▪ Abstract The 100 kyr quasiperiodic variation of continental ice cover, which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of Earth history, has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n-body effects in the Solar System on the geometry of Earth's orbit around the Sun. The impacts of the changing surface ice load upon both Earth's shape and gravitational field, as well as upon sea-level history, have come to be measurable using a variety of geological and geophysical techniques. These observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid Earth and on the detailed spatiotemporal characteristics of glaciation history. This review focuses upon the most recent advances that have been achieved in each of these areas, advances that have proven to be central to the construction of the refined model of the global process of glacial isos...

GLOBAL GLACIAL ISOSTASY AND THE SURFACE OF THE ICE-AGE EARTH: The ICE-5G (VM2) Model and GRACE

GLOBAL GLACIAL ISOSTASY AND THE SURFACE OF THE ICE-AGE EARTH: The ICE-5G (VM2)

http://szseminar.asu.edu/readings/EPSL_Schmidt_1998.pdf

Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation

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 Earth's 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.

https://science.sciencemag.org/content/sci/260/5109/771.full.pdf

Rheology of the Upper Mantle: A Synthesis

A positive temperature coefficient is the term which has been used to indicate that an increase in solubility occurs as the temperature is raised, whereas a negative coefficient indicates a decrease in solubility with rise in temperature.

Effect of Temperature

Temperature dependence of seismic wave velocities comes both from anharmonicity and anelasticity. The contribution from anelasticity is shown to be important in the Earth's 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.

Importance of anelasticity in the interpretation of seismic tomography

The influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied, using a gas-medium high-pressure deformation apparatus. The specimens used were hot-pressed, dense olivine aggregates with controlled grain size ranging from a few to 70 μm, with or without added water. Mechanical tests were made at 1573 K and 300 MPa confining pressure and at strain rates of 10−3 to 10−6 s−1. The results reveal two distinct mechanisms of deformation, depending on stress level and grain size. At relatively high stress and large grain size, the strain rate is proportional to about the cube power of the stress and is nearly independent of grain size. In this regime, microstructural observations gave evidence of intragranular deformation involving dislocation motion. At low stress and small grain size, the strain rate depends almost linearly on stress and decreases markedly with increase in grain size. In the latter regime, little evidence was found for intragranular deformation. These observations suggest that the deformation mechanism in the grain size insensitive regime is dislocation creep, while that in the grain size sensitive regime is diffusion creep. In both regimes, water was found to enhance the creep rate. The absence of grain size sensitivity in the dislocation creep regime and comparison with single-crystal data indicate that the water weakening effect is mainly an intragranular process. However, the existence also of a water weakening effect in the diffusion creep regime indicates that water also enhances diffusion. The extrapolation of the present results to coarser grain sizes indicates that the transition from dislocation to diffusion creep occurs at 0.1–1 MPa for 10-mm grain size. Therefore it is suggested that this transition may occur in the upper mantle and that, in both regimes, the presence of trace amounts of water will result in significantly lower creep strength than under strictly “dry” conditions.

Rheology of synthetic olivine aggregates: Influence of grain size and water

The interpretation of seismic anisotropy in Earth's upper mantle has traditionally been based on the fabrics (lattice-preferred orientation) of relatively water-poor olivine. Here we show that when a large amount of water is added to olivine, the relation between flow geometry and seismic anisotropy undergoes marked changes. Some of the puzzling observations of seismic anisotropy in the upper mantle, including the anomalous anisotropy in the central Pacific and the complicated anisotropy in subduction zones, can be attributed to the enrichment of water in these regions.

https://science.sciencemag.org/content/sci/293/5534/1460.full.pdf

Water-Induced Fabric Transitions in Olivine

REGIONS of the Earth's upper mantle show significant seismic ani-sotropy due to the preferred crystallographic orientation ('lattice preferred orientation') adopted by its constituent minerals in response to deformation1–6. Seismic anisotropies thus provide clues to the flow and/or stress patterns in the upper mantle, but the use of lattice preferred orientation to infer such properties from seismic data has been hampered by the lack of experimental studies relating changes in crystallographic orientation to simple-shear deformation. (Simple shear is probably the dominant mode of deformation in the upper mantle, whereas most previous experiments have focused on the effects of uniaxial compression7–9.) Here we describe the results of simple-shear deformation experiments on olivine aggregates, conducted at high temperatures and pressures (~1,500 K and 300 MPa). For large strains (up to 150%), our experiments reproduce the lattice preferred orientation observed in highly deformed upper-mantle rocks, in which the olivine [100] axes lie nearly parallel to the flow direction. But for relatively small strains, the preferred orientation is rotated with respect to the flow direction, indicating that seismic anisotropy should also be sensitive to the sense of shear in the upper mantle.

Shun-ichiro Karato

Papers

Rheology of the Upper Mantle: A Synthesis

Importance of anelasticity in the interpretation of seismic tomography

Rheology of synthetic olivine aggregates: Influence of grain size and water

Water-Induced Fabric Transitions in Olivine

Lattice preferred orientation of olivine aggregates deformed in simple shear