Shun-ichiro Karato

Rheology of the upper mantle: A synthesis

Rheological properties of the upper mantle of the Earth play an important role in the dynamics of the lithosphere and asthenosphere. However, such fundamental issues as the dominant mechanisms of flow have not been well resolved. A synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the Earths upper mantle. The hot and shallow upper mantle flows by dislocation creep, whereas cold and shallow or deep upper mantle may flow by diffusion creep. When the stress increases, grain size is reduced and the upper mantle near the transition between these two regimes is weakened. Consequently, deformation is localized and the upper mantle is decoupled mechanically near these depths.

Importance of anelasticity in the interpretation of seismic tomography

Temperature dependence of seismic wave velocities comes both from anharmonicity and anelasticity. The contribution from anelasticity is shown to be important in the Earths mantle particularly for shear waves. In the low Q (Qµ∼100) regions in the upper mantle, the correction due to anelasticity will roughly double the temperature derivatives due to anharmonicity alone. The correction for anelasticity will also be important in the deep mantle where Q is larger, if temperature derivatives due to anharmonicity will decrease significantly with pressure. These results imply that the temperature anomalies associated with low velocity anomalies in the mantle will be significantly smaller than previously considered on the basis of anharmonic effect alone and that the amplitude of velocity anomalies will be significantly larger for shear waves than for compressional waves.

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 Earths 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. We propose that, as the ascending ambient mantle (forced up by the downward flux of subducting slabs) rises out of the high-water-solubility transition zone (between the 660 km and 410 km discontinuities) into the low-solubility upper mantle above 410 km, it undergoes dehydration-induced partial melting that filters out incompatible elements. The filtered, dry and depleted solid phase continues to rise to become the source material for mid-ocean-ridge basalts. The wet, enriched melt residue may be denser than the surrounding solid and accordingly trapped at the 410 km boundary until slab entrainment returns it to the deeper mantle. The filter could be suppressed for both mantle plumes (which therefore generate wetter and more enriched ocean-island basalts) as well as the hotter Archaean mantle (thereby allowing for early production of enriched continental crust). We propose that the transition-zone water-filter model can explain many geochemical observations while avoiding the major pitfalls of invoking isolated mantle reservoirs.

Water, partial melting and the origin of the seismic low velocity and high attenuation zone in the upper mantle

Abstract The common belief that the seismic low velocity and high attenuation zone (the asthenosphere) is caused by the presence of a small amount of melt is not supported by recent mineral physics and seismological observations. A review of recent mineral physics observations suggests that water significantly reduces seismic wave velocities through anelastic relaxation and hence, at a small melt fraction expected in most of the Earths upper mantle, partial melting will increase seismic wave velocities through the removal of water from minerals such as olivine. Therefore the asthenosphere, in this model, is a layer where no significant partial melting occurs and hence a high water content is retained. We apply this model to calculate seismic wave velocities and attenuation in the upper mantle with a range of water contents. The seismic structures calculated from this model depend on geotherm, the mode of partial melting (batch or fractional melting) and the geometry of upwelling flow (passive flow or dynamic upwelling). The sharp velocity change around 60–80 km (the Gutenberg discontinuity) can be attributed to a sharp change in water content due to partial melting, if the temperature there is relatively high as implied by the plate model and if melting occurs as fractional melting but not by batch melting. However, the significant increase in seismic wave velocity with age in young oceanic upper mantle suggests rapid cooling as predicted by a cooling half-space model. Thus, the present model suggests fast cooling in the early stage but slow cooling in the later stage of evolution of the oceanic upper mantle, the latter being caused presumably by some additional heat in the old oceanic upper mantle. The seismic structures of typical oceanic upper mantle with a fast spreading rate (e.g., the Pacific) is consistent with passive spreading, whereas the greater depth of the G-discontinuity and the weaker seismic anisotropy in back-arc regions (e.g., the Philippine Sea) suggest dynamic upwelling caused presumably by a higher degree of melting due to a larger amount of water.

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

The distribution of water in the Earths 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 Earths mantle (from ∼410 to ∼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 ∼410 km, which would affect the distribution of certain elements in the Earth. However, the amount of water in the transition zone has remained unknown. Here we determined the effects of water and temperature on the electrical conductivity of the minerals wadsleyite and ringwoodite to infer the water content of the transition zone. We find that the electrical conductivity of these minerals depends strongly on water content but only weakly on temperature. By comparing these results with geophysically inferred conductivity, we infer that the water content in the mantle transition zone varies regionally, but that its value in the Pacific is estimated to be ∼0.1–0.2 wt%. These values significantly exceed the estimated critical water content in the upper mantle, suggesting that partial melting may indeed occur at ∼410 km depth, at least in this region.

Origin of lateral variation of seismic wave velocities and density in the deep mantle

Strong constraints can be placed on the origin of heterogeneity of seismic wave velocities and density if the observed ratios of various parameters are compared with mineral physics predictions. They include the shear to compressional wave velocity heterogeneity ratio, Rs/p ≡ δ log Vs/δ log Vp, the bulk sound to shear wave velocity heterogeneity ratio, Rϕ/s ≡ δ log Vϕ/δ log Vs, and the density to velocity heterogeneity ratio, Rρ/s,p ≡ δ log ρ/δ log Vs,p. Using mineral physics considerations, we calculate these ratios in the lower mantle corresponding to the thermal and chemical origin of velocity and density heterogeneity. Both anharmonic and anelastic effects are considered for thermal origin. Anharmonic effects are estimated from the theoretical calculations as well as from laboratory measurements which show a marked increase in Rs/p with pressure from ∼1.5 to ∼2.1 in the lower mantle. Such a trend is marginally consistent with seismological observations showing an increase in Rs/p with depth (from ∼1.7 to ∼3.2 in the lower mantle). However, anharmonic effect alone cannot explain inferred low Rρ/s ( 2.7) and corresponding negative values of Rϕ/s (and Rρ/s) in the deep lower mantle which cannot be accounted for by thermal or simple chemical heterogeneity such as the heterogeneity in the Fe/(Fe + Mg) and/or Mg/(Mg + Si) ratios. Possible causes of anomalies in this region are discussed, including the role of anisotropy and a combined effect of heterogeneity in Fe and Ca content.

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. Here we report a laboratory study of the dependence of the electrical conductivity of olivine aggregates on water content at high temperature and pressure. The electrical conductivity of synthetic polycrystalline olivine was determined from a.c. impedance measurements at a pressure of 4 GPa for a temperature range of 873–1,273 K for water contents of 0.01–0.08 wt%. The results show that the electrical conductivity is strongly dependent on water content but depends only modestly on temperature. The water content dependence of conductivity is best explained by a model in which electrical conduction is due to the motion of free protons. A comparison of the laboratory data with geophysical observations suggests that the typical oceanic asthenosphere contains ∼10-2 wt% water, whereas the water content in the continental upper mantle is less than ∼10-3 wt%.

Some mineral physics constraints on the rheology and geothermal structure of Earth’s lower mantle

Abstract We explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of Earth’s lower mantle. We show that MgSiO3 perovskite is significantly stronger than MgO periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase, periclase. We calculate viscosities of the lower mantle for two cases: (1) where periclase occurs as isolated grains and (2) where periclase occurs as continuous films, using mineral physics data and models of two-phase rheology. We find that the effective viscosity for the former is about ~10-1000 times higher than the latter. We therefore suggest that the rheology of the lower mantle is structureand hence strain-dependent, leading to weakening at large strains due to the formation of continuous films of periclase. Overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient. Both MgSiO3 perovskite and periclase have relatively small activation energies (E* = gRTm with g = 10-14, where R is the gas constant and Tm is melting temperature), and therefore the depth variation of viscosity is rather small, even for a nearly adiabatic temperature gradient. However, the geothermal gradients consistent with the geodynamical inference of nearly depth-independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood. A superadiabatic gradient of up to ~0.6 K/km is also consistent with mineral physics and geodynamical observations.

Grain growth kinetics in olivine aggregates

Abstract Grain growth kinetics were studied in hot-pressed fine-grained olivine aggregates. Experimental conditions include pressures of 0.1 MPa, 300 MPa and 1 GPa, and temperatures of 1473, 1573 and 1673 K, with or without the presence of water. At the initial stage of grain growth, growth of larger grains occurs rapidly, consuming smaller grains, which results in significant pore entrapment. Almost homogeneous grain-size distribution is established in this process. After this stage, grain-size distribution remains almost homogeneous in most cases. It was found that the presence of water enhances the grain growth kinetics when the amount of water is limited. However, when the amount of water is large, the water-containing pores significantly inhibit the grain boundary migration. Therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water. Abnormal grain growth was found in some samples. In these cases, normal grain growth was inhibited by the presence of a significant amount of water-filled pores, or grains with significantly larger size than the rest occurred in the starting materials. In 0.1 MPa runs, significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures. These results show that both water and pores have important effects on grain growth kinetics in olivine. When the effects of pores (or secondary phases) in inhibiting grain growth are unimportant, the grain growth rate in olivine is very fast: growth to 100 μm size will occur in 102 to 103 hours at 1500 K. Thus fine grain size in olivine will be maintained only for a short geological time, unless the grain boundaries are effectively pinned by secondary particles.

Mapping Water Content in the Upper Mantle

Variations in water (hydrogen) content in Earths (upper) mantle can be inferred from geophysical observations if the relationship between water content and relevant physical properties is known and if high-resolution geophysical measurements are available. This paper reviews the current status of mineral physics understanding of the effects of water on elastic and non-elastic deformation of minerals such as olivine and its influence on seismologically measurable properties. Important effects of water on seismic wave propagation are through indirect effects due to hydrogen-related defects in nominally anhydrous minerals as opposed to the direct effects caused by the formation of hydrous minerals. Two cases of indirect effects are reviewed: (i) effects through the enhancement of anelasticity and (ii) effects through the modifications of lattice preferred orientation. The former causes enhanced attenuation (low Q) and low velocities by the increase of water content and the latter modifies the nature of seismic anisotropy. Experimental data are reviewed to formulate ways to infer water content from seismological data and analytical equations are derived that relate velocity and attenuation anomalies to anomalies in temperature and/or water content. The results are applied to infer the distribution of water in Earths upper mantle using seismological observations. In subduction zones, the regions of high water content in the shallow upper mantle (200 km) are inferred to be localized to the mantle beneath current or recent volcanoes although wider distribution is hinted in the deeper portions (200 km). In the upper mantle beneath hot spot volcanoes such as Hawaii and Iceland, both seismic wave attenuation and anisotropy measurements suggest the presence of a column of material with a high water content, indicating that Hawaii and Iceland are not only hot spots but also wet spots.