Tomohiro Ohuchi

Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth’s upper mantle

Grain size–sensitive creep controls the flow in the middle and deep upper mantle. Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth’s upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 1019.6 to 1020.7 Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size–sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.

In situ stress-strain measurements in a deformation-DIA apparatus at P-T conditions of the upper part of the mantle transition zone

Abstract We report on technical improvements in experiments with a deformation-DIA (D-DIA) apparatus, which enable the study of the rheology of solid materials at P-T conditions of the Earth’s mantle transition zone. Dimensions of the anvil truncation, pressure medium, and gasket were optimized to achieve deformation experiments above 13 GPa with a relatively low press load (<0.7 MN) to minimize the damage of the X‑ray transparent second-stage anvils. The adoption of low X‑ray absorbing material (e.g., cubic BN anvils, graphite window in a LaCrO3 heater) along the X‑ray path enabled quantitative determination of stress and strain of a sample by means of simultaneous in situ X‑ray radial diffraction and radiography using synchrotron radiation at SPring-8. Based on the new technique, a uniaxial deformation experiment with a strain rate of 3.88 × 10−5 s−1 and strains up to 25.5% was carried out on wadsleyite at a pressure of 14.5 GPa and a temperature of 1700 K.

Rheology of fine‐grained forsterite aggregate at deep upper mantle conditions

High-pressure and high-temperature deformation experiments on fine-grained synthetic dunite (forsterite aggregate) were conducted to determine the dominant deformation mechanism in the deep upper mantle. The sintered starting material has 90% forsterite, 10% enstatite, and an average grain size of ~1 µm. Deformation experiments were performed using a deformation-DIA apparatus at pressures of 3.03–5.36 GPa, temperatures of 1473–1573 K, and uniaxial strain rates of 0.91 × 10−5 to 18.6 × 10−5 s−1 at dry circumstances <50 H/106Si. The steady state flow stress was determined at each deformation condition. Derived stress-strain rate data is analyzed together with that reported from similar but low-pressure deformation experiments using flow law equations for diffusion creep (stress exponent of n = 1, grain-size exponent of p = 2) and for dislocation-accommodated grain-boundary sliding (GBS-disl, n = 3, p = 1). The activation volume for diffusion creep (V*dif) and for GBS-disl (V*GBS) of dunite is determined to be 8.2 ± 0.9 and 7.5 ± 1.0 cm3/mol, respectively. Calculations based on these results suggest that both diffusion creep and dislocation creep play an important role for material flow at typical deformation conditions in the Earths asthenospheric upper mantle whereas the contribution of GBS-disl is very limited, and dislocation creep is the dominant deformation mechanism during the deformation of olivine in sheared peridotite xenolith. Though these conclusions are not definitive, these are the first results on potential deformation mechanisms of forsterite aggregate based on extrapolation in the pressure, temperature, stress, and grain-size space.

A new chemical etching technique for peridotites using molten anhydrous borax

Abstract A new chemical etching technique has been devised for synthetic Fe-free peridotites composed of forsterite (Fo), enstatite (En), and diopside (Di) Among the etchants were acids, molten carbonates, and borates, but it was found that only molten anhydrous borax (Na2B4O7) dissolved all phases equally. Molten anhydrous borax was found to be a successful etchant in equally enhancing all the grain (i.e., Fo-Fo, En-En, and Di-Di) and interphase (i.e., Fo-En, Fo-Di, and En-Di) boundaries. From the backscattered electron images of the etched surface, maps of grain- and interphase-boundaries can be obtained semi-automatically for microstructural analysis by using image processing software. An Febearing wehrlite was also etched successfully by molten anhydrous borax, thus showing the usefulness of this technique for enhancing the grain- and interphase-boundaries in many natural peridotites

Oxidation softens mantle rocks

Seismic waves that propagate through a layer of Earth’s upper mantle are highly attenuated. Contrary to general thinking, this attenuation seems to be strongly affected by oxidation conditions, rather than by water content. Seismic waves that propagate through a layer of Earth’s upper mantle are highly attenuated. Contrary to general thinking, this attenuation seems to be strongly affected by oxidation conditions, rather than by water content.