Synthesis of pyrope-knorringite solid solution series

The garnet solid solution series between pyrope Mg₃Al₂Si₃O₁₂ and knorringite Mg₃Cr₂Si₃O₁₂ has been synthesized from oxide mixtures at pressures of 60–80 kbars and 1400–1500°C. Lattice parameters and refractive indices of solid solutions vary linearly with (molecular) composition within the limits of measurement. The lattice parameter of pure knorringite is 11.600Åand its refractive index is 1.83. The genetic significance of mineral inclusions in natural diamonds is discussed, particularly in the light of the very high knorringite contents often found in garnet inclusions. It is suggested that the most common mineral assemblage occurring as inclusions in diamonds (olivine + knorringite-rich garnet + enstatite) might be explained in terms of subduction into the mantle of olivine + chrome-spinel + enstatite cumulates originally formed by crystallization of mafic magmas within the oceanic crust. The cumulate assemblage experienced alteration by circulating hydrothermal solutions, resulting in the introduction of some carbonate and serpentine minerals. During subduction, this assemblage was partially melted at depth below 150 km, accompanied by reduction of carbonate, to form a reconstituted assemblage consisting of olivine + knorringite-rich garnet + enstatite ± diamond.     

Elasticity of single crystal pyrope and implications for garnet solid solution series

The elastic moduli of a synthetic single crystal of pyrope (Mg₃Al₂Si₃O₁₂) have been determined using a technique based on Brillouin scattering. These results are used in an evaluation of the effect of composition on the elastic properties of silicate garnet solid solution series (Mg, Fe, Mn, Ca)₃ (Al, Fe, Cr)₂ Si₃O₁₂. In the pyralspites (Mg FeMn aluminum garnets), for which a large amount of data is available, this analysis indicates that the bulk modulus K is independent of the Fe²⁺/Mg²⁺ ratio, which is similar to the behavior observed in olivines and pyroxenes. However, the shear modulus μ of the garnets increases by 10% from the Mg to the Fe end member, in contrast to the decrease of μ with Fe content which is observed in olivines and pyroxenes. This contrasting behavior is most probably related to the oxygen coordination of the cation site occupied by Mg²⁺ and Fe²⁺ in these different minerals.     

An experimental study of solid matrix weakening in water‐saturated Savonnières limestone

Petrophysical properties of carbonate reservoirs are less predictable than that of siliciclastic reservoirs. One of the main reasons for this is the physical and chemical interactions of carbonate rocks with pore fluids. Such interactions can significantly change the elastic properties of the rock matrix and grains, making the applicability of Gassmann's fluid substitution procedure debatable. This study is an attempt to understand the mechanisms of fluid‐rock interactions and the influence of these interactions on elastic parameters of carbonates. We performed precise indentation tests on Savonnières limestone at a microscale level under dry, distilled water, and n‐Decane saturated conditions. Our experiments display softening of the rock matrix after water saturation. We have found that mainly the ooid cortices, peloid nuclei and prismatic intergranular cement are affected by water flooding. We also observed a shear modulus reduction in Savonnières limestone in an experiment performed at ultrasonic frequencies. One of the most important results obtained in our experimental study is that the Gassmann fluid substitution theory might not always be applicable to predict the elastic moduli of fluid‐saturated limestones.     

An experimental study of triggered stick-slip

Stick-slips have been studied in the laboratory on granite, labrodorite and sandstone samples of two different sizes. Different roughness was achieved on the sawcut surfaces by finishing them with different grinding compounds ranging from grit 40 to grit 1000. Stick-slips occurred as a result of 1) slowly increasing the shear and normal stresses, 2) superimposing a sinusoidal stress modulation (0.1 and 10 Hz) on the slowly increasing stresses, 3) triggering by a stress impulse when the shear stress was well below the levels where stick-slips occurred without the impulse, and 4) foreshocks.Stick-slips triggered by impulses or foreshocks occurred long after the beginning of the triggering events, i.e., long in comparison with elastic wave travel times through the sample. All triggered events were very rich in high frequencies (corner frequency of 100 kHz). The untriggered stick-slips did not contain much energy at the high frequencies (corner frequency of 10 kHz). The dynamic friction coefficients for the triggered stick-slips were smaller than for the untriggered events.The long delay between the onset of the trigger and the stick-slip, and the high frequencies may be a consequence of corrosion of asperities. The ultimate triggering and the rate of corrosion are likely related to the interplay of the normal and shear stresses as they load and unload the fault surface. The consistent shape of the high frequency spectra is probably due to sample resonances which are excited rather than being characteristic of the details of the stick-slips. If these laboratory observations are directly applicable to earthquake seismology, the spectra of earthquakes which were triggered by other earthquakes should be anomalously rich in high frequencies.     

An experimental study of magnetite-titanomagnetite interdiffusion

Interdiffusion experiments were performed between Fe₃O₄ (single crystal) and Fe_2.8Ti_0.2O₄ (powder), under self-buffering conditions (temperature range 600–1034°C), and for various oxygen potentials at 1400°C. Profiles of Fe and Ti were obtained by electronprobe microanalysis, and the interdiffusion coefficient $\text{D}$ was calculated by the Boltzmann-Matano method. Low-temperature data at 3 mole% Ti could be described by $\text{D}\text{= (3.85}{}_{\mathrm{?1.11}}{}^{\mathrm{+1.68}}\text{) × 10}{}^{\mathrm{?3}}\text{exp}\text{(2.23 ± 0.04}\text{eV}\text{/kT)}\text{cm}{}^2\text{/}\text{s}$. An estimate is given for the time to interdiffuse 2μm at various temperatures, and the results compared with recent experiments.     

Electron probe microanalysis study on processes of low-temperature oxidation of titanomagnetite

Partially oxidized titanomagnetite grains in various kinds of volcanic rocks were investigated by electron probe microanalyzer (EPMA) in order to clarify the process of oxidation at low temperature. The following results were obtained by the present investigation: (1) Primary composition of titanomagnetite is homogeneous in individual grains, although variation in composition among different grains is observed on each thin section. (2) Migration of Fe cations during low-temperature oxidation is clearly seen in all oxidized grains. Some other constituent cations are also bleached and consequently the relative content of the remaining cations becomes large. (3) The detailed internal structures of titanomagnetite grains are observed as backscattered electron images (BEI) with an electronprobe microanalyzer, and it seems likely that the structures depend upon the degree of low-temperature oxidation. (4) The chemical and physical properties of oxidized titanomagnetites imply that low-temperature oxidation is not a simple process but a complex one. Such an oxidation process is correlatable to both the mobility of cation and the oxidation condition such as a circulation of some active hydrothermal materials at low temperature.     

An experimental study of the seismic ringing of the moon

Summary The seismic records obtained from the seismic station ALSEP (Apollo Lunar Surface Experiments Package) on the moon, due to the impact of the LM (Lunar Module) of the Apollo XII and the S-IVB rocket state of the Apollo XIII moon missions show the seismic signals decaying in amplitude slowly with time. The observed ringing phenomenon on these records along with the fact that the disturbances lasted for a long period were unexpected. Laboratory experiments on homogeneous spheres have produced this ringing. The physical explanation of this may be complicated, and it is quite possible that this observed ringing is due to the combined effect of the filtering, reflections and scattering of energy due to the curvature of the homogeneous (in the seismic sense) sphere. The explanation of the ringing of the moon may very well lie in such a study.     

An experimental study of Stewartson layer separation

Abstract

The separation of sidewall boundary layers in a rotating annulus of homogeneous fluid is studied experimentally. The flow is driven by a differentially rotating lid, and a very small uniform slope of the bottom causes a weak mountain vortex pair to form in the interior, away from the sidewalls. A necessary condition for aerodynamic separation of the sidewall boundary layers is derived and compared with the experimental results. The laboratory flow separates for parameters that are just slightly more inviscid than those required by the necessary condition for the existence of adverse pressure gradients at the wall. As the bottom friction is decreased further, the flow becomes unsteady and chaotic. The most interesting aspect of this problem is that chaotic interior behavior, associated with the separated boundary layer, is observed for parameter values for which the interior topographically forced flow is, by itself, essentially linear.     

An experimental study of in-plane arch-shaped dampers

As an effort to minimize material utilization, seismic steel dampers designed to deform inelastically in an in-plane flexural mode have attracted serious attention recently. This paper presents a new type of metallic yielding damper referred to as the in-plane arch-shaped damper modified from its portal frame-shaped counterpart by replacing the straight beam with a circular arch to minimize the effects of stress concentration and warping, and therefore to avoid premature failure. Component tests of both the portal frame-shaped and arch-shaped in-plane dampers were conducted for comparison. Hysteresis loops obtained from the component tests under cyclic loads indicate substantial improvement on the energydissipative characteristics of the proposed damper. Moreover, seismic performance assessment of the proposed damper was carried out further via shaking table tests of a five-story model frame. Encouraging results have been achieved in terms of acceleration reduction, damping enhancement and peak suppression of the frequency response functions, suggesting the potential of the proposed device to be used in earthquake-resisting systems.     

An experimental study of sapped drainage network development

In a basin developed on a stream table, effluent subsurface flow supported a channel network that evolved by a combination of headward growth, lateral widening and divide decay. The area occupied by the developing network increased with time. Circularity was used to characterize network evolution which occurred in three phases (initiation, extension and abstraction). Basin sediment discharge declined exponentially with time. Pronounced quasi-cyclic variability was superimposed upon this general trend. Some of the variability was directly linked to changes in the amount of sediment supplied to the channel. The variation of mean network sediment yield (mean sediment discharge scaled by network area) with time adequately described the general decline in sediment discharge as the network evolved.     

On the theory of low-temperature oxidation of titanomagnetite grains

It is shown that the space-time distribution of vacancies in the process of formation of cation-deficient spinels is determined mainly by the boundary kinetics. At low rates of vacancy appearance in the surface layer (e.g., owing to oxygen deficit), the degree of grain oxidation remains invariant over the entire volume. The rate of oxidation in this case does not depend on the diffusion coefficient, but is determined by the boundary processes. With an excess of oxygen, the process of oxidation occurs inhomogeneously over the grain volume. In this case the rate of oxidation is determined by the diffusion coefficient. Approximate estimates have been made of the physical parameters characterising the two types of oxidation.     

An experimental study of forced barotropic rossby waves

Abstract

Barotropic Rossby waves are studied in a homogeneous fluid contained in a rotating cylindrical annulus with a radially sloping bottom boundary. The waves are forced by a simple source-sink distribution which can be rotated differentially relative to the annulus. When the speed of the source-sink distribution is close to the phase speed for a free Rossby wave of a given mode, resonant amplification occurs. The experimental results are in qualitative agreement with the predictions of a simple linear theory, but certain systematic differences between theory and experiment were observed.     

An experimental study on the ripple–dune transition

Flume experiments were conducted on different bed stages across the ripple–dune transition. As flow velocity increases, an initially flat bed surface (made of fairly uniform sandy material) is gradually transformed into a two‐dimensional rippled bed. With further increase in velocity, two‐dimensional ripples are replaced by irregular, linguoid ripples. As the average velocity necessary for the ripple–dune transition to occur is imposed on the bed surface, these non‐equilibrium linguoid ripples are further transformed into larger, two‐dimensional dunes. For each of these stages across the transition, a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated. An acoustic Doppler velocimeter was used to study the flow characteristics above each bed surface. Detailed profiles were used along a transect located in the middle of the channel. Results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields (section view). They clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition. Turbulence intensity and Reynolds stresses gradually increase throughout the transition. Two‐dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed. Linguoid ripples induce three‐dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region. A very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes. The turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability (when compared with the rippled beds). Further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis, together with an emphasis on quadrant analysis. Copyright © 2001 John Wiley & Sons, Ltd.     

Magnetic properties of monodomain aluminium-substituted titanomagnetite

A suite of synthetic titanomagnetites were prepared with compositions Fe_2.6?δTi_0.4Al_δO₄ and Fe_2.4?δTi_0.6Al_δO₄ (δ = 0, 0.1, 0.2 in both cases). Ball-milling of the synthesized samples produced material in the magnetic monodomain state as indicated by hysteresis loops and the Lowrie-Fuller test. The coercive force of the specimens depends on the Al concentration and lies in the range 1–2 kOe. The TRM induced in the samples is correspondingly “hard”. The low-field (0–1 Oe) TRM acquisition curve is linear. The higher field TRM-H curve is not in agreement with either monodomain or two-domain theoretical models.     

On a power series solution to the Boussinesq equation

For certain initial and boundary conditions the Boussinesq equation, a nonlinear partial differential equation describing the flow of water in unconfined aquifers, can be reduced to a boundary value problem for a nonlinear ordinary differential equation. Using Song et al.'s (2007) [7] approach, we show that for zero head initial condition and power-law flux boundary condition at the inlet boundary, the solution in the form of power series can be obtained with Barenblatt's (1990) [2] rescaling procedure applied to the power series solution obtained in Song et al. (2007) [7] for the power-law head boundary condition. Polynomial approximations can then be obtained by taking terms from the power series. Although for a small number of terms the newly obtained approximations may be worse than polynomial approximations obtained by other techniques, any desired accuracy can be achieved by taking more terms from the power series.     

An analytical solution to study substrate-microbial dynamics in soils

We provide an approximate analytical solution for the substrate-microbial dynamics of the organic carbon cycle in natural soils under hydro-climatic variable forcing conditions. The model involves mass balance in two carbon pools: substrate and biomass. The analytical solution is based on a perturbative solution of concentrations, and can properly reproduce the numerical solutions for the full non-linear problem in a system evolving towards a steady state regime governed by the amount of labile carbon supplied to the system. The substrate and the biomass pools exhibit two distinct behaviors depending on whether the amount of carbon supplied is below or above a given threshold. In the latter case, the concentration versus time curves are always monotonic. Contrarily, in the former case the C-pool concentrations present oscillations, allowing the reproduction of non-monotonic small-scale biomass concentration data in a natural soil, observed so far only in short-term experiments in the rhizosphere. Our results illustrate the theoretical dependence of oscillations from soil moisture and temperature and how they may be masked at intermediate scales due to the superposition of solutions with spatially variable parameters.     

An experimental study of the application of kinematic-wave theory to overland flow

An experimental test to evaluate the accuracy of kinematic-wave theory as applied to overland flow over a rough, uneven surface is described. The depression storage, and the hydraulic roughness for flows without rain were measured in separate tests; the hydraulic roughness for flows under rain was calculated from the rising limbs of the runoff hydrographs from a series of simulated storms. Recession curves were calculated using kinematic wave theory. The close agreement between experimental and calculated recessions showed the accuracy of this theory in predicting the behaviour of flows over the rough uneven surface. Hysteresis was observed in the storage-discharge curves; this was also explained in terms of kinematic wave theory.     

An experimental study on iron accumulation in the clam Mactra lilacea

Iron concentration in laboratory-maintained Mactra lilacea when exposed to sea water with a particle load of 1 kg m^?3 of iron ore dust was found to increase rapidly due to the filtering activity of these bivalves. This temporary store of iron diminished as iron ore was eliminated as faeces and also when the animals were allowed to purge themselves in clean sea water.     

An experimental study on the effects of phenocrysts on convection in magmas

The effect of phenocrysts on convection in magma chambers is investigated experimentally using small heavy particles in convecting fluids. The particles are initially uniformly distributed in a fluid which is either heated from below or cooled from above. The system is allowed to evolve, and temperature and particle concentration profiles are measured as functions of time. When the concentration of particles is sufficiently small, convection is basically unaffected by their presence. When the concentration is above a critical value, however, the convective motion is considerably altered. The effect of particles on the subsequent fluid behaviour is different in the cases of heating from below and cooling from above. In the former case, there are strong convective motions confined to a sedimentary layer of decreasing thickness beneath a clear layer which displays rather weak convective motions. With time, the destabilizing increase of temperature in the lower layer overcomes the stabilizing contribution to the bulk density due to the particles and the layer overturns quite suddenly. In the situation of cooling from above, a critical condition separates a case of continual overturn from a case of no overturn at all, with the sedimentary layer falling unimpeded to the bottom. Theoretical analysis suggests that the critical value is determined primarily by the ratio of the contribution to the bulk density of the suspension due to particles to the change in fluid density due to the thermal effect. The size distribution of the particles can also modify the fluid behaviour. Applying our general results to geological situations, we suggest that the presence of relatively small concentrations of phenocrysts can critically influence the mode of convection in magmas.