Takehiko Hiraga

Influence of melt on the creep behavior of olivine–basalt aggregates under hydrous conditions

The influence of melt on the creep behavior of olivine–basalt aggregates under hydrous conditions has been investigated by performing a series of high-temperature triaxial compression experiments. Samples with melt fractions of 0.02≤φ≤0.12 were deformed under water-saturated conditions at temperatures between 1373 and 1473 K and a confining pressure of 300 MPa in a gas-medium apparatus. At constant differential stress and temperature, the rate of deformation increased rapidly but systematically with increasing melt fraction. In the diffusion creep regime, at a given differential stress, samples with melt fractions of 0.02 and 0.12 deformed a factor of ∼2 and ∼20, respectively, faster than a melt-free sample. In the dislocation creep regime, a sample with a melt fraction of 0.12 deformed a factor of ∼40 faster than a melt-free sample. For partially molten olivine–basalt aggregates deformed under hydrous conditions, the dependence of creep rate on melt fraction can be expressed in the form ϵ(φ)=ϵ(0) exp(αφ), where α≈26 for diffusion creep and α≈31 for dislocation creep. The results of this study, combined with reasonable estimates for the spatial variation in the concentrations of water and melt (as well as for the geotherm and the activation volume for creep), provide constraints on the viscosity structure of Earth’s upper mantle. As an example, we present a viscosity profile for the mantle wedge above a subducting plate, demonstrating that the viscosity in that region can vary by ∼3 orders of magnitude over a depth of ∼60 km due to the combined effects of water and melt weakening.

Grain boundaries as reservoirs of incompatible elements in the Earth's mantle.

The concentrations and locations of elements that strongly partition into the fluid phase in rocks provide essential constraints on geochemical and geodynamical processes in Earths interior. A fundamental question remains, however, as to where these incompatible elements reside before formation of the fluid phase. Here we show that partitioning of calcium between the grain interiors and grain boundaries of olivine in natural and synthetic olivine-rich aggregates follows a thermodynamic model for equilibrium grain-boundary segregation. The model predicts that grain boundaries can be the primary storage sites for elements with large ionic radius—that is, incompatible elements in the Earths mantle. This observation provides a mechanism for the selective extraction of these elements and gives a framework for interpreting geochemical signatures in mantle rocks.

Olivine crystals align during diffusion creep of Earth/'s upper mantle

The crystallographic preferred orientation (CPO) of olivine produced during dislocation creep is considered to be the primary cause of elastic anisotropy in Earth’s upper mantle and is often used to determine the direction of mantle flow. A fundamental question remains, however, as to whether the alignment of olivine crystals is uniquely produced by dislocation creep. Here we report the development of CPO in iron-free olivine (that is, forsterite) during diffusion creep; the intensity and pattern of CPO depend on temperature and the presence of melt, which control the appearance of crystallographic planes on grain boundaries. Grain boundary sliding on these crystallography-controlled boundaries accommodated by diffusion contributes to grain rotation, resulting in a CPO. We show that strong radial anisotropy is anticipated at temperatures corresponding to depths where melting initiates to depths where strongly anisotropic and low seismic velocities are detected. Conversely, weak anisotropy is anticipated at temperatures corresponding to depths where almost isotropic mantle is found. We propose diffusion creep to be the primary means of mantle flow.

Water weakening of clinopyroxene in the dislocation creep regime

We performed a series of triaxial compressive creep experiments at two different water fugacities to investigate the effect of water on the creep strength of a natural clinopyroxenite. Samples were deformed under water-saturated conditions at temperatures between 1373 and 1473 K, confining pressures of 150 and 300 MPa, and differential stresses from 34 to 261 MPa. Strain rates were in the range 10 -7 to 10 -5 s -1 . Water fugacity was controlled at either 140 or 280 MPa. The creep results yield a stress exponent of 2.7 ± 0.3 and an activation energy of 670 ± 40 kJ/mol. Compared to dry clinopyroxene, wet samples creep over 100 times faster at a given temperature, confining pressure, water fugacity, and differential stress. The creep rate of clinopyroxene is proportional to the water fugacity to the 3.0 ± 0.6 power, with an activation volume of 0 m 3 /mol. One possible water-weakening mechanism is an enhancement of the rate of dislocation climb associated with increases in the concentration of jogs and the diffusivity of silicon ions. Compared to other major minerals in Earths lower crust, specifically olivine and plagioclase, the water-weakening effect is most significant for clinopyroxene. Under hydrous conditions the strengths of clinopyroxene and anorthite are comparable over the investigated stress range, and both phases are weaker than olivine. Since the mineral assemblages in Earths lower continental crust are often dominated by plagioclase and pyroxene, in places where a wet flow law applies, the mechanical behavior of clinopyroxene will have a substantial effect on creep strength.

Experimental study of attenuation and dispersion over a broad frequency range: 2. The universal scaling of polycrystalline materials

attenuation QE1 were measured accurately over a broad frequency range (10 −4 ≤ f (Hz) ≤ 2.15) and at low strain amplitude (10 −5 –10 −6 ). Creep experiments were performed with the same apparatus to measure the steady state viscosity. Anelasticity and viscosity were measured at high homologous temperatures (T =2 2 –48°C; T/Tm = 0.61–0.67) and various grain sizes (3–22 mm), the growth of which was controlled by annealing. Using the measured viscosities h and the unrelaxed modulus EU determined from ultrasonic experiments, the frequency of the entire data set was normalized by the Maxwell frequency fM = EU/h, resulting in E and Q −1 master curves. The Q −1 data from previous studies on olivine‐dominated samples also collapse onto the same curve when scaled by fM,, demonstrating the universality of anelasticity for polycrystalline materials. The similitude by the Maxwell frequency scaling indicates that the dominant mechanism for the anelasticity observed in this study and in previous studies is diffusionally accommodated grain boundary sliding. A generalized formulation for this similitude is provided to extrapolate the experimental data to velocity and attenuation of seismic shear waves.

Chemistry of grain boundaries in mantle rocks

Abstract The compositions of olivine grain boundaries have been analyzed with scanning transmission electron microscopy (STEM) via energy dispersive X-ray (EDX) spectrum profiling in three specimens: a peridotite ultramylonite, olivine phenocrysts in a basaltic rock, and synthesized compacts of olivine + diopside. Composition profiles across grain boundaries in both natural and synthetic samples exhibit a characteristic width of 5 nm and a depletion of Mg and concomitant enrichments of Ca, Al, Ti, and Cr. Chemical segregation is known to affect grain boundary processes such as grain boundary diffusion, sliding, fracture, and migration, all of which influence the rheological properties of polycrystalline aggregates. Also, because grain boundaries are enriched in trace elements, the boundaries can be important storage sites for such elements in mantle rocks. Mantlederived melts with unusual compositions, such as those rich in Ca and/or Ti, might be explained by preferential melting of olivine grain boundaries enriched in these elements. The common chemical signatures at grain boundaries in all samples indicate that chemical segregation is an energetically favorable phenomenon and thus should occur elsewhere in Earth’s mantle. Segregation of trace elements to grain boundaries may play an important role in dynamical and geochemical processes in Earth’s mantle.

Mantle superplasticity and its self-made demise

The unusual capability of solid crystalline materials to deform plastically, known as superplasticity, has been found in metals and even in ceramics. Such superplastic behaviour has been speculated for decades to take place in geological materials, ranging from surface ice sheets to the Earth’s lower mantle. In materials science, superplasticity is confirmed when the material deforms with large tensile strain without failure; however, no experimental studies have yet shown this characteristic in geomaterials. Here we show that polycrystalline forsterite + periclase (9:1) and forsterite + enstatite + diopside (7:2.5:0.5), which are good analogues for Earth’s mantle, undergo homogeneous elongation of up to 500 per cent under subsolidus conditions. Such superplastic deformation is accompanied by strain hardening, which is well explained by the grain size sensitivity of superplasticity and grain growth under grain switching conditions (that is, grain boundary sliding); grain boundary sliding is the main deformation mechanism for superplasticity. We apply the observed strain–grain size–viscosity relationship to portions of the mantle where superplasticity has been presumed to take place, such as localized shear zones in the upper mantle and within subducting slabs penetrating into the transition zone and lower mantle after a phase transformation. Calculations show that superplastic flow in the mantle is inevitably accompanied by significant grain growth that can bring fine grained (≤1 μm) rocks to coarse-grained (1–10 mm) aggregates, resulting in increasing mantle viscosity and finally termination of superplastic flow.

Effect of H+ on Fe–Mg interdiffusion in olivine, (Fe,Mg)2SiO4

To quantify the effect of hydrogen on the kinetics of Fe–Mg interdiffusion in olivine, diffusion couples composed of crystals with Mg∕(Mg+Fe) ratios of 0.91 and 0.83 were annealed under water-saturated conditions at T=1373K and P=300MPa. With fO2 buffered at the Ni–NiO phase boundary, fH2O≈300MPa. The resulting interdiffusivity, DFe–Mg≈1×10−16m2∕s, is approximately one order of magnitude larger than that measured under anhydrous conditions. The enhancement in diffusivity in the presence of water results from an increase in the concentration of cation vacancies associated with the introduction of high concentrations of protons as point defects into the olivine structure.

Comparison of microstructures in superplastically deformed synthetic materials and natural mylonites: Mineral aggregation via grain boundary sliding

We conducted compressional, tensile, and torsional creep experiments on fine-grained forsterite plus Ca-bearing pyroxene aggregates. A distinct microstructure with aggregation of the same phase in the direction of compression was formed in our samples after all the experiments. The stress–strain rate relationship, grain-size dependent flow strength, and the achievement of large tensile strain all indicate that samples underwent creep due to grain boundary sliding (GBS). As a result of GBS, grain-switching events allow dispersed phases to contact grains of the same phase and orient in the direction of compression. We identify similar aggregated microstructures in previously reported micrographs of polymineralic granite-origin ultramylonites. Mineral phase mixing through GBS, which helps to retain fine grain size in rocks due to grain boundary pinning, has been speculated to occur during formation of mylonites. However, our results contradict this hypothesis because mineral aggregation through GBS promotes demixing rather than mixing of the mineral phases. GBS processes alone will not promote a transformation of well-developed monomineralic bands to polymineralic bands during mylonitization.

Plastic deformation of quartz at room temperature: A Vickers Nano‐Indentation Test

Vickers indentation tests of natural quartz were performed with a load of 98 mN at room temperature and one atmosphere. Atomic force microscopy revealed no evidence of fracturing during indentation. Transmission electron microscope observations indicate that no dislocations were generated during the indentation tests. However, high resolution electron microscopy revealed that sharp creases of crystal lattices had developed. These observations lead to the conclusion that quartz deformed plastically even at room temperature. The plastic strain was accommodated by the mechanism of lattice creasing, which is described here for the first time.