Polycrystal anelasticity at near‐solidus temperatures

Abstract

Elasticity, anelasticity, and viscosity of polycrystalline aggregates were measured at the near‐solidus temperatures ranging from below to above the solidus temperature (Tm). The result shows that the mechanical effects of the partial melting are twofold; changes just below the solidus temperature in the absence of melt and changes at the solidus temperature due to the onset of partial melting. As homologous temperature (T/Tm) increases from about 0.92 to 1, high‐frequency part of the attenuation spectrum significantly grows. Viscosity of the grain boundary diffusion creep is also reduced in this temperature range. These changes are caused by a solid‐state mechanism and have a large amplitude even for the samples which can generate very small amounts of melt at the solidus temperature. At the onset of melting, further increases in the elastic, anelastic, and viscous compliances occur. These changes are caused by the direct effects of the melt phase and are very small for the samples with very small melt fractions. Mechanical properties of a partially molten aggregate are determined by these twofold changes, and when melt fraction is small, the former changes are dominant. We performed a parameterization of the present experimental results and applied the obtained empirical formula to the seismic tomographic data in the upper mantle. The present model explains well the steep reduction of the seismic shear wave velocity in the oceanic lithosphere just below the solidus temperature.

Phase‐Field Modeling of Grain Boundary Premelting

“…In binary systems, even for a very small concentration of the secondary component (e.g., c σ =3×10 −4 for d =10 μm and 3×10 −6 for d =1 mm), the enhancement of grain boundary disorder occurs near the eutectic temperature. This explains well the experimental result by Yamauchi and Takei () that, in the borneol + diphenylamine binary eutectic system including only 0.3 mol% diphenylamine, grain boundary diffusion creep and shear stress relaxation at the grain boundaries are significantly enhanced just below the eutectic temperature (43 °C), which is much lower than the melting temperature of borneol (204.5 °C). The model result also explains well the experimental result that the amplitude of these subsolidus changes are insensitive to the diphenylamine concentration from 0.3% to 1.7% (Figures 16 and 17 in Yamauchi & Takei, ).…”

“…This explains well the experimental result by Yamauchi and Takei () that, in the borneol + diphenylamine binary eutectic system including only 0.3 mol% diphenylamine, grain boundary diffusion creep and shear stress relaxation at the grain boundaries are significantly enhanced just below the eutectic temperature (43 °C), which is much lower than the melting temperature of borneol (204.5 °C). The model result also explains well the experimental result that the amplitude of these subsolidus changes are insensitive to the diphenylamine concentration from 0.3% to 1.7% (Figures 16 and 17 in Yamauchi & Takei, ). The results shown in Figure also explain well the experimental results of Takei et al () that at a given temperature, anelastic relaxation by grain boundary sliding was always larger in the binary samples than in the pure samples (Figure 16 in Takei et al, ).…”

“…From the thermal and seismological models of the Pacific mantle, Priestley and McKenzie (, ) captured a steep reduction of the seismic shear wave velocity below the dry peridotite solidus. This velocity reduction can be explained well by the experimentally captured subsolidus effect, suggesting the applicability of the laboratory data to the mantle (Takei, ; Yamauchi & Takei, ). However, the underlying physics causing the subsolidus change remains unclear.…”

“…Steady‐state creep and anelastic relaxation, which showed the precursor indications of partial melting, are both related to grain boundary processes (Takei et al, ; Yamauchi & Takei, ). The creep mechanism is grain boundary diffusion creep, and the mechanism of anelastic relaxation is grain boundary sliding (Raj & Ashby, ), both of which are strongly influenced by the structure and matter diffusivity in the grain boundary.…”

Structure of the oceanic lithosphere and upper mantle north of the Gloria Fault in the eastern mid‐Atlantic by receiver function analysis

“…Most of the discussed models of the LAB [ Kawakatsu et al , ; Fischer et al , ; Olugboji et al , ] include thermal control, changes in rheology, dehydration, anisotropy, or partial melt. Experiments with polycrystalline materials [ Takei et al , ; Yamauchi and Takei , ] at subsolidus temperatures indicate that solid state mechanism such as diffusionally accommodated grain boundary sliding play an important role for S wave velocity decrease with rising temperature in the oceanic lithosphere [ Yamauchi and Takei , , Figure 20]. Besides the depth of the LAB, the sharpness of the discontinuity is of interest.…”