Masaki Ogawa

High-pressure recovery of olivine: implications for creep mechanisms and creep activation volume

High-temperature and high-pressure recovery experiments were made on experimentally deformed olivines at temperatures of 1613–1788 K and pressures of 0.1 MPa to 2.0 GPa. In the high-pressure experiments, a piston cylinder apparatus was used with BN and NaCl powder as the pressure medium, and the hydrostatic condition of the pressure was checked by test runs with low dislocation density samples. No dislocation multiplication was observed. The kinetics of the dislocation annihilation process were examined by different initial dislocation density runs and shown to be of second order, i.e. dρdt= −p2K0exp[−(E∗+PV∗RT] where ρ is the dislocation density, k0 is a constant, E∗ and V∗ are the activation energy and volume respectively, and P, R and T are pressure, gas constant and temperature, respectively. Activation energy and volume were estimated from the temperature and pressure dependence of the dislocation annihilation rate as E∗=389±59 kJ mol−1 and V∗=14±2 cm3 mol−1, respectively. The diffusion constants relevant to the dislocation annihilation process were estimated from a theoretical relation k=αD where k=k0 exp[−(E∗ + PV∗)/RT], D is the diffusion constant and α is a non-dimensional constant of ca. 300. The results agree well with the self-diffusion constant of oxygen in olivine. This suggests that the dislocation annihilation is rate-controlled by the (oxygen) diffusion-controlled dislocation climb. The mechanisms of creep in olivine and dry dunite are examined by using the experimental data of static recovery. It is suggested that the creep of dry dunite is rate-controlled by recovery at cell walls or at grain boundaries which is rate-controlled by oxygen diffusion. Creep activation volume is estimated to be 16±3 cm3 mol−1.

Transitions in thermal convection with strongly temperature-dependent viscosity in a wide box

Numerical models are systematically presented for time-dependent thermal convection of Newtonian fluid with strongly temperature-dependent viscosity in a two-dimensional rectangular box of aspect ratio 3 at various values of the Rayleigh number Rab defined with viscosity at the bottom boundary up to 1.6×108 and the viscosity contrast across the box rη up to 108. We found that there are two different series of bifurcations that take place as rη increases. One series of bifurcations causes changes in the behavior of the thermal boundary layer along the surface boundary from small-viscosity-contrast (SVC) mode, through transitional (TR) mode, to stagnant-lid (ST) mode, or from SVC mode directly to ST mode, depending on Rab. Another series of bifurcations causes changes in the aspect ratio of convection cells; convection with an elongated cell can take place at moderate rη (103–105.5 at Rab=6×106), while only convection of aspect ratio close to 1 takes place at small rη and large rη. The parameter range of rη and Rab for elongated-cell convection overlaps the parameter range for SVC and ST modes and include the entire parameter range for TR mode. In the elongated-ST regime, the lid of highly viscous fluid along the top boundary is not literally ‘stagnant’ but can horizontally move at a velocity high enough to induce a convection cell with aspect ratio much larger than 1.

GaAs dual-gate MESFET's

Performance of GaAs dual-gate MESFET, including high-frequency noise behavior, was analyzed on the basis of Statzs model. Under the design considerations developed from the analysis, fabrication and characterization of a prototype device were carried out. The present analysis was confirmed to reproduce satisfactorily the performance observed. Minimum noise figure and associated gain observed in the device with two 1-µm gates were; 1.2 dB and 16.7 dB at 4 GHz, 2.2 dB and 16.3 dB at 8 GHz, and 3.2 dB and 12.6 dB at 12 GHz, respectively. More than 35-dB gain controllability was also obtained at 8 GHz.

Thermochemical regime of the early mantle inferred from numerical models of the coupled magmatism‐mantle convection system with the solid‐solid phase transitions at depths around 660 km

A numerical model is presented for the coupled magmatism-mantle convection system in the entire mantle to study how the coupled system controls the thermochemical state of the mantle under the influence of the solid-solid phase transitions at depths around 660 km depending on the internal heating rate. The solid-state convection in the mantle is modeled by a convection of a binary eutectic material with constant viscosity in a two-dimensional square box uniformly heated by an internal heat source. One of the end-members of the material stands for an olivine-rich material in the uppermost mantle and is transformed into its high-pressure phase at depths greater than a threshold around 660 km. The phase boundary has a negative Clausius-Clapeyron slope. Another end-member stands for a garnet-rich material in the uppermost mantle and is gradually transformed into its high-pressure phase with increasing depth in a depth range around 660 km. The density of the material depends on its composition, phase, melt content, and temperature. Magmatism is modeled by a permeable flow of melt generated by a pressure release partial melting of the material. The permeable flow is driven by the buoyancy of the melt. There are two regimes in the thermochemical state of the mantle regardless of the strength of the barrier against mass exchange between the upper mantle and the lower mantle due to the solid-solid phase transitions. On one regime called the TC branch, magmatism occurs only mildly at most, the mantle remains chemically homogeneous as a whole, and the solid-state convection occurs dominantly as a thermal convection. The convective circulation is mantle-wide or layered depending on the strength of the barrier due to the solid-solid phase transitions. The TC branch is stable only when the internal heating rate is lower than a threshold. A bifurcation occurs on the TC branch at the threshold, and the thermochemical state falls on another regime called the CS branch above the threshold. An episodic magmatism actively occurs to make the mantle chemically stratified with the upper mantle largely occupied by olivine-rich residual materials and the deeper part of the lower mantle occupied by magmatic products of basaltic composition on the CS branch. The solid-state convection occurs as a whole mantle convection when the barrier is weak, while it occurs as a layered convection punctuated by flushing events that induce particularly vigorous magmatic activities when the barrier due to the phase transition of the garnet-rich material is sufficiently strong. The overall features of the thermochemical state on the CS branch mesh in many observations from the Archean and early Proterozoic continents.

Numerical models of magmatism in convecting mantle with temperature‐dependent viscosity and their implications for Venus and Earth

Numerical models are presented for magmatism in a convecting mantle that contains internal heat source. Mantle convection is modeled by a convection of binary eutectic material with Newtonian temperature-dependent rheology driven by thermal, compositional, and melt buoyancy as well as the buoyancy from the “660-km” phase transitions. Mantle magmatism is modeled by a gravitationally induced permeable flow of magma generated by pressure release melting through matrix. Magma mostly has the eutectic composition, that is, basaltic composition. The numerical models suggest that magmatism and mantle convection strongly influence each other to form a coupled system when the internal heating is sufficiently strong. Episodic magmatism takes place to chemically differentiate the mantle; basaltic materials occupy deeper part of the lower mantle, while magma residue occupies the uppermost mantle. Compositional buoyancy of the differentiated materials makes mantle convection sluggish except when magmatic activities take place. The style of magmatism and mantle convection depends on the viscosity contrast across the cold thermal boundary layer (TBL) along the top surface boundary. When the viscosity contrast is large, the fluid in the coldest part of top TBL becomes a stagnant lid and a conspicuous lateral heterogeneity arises in the upper mantle. Magmatism is induced by hot uprising diapirs originating in the top of the lower mantle. The diapirs induce convective circulation within the upper mantle when the lower mantle is rather cold but induce flushing event, that is, a massive flow across the 660-km phase boundary and the resulting vigorous magmatism, when the lower mantle becomes sufficiently hot owing to the internal heating. When the viscosity contrast is moderate, the fluid in the coldest part of top TBL behaves as a moving lid. Both the moving lid and hot uprising diapirs induce magmatic activity; the magmatic activity due to a moving lid resembles ridge volcanism. The mantle of Venus and that of the early Earth are suggested to have been on the regime modeled here, and the magmatic activity by flushing event is compared to the magmatism of volcanic plain formation on Venus.

A thermo-chemical regime in the upper mantle in the early Earth inferred from a numerical model of magma-migration in a convecting upper mantle

Abstract A numerical model of mantle magmatism in a convecting upper mantle has been developed to study the thermo-chemical evolution of the upper mantle of the early Earth. The solid-state convection in the upper mantle is modeled by a convection of a binary eutectic material with a Newtonian temperature-dependent rheology in a two-dimensional rectangular box placed on a heat bath as a model of the lower mantle. The density depends on the chemical composition and melt-content as well as temperature of the material. The material contains heat-producing elements that are incompatible and exponentially decay with time. Mantle magmatism is modeled by a permeable flow of melt generated by a pressure-release melting induced by the solid-state convection through matrix. The permeable flow is driven by a buoyancy due to the density difference between the melt and the matrix. The thermo-chemical evolution in the box occurs in two stages if the deeper part of the box is not so strongly depleted in heat-producing elements in spite of the upward migration and concentration of heat-producing elements into a crustal layer along the top surface boundary due to magmatism. In the earlier stage, active magmatism occurs because of a strong internal heating due to the heat-producing elements, a chemically stratified structure develops well in the box with dense magmatic products in the deeper part and less dense residual materials in the shallower part, and the temperature distribution becomes strongly superadiabatic over the entire box. The temperature at the base of the box becomes as high as the solidus temperature. The chemically stratified structure is, however, suddenly destroyed by convective mixing and the temperature in the deeper part of the box suddenly drops by several hundred degrees when the internal heat source becomes too weak owing to the decay of heat-producing elements which sustain the active magmatism and hence keep the effect of chemical differentiation due to the magmatism stronger than the effect of convective mixing. In the later stage of the evolution, the box becomes chemically homogeneous and magmatism occurs only mildly. If heat-producing elements are efficiently transported into the crustal layer and the deeper part of the box becomes strongly depleted in heat-producing elements owing to the magmatism, only a mild magmatism occurs even at the beginning, a chemically stratified structure does not develop well, and the temperature in the box rapidly decreases to a stationary value. The regime of hot and chemically stratified upper mantle suggested from the earlier stage of the case with mild depletion of heat-producing elements at depth fits in with many observations of the Archean continental crust.

ON THE VIGOR OF MANTLE CONVECTION IN SUPER-EARTHS

Numerical models are presented to clarify how adiabatic compression affects thermal convection in the mantle of super-Earths ten times the Earths mass. The viscosity strongly depends on temperature, and the Rayleigh number is much higher than that of the Earths mantle. The strong effect of adiabatic compression reduces the activity of mantle convection; hot plumes ascending from the bottom of the mantle lose their thermal buoyancy in the middle of the mantle owing to adiabatic decompression, and do not reach the surface. A thick lithosphere, as thick as 0.1 times the depth of the mantle, develops along the surface boundary, and the efficiency of convective heat transport measured by the Nusselt number is reduced by a factor of about four compared with the Nusselt number for thermal convection of incompressible fluid. The strong effect of adiabatic decompression is likely to inhibit hot spot volcanism on the surface and is also likely to affect the thermal history of the mantle, and hence, the generation of magnetic field in super-Earths.

Two-stage evolution of the Earth's mantle inferred from numerical simulation of coupled magmatism-mantle convection system with tectonic plates

Self-consistent numerical models are developed for a coupled magmatism-mantle convection system with tectonic plates in a two-dimensional rectangular box to understand the Earths mantle evolution. The mantle evolves in two stages owing to decaying internal and basal heating, provided that the lithosphere is mechanically strong enough to inhibit spontaneous formation of new subduction zones by ridge push force. On the earlier stage that continues for the first 1–2 Gyr, the deep mantle is strongly heated, and hot materials there frequently ascend to the surface as bursts. The mantle bursts cause vigorous magmatism and make the lithosphere move chaotically. The thermostat effect of the vigorous magmatism keeps the average temperature in the upper mantle below about 1800 K no matter how strongly the mantle is heated. As the heating rate of the mantle declines, however, the mantle evolves into the later stage where mantle bursts subside, rigid tectonic plates emerge to move rather steadily, and subducted basaltic crusts accumulate on the core-mantle boundary to form compositionally dense piles. Hot plumes occasionally ascend from the basaltic piles to cause magmatism. It takes time on the order of one billion years for the slabs that sink into the lower mantle to return back to the upper mantle, and the long overturn time makes the thermal history of the upper mantle, which has been petrologically constrained for the Earth, distinct from that of the whole mantle. The long overturn time also makes water injected into the mantle by slabs distribute heterogeneously.

Evolution of the interior of Mercury influenced by coupled magmatism‐mantle convection system and heat flux from the core

To discuss mantle evolution in Mercury, I present two-dimensional numerical models of magmatism in a convecting mantle. Thermal, compositional, and magmatic buoyancy drives convection of temperature-dependent viscosity fluid in a rectangular box placed on the top of the core that is modeled as a heat bath of uniform temperature. Magmatism occurs as a permeable flow of basaltic magma generated by decompression melting through a matrix. Widespread magmatism caused by high initial temperature of the mantle and the core makes the mantle compositionally stratified within the first several hundred million years of the 4.5 Gyr calculated history. The stratified structure persists for 4.5 Gyr, when the reference mantle viscosity at 1573 K is higher than around 1020 Pa s. The planet thermally contracts by an amount comparable to the one suggested for Mercury over the past 4 Gyr. Mantle upwelling, however, generates magma only for the first 0.1–0.3 Gyr. At lower mantle viscosity, in contrast, a positive feedback between magmatism and mantle upwelling operates to cause episodic magmatism that continues for the first 0.3–0.8 Gyr. Convective current stirs the mantle and eventually dissolves its stratified structure to enhance heat flow from the core and temporarily resurrect magmatism depending on the core size. These models, however, predict larger contraction of the planet. Coupling between magmatism and mantle convection plays key roles in mantle evolution, and the difficulty in numerically reproducing the history of magmatism of Mercury without causing too large radial contraction of the planet warrants further exploration of this coupling.

Mantle evolution in Venus due to magmatism and phase transitions: From punctuated layered convection to whole‐mantle convection

A series of numerical models of magmatism and mantle convection with a stagnant lithosphere are developed to understand the mantle evolution in Venus. Magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting, and the solid-state convection of mantle materials with temperature-dependent Newtonian rheology is affected by the garnet-perovskite transition and the postspinel transition. In our preferred models, the mantle evolves in two stages: The earlier stage is characterized by layered mantle convection punctuated by repeated bursts of hot material from the deep mantle to the surface. Mantle bursts induce vigorous magmatism and also cause the basaltic crust, enriched in heat-producing elements (HPEs), to recycle into the mantle. A part of the recycled basaltic crusts accumulates along the postspinel boundary to form a barrier, and this basalt barrier causes mantle convection to become layered. At a later stage, when the HPEs have already decayed, in contrast, the basalt barrier disappears and whole-mantle convection occurs more steadily. Mild magmatism is induced by small-scale partial melting at the base of the crust and hot plumes from the deep mantle. The internal heating by the HPEs that recycled into the mantle in the earlier stage allows the magmatism of the later stage to continue throughout the calculated history of mantle evolution. The two stages arise when the barrier effect of the postspinel transition is weak and the lithosphere is mechanically strong enough. The two-stage evolution model meshes with the observed history of magmatism and the lithosphere on Venus.