperatures of the samples used in the previous diffusion creep experiments. These temperatures were used to compare previously reported diffusion creep rates for olivine with our established diffusion creep law. We found that the law explains a difference of up to two orders of magnitude in olivine creep rates at the same temperatures, stresses, grain sizes, and water contents in the previous studies. The geotherm normalized by the mantle solidus was calculated for the upper mantle with water contents up to 200 wt. ppm, which predicts depths where weak (low-viscosity) mantle is expected to occur due to enhanced grain-boundary diffusion creep.
Constructed viscosity–depth profiles based on our diffusion creep law reveal a very thin mantle lithosphere beneath mid-ocean ridges, with development of the lithosphere away from the ridge, leaving a low-viscosity region below. Given a grain size of 1 mm and depending on the water content, a viscosity of 2–5*1019 Pa s is predicted for the low-viscosity asthenospheric mantle beneath 50-million-year-old seafloor.
Diffusion creep of olivine is, however, a subject of some debate. Differences of up to two orders of magnitude in viscosity at the same grain sizes and temperatures have been reported from experimental diffusion creep studies. Impurity content can range significantly among samples synthesized by different methods and is known to affect diffusional properties through grain-boundary segregation.
Motivated by our previous findings on the segregation of Ca and Al at olivine grain boundaries, we synthesized and reported experimental creep results for fine-grained Fe-bearing olivine (Mg1.8Fe0.2SiO4; so-called Fo90 olivine) aggregates that were variably doped with CaO±Al2O3 to explore the effect of Ca and Al on creep rate. By comparing creep properties of undoped and doped olivine aggregates, we identified the roles of Ca and Al in enhancing grain-boundary diffusion creep at >0.92Ts (Ts: solidus in K), an effect which became significant with increasing temperature. We considered the enhancement to result from grain-boundary disordering promoted by grain-boundary segregation. Based on the Arrhenius-like behavior of the disordering effect, we established an olivine diffusion creep law that suitably describes the creep rates in both temperature ranges of <0.92Ts and >0.92Ts.
We estimated solidus temperatures of the samples used in the previous diffusion creep experiments. These temperatures were used to compare previously reported diffusion creep rates for olivine with our established diffusion creep law. We found that the law explains a difference of up to two orders of magnitude in olivine creep rates at the same temperatures, stresses, grain sizes, and water contents in the previous studies. The geotherm normalized by the mantle solidus was calculated for the upper mantle with water contents up to 200 wt. ppm, which predicts depths where weak (low-viscosity) mantle is expected to occur due to enhanced grain-boundary diffusion creep.
Constructed viscosity–depth profiles based on our diffusion creep law reveal a very thin mantle lithosphere beneath mid-ocean ridges, with development of the lithosphere away from the ridge, leaving a low-viscosity region below. Given a grain size of 1 mm and depending on the water content, a viscosity of 2–5*1019 Pa s is predicted for the low-viscosity asthenospheric mantle beneath 50-million-year-old seafloor.