Yasuko Takei

Seismic Evidence for Sharp Lithosphere-Asthenosphere Boundaries of Oceanic Plates

Detection of the presence of melt at variable depth beneath two oceanic plates reveals the vertical extent of old oceanic plates. Boundary Issues of the Lithosphere The depth of Earths tectonic plates is defined by the lithosphere-aesthenosphere boundary (LAB), but its seismic signature is more subtle compared with other deeper boundaries within Earth (see the Perspective by Romanowicz). Under oceanic plates, the LAB is often defined by where temperatures are hot enough to cause some melting. This boundary has been hard to detect in older oceanic plates, but it is important for understanding how these plates thicken with age or distance from ocean ridges, and for assessing heat flow through the oceanic crust. Kawakatsu et al. (p. 499) use a detailed seismic array to detect a seismic velocity reduction beneath the Philippine Sea and Pacific plates. The data imply that 5%, or less, melt forms horizontal layers, and that oceanic plate thicknesses do indeed deepen with age. Rychert and Shearer (p. 495) used 15 years of seismic data to explore the global distribution of an anomaly imaged by conversion of pressure waves to shear waves (waves associated with a sharp velocity drop). The data reveal a broad signal at depths of 70 kilometers (km) beneath ocean islands to 95 km beneath Precambrian shields. It is not clear whether this boundary is the lithosphere-aesthenosphere boundary or a layer with a distinct horizontal fabric. The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.

Effect of pore geometry on VP/VS: From equilibrium geometry to crack

Seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes, including oblate spheroids, tubes, cracks, and an equilibrium geometry controlled by a dihedral angle. The relative role of liquid compressibility and pore geometry on the V P /V S velocity ratio is clarified. The result clearly indicates that P and S velocity structures determined by seismic tomography can be used to verify whether interfacial energy-controlled melt or fluid geometry (equilibrium geometry) is achieved. Relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio. As a function of the aspect ratio, a significant effect of pore geometry on d In V S /d In V P , the ratio of the fractional changes in V S and V P , is shown. Equilibrium geometry of the partially molten rocks, characterized by a dihedral angle of 20°-40°, corresponds to an aspect ratio of 0.1-0.15. The value of d In V S /d In V P expected for the texturally equilibrated partially molten rocks is shown to be 1-1.5, which is much smaller than that expected for cracks and dikes with an aspect ratio of <10 -2 -10 -3 . In the upper mantle low-velocity regions the seismologically obtained value of d In V S /d In V P is within this range beneath the Bolivian Andes (1.1-1.4) but is as high as 2 beneath Iceland (1.7-2.3) and beneath northeastern Japan (2.0). The former region can be regarded as a region where equilibrium geometry is achieved, and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate.

Constitutive mechanical relations of solid‐liquid composites in terms of grain‐boundary contiguity

Macroscopic constitutive relations of solid-liquid composites are derived as functions of microscopic geometry described by grain-boundary contiguity. The model consists of solid grains, which make a framework through grain-to-grain contact, and an interstitial liquid phase. Contiguity is the essential geometric factor that determines the macroscopic mechanical properties of the granular composites, while the other factors such as liquid volume fraction affect the properties only indirectly through the contiguity. Contiguity is defined by the portion of the grain surface being in contact with the neighboring grains, and takes a value between 0 and 1. Total length of the solid-solid and solid-liquid phase boundaries observed in cross section can be quantitatively related to the contiguity. Contiguity φ can be extended to a tensor φij to cover anisotropic grain contact. The full anisotropic macroscopic elasticities are quantitatively derived as functions of contiguity. Also, as an attempt to apply the present formulation to viscous behaviors of solid-liquid composites, bulk and shear viscosities are derived as functions of contiguity. The results successfully predict the large structural sensitivities of these properties; the bulk and shear moduli (or viscosities) of the skeleton are drastically reduced by decreased contiguity and vanish when contiguity vanishes. For partially molten media under thermodynamic equilibrium, geometrical variation according to melt fraction and dihedral angle is quantitatively related to the variation of contiguity, and the acoustic properties are derived as functions of melt fraction and dihedral angle.

Acoustic properties of partially molten media studied on a simple binary system with a controllable dihedral angle

Borneol-diphenylamine, a binary eutectic system of the organic compounds, provides an appropriate analogue of melting in the Earths mantle. Eutectic temperature is just above room temperature (43°C), and at this temperature the dihedral angle is about 40°. As the temperature increases, the dihedral angle gradually decreases at a rate of about 1.5° per 1°C, and becomes nearly zero at 70°C. Melt fraction change is small at this temperature range; this system is therefore appropriate in investigating a systematic effect of dihedral angle. Using this system, reduction of the shear and longitudinal wave velocities caused by grain boundary melt, having nearly equilibrium textures, was measured accurately as functions of both melt fraction and dihedral angle. The results demonstrate the significant effect of equilibrium melt geometry on shear wave velocities, while also showing that the effects of melting and dihedral angles are much smaller on the longitudinal waves. The quantitative effects of the melt fraction and dihedral angles on the acoustic wave velocities can be predicted theoretically using the elasticities of granular media derived as functions of grain-boundary contiguity [Takei, 1998]. The present experimental results described in this paper agree well with the theoretical predictions and demonstrate the validity of the theory. Clarifying the analogy and difference between the present organic system and the Earths materials, the shear and longitudinal wave velocities of the partially molten rocks in the Earth were predicted as functions of melt fraction, dihedral angle, and the compressibility ratio between solid and melt.

Experimental study of attenuation and dispersion over a broad frequency range: 2. The universal scaling of polycrystalline materials

attenuation QE1 were measured accurately over a broad frequency range (10 −4 ≤ f (Hz) ≤ 2.15) and at low strain amplitude (10 −5 –10 −6 ). Creep experiments were performed with the same apparatus to measure the steady state viscosity. Anelasticity and viscosity were measured at high homologous temperatures (T =2 2 –48°C; T/Tm = 0.61–0.67) and various grain sizes (3–22 mm), the growth of which was controlled by annealing. Using the measured viscosities h and the unrelaxed modulus EU determined from ultrasonic experiments, the frequency of the entire data set was normalized by the Maxwell frequency fM = EU/h, resulting in E and Q −1 master curves. The Q −1 data from previous studies on olivine‐dominated samples also collapse onto the same curve when scaled by fM,, demonstrating the universality of anelasticity for polycrystalline materials. The similitude by the Maxwell frequency scaling indicates that the dominant mechanism for the anelasticity observed in this study and in previous studies is diffusionally accommodated grain boundary sliding. A generalized formulation for this similitude is provided to extrapolate the experimental data to velocity and attenuation of seismic shear waves.

Temperature, grain size, and chemical controls on polycrystal anelasticity over a broad frequency range extending into the seismic range

Recent experimental studies have shown that anelasticity of polycrystalline materials is subject to the Maxwell frequency (fM) scaling: Q−1(f/fM). However, the applicability of this scaling to the seismic waves has not been guaranteed because experimental frequencies normalized to fM of the laboratory samples are usually much lower than the seismic frequencies normalized to fM in the upper mantle (106≤f/fM≤109). In this study, by using polycrystalline organic borneol as an analogue to mantle rock, we measured anelasticity up to f/fM≃108 and found that the Maxwell frequency scaling is not fully applicable at f/fM>104. A closer examination showed that each of the relaxation spectra obtained under various temperature, grain size, and chemical composition can be represented by the superposition of a background dissipation subject to the Maxwell frequency scaling and a peak dissipation centered at f/fM≃103. Significant increases of the peak amplitude and width with increasing temperature, grain size, and impurity content result in failure of the Maxwell frequency scaling at f/fM>104, where the peak dissipation dominates over the background dissipation. The peak is significantly broadened near the solidus temperature (T/Tmelt=0.93), producing an absorption band toward the normalized seismic frequencies. The absorption band suggested by the present data is characterized by variable amplitude and width, indicating a nonlinear reduction of seismic velocity near the solidus.

Polycrystal anelasticity at near-solidus temperatures†

Elasticity, anelasticity, and viscosity of polycrystalline aggregates were measured at the near-solidus temperatures ranging from below to above the solidus temperature (Tm). The result shows that the mechanical effects of the partial melting are twofold; changes just below the solidus temperature in the absence of melt and changes at the solidus temperature due to the onset of partial melting. As homologous temperature (T/Tm) increases from about 0.92 to 1, high-frequency part of the attenuation spectrum significantly grows. Viscosity of the grain boundary diffusion creep is also reduced in this temperature range. These changes are caused by a solid-state mechanism and have a large amplitude even for the samples which can generate very small amounts of melt at the solidus temperature. At the onset of melting, further increases in the elastic, anelastic, and viscous compliances occur. These changes are caused by the direct effects of the melt phase and are very small for the samples with very small melt fractions. Mechanical properties of a partially molten aggregate are determined by these twofold changes, and when melt fraction is small, the former changes are dominant. We performed a parameterization of the present experimental results and applied the obtained empirical formula to the seismic tomographic data in the upper mantle. The present model explains well the steep reduction of the seismic shear wave velocity in the oceanic lithosphere just below the solidus temperature.

Stress-induced anisotropy of partially molten media inferred from experimental deformation of a simple binary system under acoustic monitoring

Microstructural changes of partially molten media under deviatoric stress were investigated in a newly developed apparatus by deforming a large sample (a 70-mm cube) under a uniform pure shear stress. Borneol + melt system having a moderate dihedral angle and texturally equilibrated under hydrostatic stress was used as a partially molten rock analogue. The applied stress was small enough not to involve cataclastic-plastic deformation of the solid grains. Shear strain rate was about 10 -8 s -1 , and a stress exponent indicative of diffusion creep was obtained. During the deformation, sample microstructure was observed in situ by means of ultrasonic shear waves. The development of stress-induced anisotropy was successfully detected by shear wave splitting. The results obtained indicate that grain boundary contiguity in the direction of the least compressive stress (σ 3 ) was reduced with respect to the equilibrium texture and also that the relative values of liquid pressure and σ 3 play an essential role for development of anisotropy. The developed anisotropy persisted as long as deviatoric stress was applied, but the initial isotropic structure was recovered by releasing this stress. Several interesting phenomena were involved in the structural change; these include shear creep-induced dilatancy, strong dependence of the timescale of structural recovery on the amount of deformation (memory effect), and relaxation creep after releasing stress. Scaling considerations using the Griffith theory shows that the structural changes observed in the present experimental system are expected to occur in the Earth as well.

Experimental test of the viscous anisotropy hypothesis for partially molten rocks.

Significance Partially molten regions of Earth link mantle convection with surface volcanism. We present laboratory experiments and theory that are at the heart of understanding the connection between these interior and surface processes. The theory proposes that the presence of melt fundamentally changes the style of deformation, making it anisotropic with leading-order, testable consequences. The laboratory results are in agreement with theoretical predictions. These results are novel at a foundational level and profoundly surprising. Together they make the case that the creep rheology of partially molten rocks is more than the sum of its parts. Moreover, this article sets forth a framework that will guide a broad swath of future research in rock mechanics and mantle flow. Chemical differentiation of rocky planets occurs by melt segregation away from the region of melting. The mechanics of this process, however, are complex and incompletely understood. In partially molten rocks undergoing shear deformation, melt pockets between grains align coherently in the stress field; it has been hypothesized that this anisotropy in microstructure creates an anisotropy in the viscosity of the aggregate. With the inclusion of anisotropic viscosity, continuum, two-phase-flow models reproduce the emergence and angle of melt-enriched bands that form in laboratory experiments. In the same theoretical context, these models also predict sample-scale melt migration due to a gradient in shear stress. Under torsional deformation, melt is expected to segregate radially inward. Here we present torsional deformation experiments on partially molten rocks that test this prediction. Microstructural analyses of the distribution of melt and solid reveal a radial gradient in melt fraction, with more melt toward the center of the cylinder. The extent of this radial melt segregation grows with progressive strain, consistent with theory. The agreement between theoretical prediction and experimental observation provides a validation of this theory.