Tomoeki Nakakuki

Interaction of the upwelling plume with the phase and chemical boundary at the 670 km discontinuity: effects of temperature-dependent viscosity

Abstract Numerical simulations have been performed to investigate the interaction of upwelling plumes with the 670 km discontinuity. Temperature-dependent rheology is employed to introduce the low viscosity and strength of the hot plume. The 670 km discontinuity is taken as a phase and/or chemical boundary. The condition for penetration of the plume into the upper mantle is examined by varying the value of the Clapeyron slope of the phase transition and the compositional density difference of the chemical boundary. Possible styles of the mantle convection are classified into six types depending on the nature of the 670 km discontinuity. The plume in the lower mantle can penetrate into the upper mantle and reach the bottom of the lithosphere when the Clapeyron slope is greater than −3 to −4 MPa/K for the pure phase boundary, and when the compositional density difference is smaller than 0.5–1.5% for the pure chemical boundary. The plume can barely penetrate into the upper mantle when the 670 km discontinuity possesses even a small amount of the compositional density contrast (less than 0.5–1.0%) together with the phase transition of the experimentally determined Clapeyron slope of −3 ± 1 MPa/K. This result suggests that the mantle may be of uniform composition or at most has a weak compositional layering if the origin of the plume is in the deep mantle.

Dynamical modeling of trench retreat driven by the slab interaction with the mantle transition zone

We present the 2-D self-consistent dynamical model of interactions of a subducting slab with the 410-km and 660-km phase boundaries to further our understanding of the relation between the slab stagnation/penetration and the trench migration. Our model takes into account freely-movable plate boundaries and the difference between tensional and compressional yield strengths in the lithosphere. For the case in which the tensional strength is weaker than the compressional one, the negative buoyancy of the subducting slab produces extension of the overriding lithosphere and, accordingly, the trench retreats. Interactions with the 410-km and 660-km phasetransition boundaries further promote the trench retreat, and the dip angle of the slab is substantially decreased. This enhances the resistance of the 660-km phase boundary against the slab penetration. Slab weakening caused by the grain-size reduction in the transition zone may result in a horizontally-lying slab and trench retreat.

MANTLE VISCOSITY DERIVED BY GENETIC ALGORITHM USING OCEANIC GEOID AND SEISMIC TOMOGRAPHY FOR WHOLE-MANTLE VERSUS BLOCKED-FLOW SITUATIONS

Abstract We have applied the genetic algorithm (GA) technique, a nonlinear global optimization method, to determine the radial viscosity structure of the mantle from regional geoidal patterns. From numerical simulations of 2-D mantle convection, we examine the horizontal spectra of the vertical mass flux at 660 km depth and find that for long wavelengths there are minor differences between partially layered convection induced by the phase transitions and mantle convection without any phase transition. The differences in the spectra of the vertical mass flux become more prominent at shorter wavelengths. This result has led us to study mantle viscosity for the intermediate wavelength geoid from the whole-mantle and blocked-flow situations, in which the appropriate boundary condition is imposed on the radial velocity at 660 km depth. In order to confirm the robustness of this study, two different density models have been used, which were constructed from three tomographic models and appropriate velocity-to-density scaling relations based on recent results from mineral physics. We have analyzed only oceanic geoid spanning between spherical harmonic degree l=12–25. The correlation of the predicted geoid with the observations over the Atlantic, Indian, and Pacific Oceans have been employed as the fitting function in our GA approach, which has been modified from the common algorithm. In constructing the families of suitable viscosity profiles, we have used 100 parents, which have been iterated for 100 generations, and have been started with 10 different sets of initial parents. Convergence to acceptable viscosity solutions is obtained for all the three oceans and for both the whole-mantle and layered models. In some cases multiple viscosity solutions are found acceptable by using the correlation criteria. The outstanding feature of these models is the nearly ubiquitous presence of two low viscosity zones, one lying under the lithosphere, the other right under the bottom of the spinel to perovskite phase change. The solutions for the whole-mantle model can fit better and are preferred over the solutions with the layered boundary condition, which generally result in unrealistic viscosity profiles. Our results would suggest a more complex mantle viscosity structure, which has not been detected previously from geoid signals with longer wavelengths, and also reveal the potential difficulties in treating the dynamical boundary condition at the 660 km discontinuity.

Temporal and vertical distributions of anthropogenic 236U in the Japan Sea using a coral core and seawater samples

The input history of 236U to the surface water of the Japan Sea was reconstructed through measurement of the 236U/238U atom ratio in annual bands of a coral skeleton which was collected at Iki Island in the Tsushima Strait, the main entrance to the Japan Sea. The 236U/238U atom ratios and concentrations of U isotopes were measured for the period 1935–2010 using AMS and ICP-MS. The 236U/238U atom ratios revealed three prominent peaks: 4.51 × 10−9 in 1955, 6.15 × 10−9 in 1959 and 4.14 × 10−9 in 1963; thereafter the isotope ratios gradually decreased over the next several decades, attaining a value of ca.1.3 × 10−9 for the present day. A simplified depth profile model for 236U in the Japan Sea, using the reconstructed 236U value for the surface water together with observed depth profiles for 236U in the water column in 2010, yielded diffusion coefficients of 3.4–5.6 cm2/s for 6 sampling points. The diffusion coefficient values obtained for the northern stations were relatively large, and fitting uncertainty was also larger for stations in the northern region. It may be presumed that the distribution of 236U in the water columns have been influenced not only by diffusion but also by subduction of the surface water in the Japan Sea.

Water circulation and global mantle dynamics: Insight from numerical modeling

We investigate water circulation and its dynamical effects on global-scale mantle dynamics in numerical thermochemical mantle convection simulations. Both dehydration-hydration processes and dehydration melting are included. We also assume the rheological properties of hydrous minerals and density reduction caused by hydrous minerals. Heat transfer due to mantle convection seems to be enhanced more effectively than water cycling in the mantle convection system when reasonable water dependence of viscosity is assumed, due to effective slab dehydration at shallow depths. Water still affects significantly the global dynamics by weakening the near-surface oceanic crust and lithosphere, enhancing the activity of surface plate motion compared to dry mantle case. As a result, including hydrous minerals, the more viscous mantle is expected with several orders of magnitude compared to the dry mantle. The average water content in the whole mantle is regulated by the dehydration-hydration process. The large-scale thermochemical anomalies, as is observed in the deep mantle, is found when a large density contrast between basaltic material and ambient mantle is assumed (4–5%), comparable to mineral physics measurements. Through this study, the effects of hydrous minerals in mantle dynamics are very important for interpreting the observational constraints on mantle convection.

Rheological decoupling at the Moho and implication to Venusian tectonics

Plate tectonics is largely responsible for material and heat circulation in Earth, but for unknown reasons it does not exist on Venus. The strength of planetary materials is a key control on plate tectonics because physical properties, such as temperature, pressure, stress, and chemical composition, result in strong rheological layering and convection in planetary interiors. Our deformation experiments show that crustal plagioclase is much weaker than mantle olivine at conditions corresponding to the Moho in Venus. Consequently, this strength contrast may produce a mechanical decoupling between the Venusian crust and interior mantle convection. One-dimensional numerical modeling using our experimental data confirms that this large strength contrast at the Moho impedes the surface motion of the Venusian crust and, as such, is an important factor in explaining the absence of plate tectonics on Venus.

A simple model of mantle convection including a past history of yielding

A two-dimensional dynamic model of mantle convection, showing plate-like behavior, is constructed by considering the effects of the previously yielded segments. Only within an uppermost thin layer is the material assumed to yield at a constant stress. The advection of the segments affected by yielding is calculated. The area covered by these segments continues to yield until it reaches a certain depth. The resultant flow shows movement of the top layer in the parameter range in which, without yielding, it becomes sluggish. A comparison of the results with and without historical effects shows the enhancement of the plate-like behavior. We consider the horizontal variation of the yield stress, which may be caused by the difference in the amount of H 2 O in the crust. The place of sinking is controlled by the lateral change in the yield stress and the flow behavior becomes different from the case with uniform yield stress.