Small effect of partial melt on electrical anomalies in the asthenosphere.

High conductivity anomalies in the shallow mantle are frequently attributed to minor partial melt (basalt or carbonatite) in the olivine-dominated peridotites. Conductivity of a melt-mineral mixture depends on the configuration of melt that could be affected by grain size of the constitutive mineral(s), but this has rarely been explored. Here, we provide experimental evidence using a conductive carbonatite analog and olivine that the bulk conductivity decreases systematically with increasing olivine grain size.

Behavior and properties of water in silicate melts under deep mantle conditions.

Water (HO) as one of the most abundant fluids present in Earth plays crucial role in the generation and transport of magmas in the interior. Though hydrous silicate melts have been studied extensively, the experimental data are confined to relatively low pressures and the computational results are still rare. Moreover, these studies imply large differences in the way water influences the physical properties of silicate magmas, such as density and electrical conductivity.

Dynamics of fault motion and the origin of contrasting tectonic style between Earth and Venus.

Plate tectonics is one mode of mantle convection that occurs when the surface layer (the lithosphere) is relatively weak. When plate tectonics operates on a terrestrial planet, substantial exchange of materials occurs between planetary interior and its surface. This is likely a key in maintaining the habitable environment on a planet.

Upper-mantle water stratification inferred from observations of the 2012 Indian Ocean earthquake.

Water, the most abundant volatile in Earth's interior, preserves the young surface of our planet by catalysing mantle convection, lubricating plate tectonics and feeding arc volcanism. Since planetary accretion, water has been exchanged between the hydrosphere and the geosphere, but its depth distribution in the mantle remains elusive. Water drastically reduces the strength of olivine and this effect can be exploited to estimate the water content of olivine from the mechanical response of the asthenosphere to stress perturbations such as the ones following large earthquakes.

Shear deformation of bridgmanite and magnesiowüstite aggregates at lower mantle conditions.

Rheological properties of the lower mantle have strong influence on the dynamics and evolution of Earth. By using the improved methods of quantitative deformation experiments at high pressures and temperatures, we deformed a mixture of bridgmanite and magnesiowüstite under the shallow lower mantle conditions. We conducted experiments up to about 100% strain at a strain rate of about 3 × 10(-5) second(-1).

Asymmetric shock heating and the terrestrial magma ocean origin of the Moon.

One of the difficulties of the current giant impact model for the origin of the Moon is to explain the marked similarity in the isotopic compositions and the substantial differences in the major element chemistry. Physics of shock heating is analyzed to show that the degree of heating is asymmetric between the impactor and the target, if the target (the proto-Earth) had a magma-ocean but the impactor did not. The magma ocean is heated much more than the solid impactor and the vapor-rich jets come mainly from the magma-ocean from which the Moon might have been formed.

Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization.

For plate tectonics to operate on a terrestrial planet, the surface layer (the lithosphere) must have a modest strength (Earth, ≤ 200 MPa), but a standard strength profile based on olivine far exceeds this threshold value. Consequently, it is essential to identify mechanisms that reduce the strength of the lithosphere on Earth. Here we report results of high-strain laboratory deformation experiments on a representative olivine-orthopyroxene composition that show the addition of orthopyroxene substantially reduces the strength in the ductile regime within a certain temperature window.

Deep penetration of molten iron into the mantle caused by a morphological instability.

The core-mantle boundary of Earth is a region where iron-rich liquids interact with oxides and silicates in the mantle. Iron enrichment may occur at the bottom of the mantle, leading to low seismic-wave velocities and high electrical conductivity, but plausible physical processes of iron enrichment have not been suggested. Diffusion-controlled iron enrichment is inefficient because it is too slow, although the diffusion can be fast enough along grain boundaries for some elements.

Electrical conductivity of orthopyroxene: implications for the water content of the asthenosphere.

Electrical conductivity of minerals is sensitive to water content and hence can be used to infer the water content in the mantle. However, previous studies to infer the water content in the upper mantle were based on pure olivine model of the upper mantle. Influence of other minerals particularly that of orthopyroxene needs to be included to obtain a better estimate of water content in view of the high water solubility in this mineral.

The effect of water on the electrical conductivity of olivine.

It is well known that water (as a source of hydrogen) affects the physical and chemical properties of minerals--for example, plastic deformation and melting temperature--and accordingly plays an important role in the dynamics and geochemical evolution of the Earth. Estimating the water content of the Earth's mantle by direct sampling provides only a limited data set from shallow regions (<200 km depth). Geophysical observations such as electrical conductivity are considered to be sensitive to water content, but there has been no experimental study to determine the effect of water on the electrical conductivity of olivine, the most abundant mineral in the Earth's mantle.

Density of hydrous silicate melt at the conditions of Earth's deep upper mantle.

The chemical evolution of the Earth and the terrestrial planets is largely controlled by the density of silicate melts. If melt density is higher than that of the surrounding solid, incompatible elements dissolved in the melt will be sequestered in the deep mantle. Previous studies on dry (water-free) melts showed that the density of silicate melts can be higher than that of surrounding solids under deep mantle conditions.

Water content in the transition zone from electrical conductivity of wadsleyite and ringwoodite.

The distribution of water in the Earth's interior reflects the way in which the Earth has evolved, and has an important influence on its material properties. Minerals in the transition zone of the Earth's mantle (from approximately 410 to approximately 660 km depth) have large water solubility, and hence it is thought that the transition zone might act as a water reservoir. When the water content of the transition zone exceeds a critical value, upwelling flow might result in partial melting at approximately 410 km, which would affect the distribution of certain elements in the Earth.

Whole-mantle convection and the transition-zone water filter.

Because of their distinct chemical signatures, ocean-island and mid-ocean-ridge basalts are traditionally inferred to arise from separate, isolated reservoirs in the Earth's mantle. Such mantle reservoir models, however, typically satisfy geochemical constraints, but not geophysical observations. Here we propose an alternative hypothesis that, rather than being divided into isolated reservoirs, the mantle is filtered at the 410-km-deep discontinuity.

Development of anisotropic structure in the Earth's lower mantle by solid-state convection.

Seismological observations reveal highly anisotropic patches at the bottom of the Earth's lower mantle, whereas the bulk of the mantle has been observed to be largely isotropic. These patches have been interpreted to correspond to areas where subduction has taken place in the past or to areas where mantle plumes are upwelling, but the underlying cause for the anisotropy is unknown-both shape-preferred orientation of elastically heterogeneous materials and lattice-preferred orientation of a homogeneous material have been proposed. Both of these mechanisms imply that large-strain deformation occurs within the anisotropic regions, but the geodynamic implications of the mechanisms differ.