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1.
Electrical conductivity of olivine, wadsleyite, and ringwoodite under upper-mantle conditions 总被引：1，自引：0，他引：1
Geophysical models show that electrical conductivity in Earth's mantle rises about two orders of magnitude through the transition zone in the depth range 410 to 660 kilometers. Impedance measurements obtained on Mg1.8Fe0.2SiO4 olivine, wadsleyite, and ringwoodite at up to 20 gigapascals and 1400 degreesC show that the electrical conductivities of wadsleyite and ringwoodite are similar and are almost two orders of magnitude higher than that of olivine. A conductivity-depth profile to 660 kilometers, based on these laboratory data, shows a conductivity increase of almost two orders of magnitude across the 410-kilometer discontinuity; such a profile favors a two-layer model for the upper mantle. Activation enthalpies of 1.2 to 1.7 electron volts permit appreciable lateral variations of conductivity with lateral temperature variations. 相似文献
2.
In experiments at 13.5 gigapascals and 1030 degreesC, the growth rate of wadsleyite, which forms from transformation of olivine, was substantially enhanced by the presence of water. Wadsleyite had a low dislocation density and subgrain boundaries in wet runs. Water enhanced the dislocation recovery in wadsleyite and therefore caused inelastic relaxation of the localized pressure drop associated with the transformation, resulting in an increase of the growth rate in wet runs. These results imply that even a small amount of water of 0. 05 weight percent can weaken wadsleyite in the mantle. 相似文献
3.
The single-crystal elastic moduli of the modified spinel structure (beta phase) of magnesium orthosilicate (Mg(2)SiO(4)) have been measured by Brillouin spectroscopy under ambient conditions. Single crystals with dimensions up to 500 micrometers were grown at 22 gigapascals and 2000 degrees C over a period of 1 hour. Growth of crystals larger than 100 micrometers was achieved only when the pressure was within 5 percent of the pressure of the phase boundary separating the beta- and gamma-phase stability fields. A comparison of the elastic properties of the modified spinel phase with those of the olivine phase suggests that the 400-kilometer seismic discontinuity in the earth's mantle can be described by a mantle with 40 percent olivine. These results confirm that the 400-kilometer discontinuity can be due to the transition from olivine to modified spinel. The amount of olivine that must be present is less than that in a pyrolite model, although the results do not exclude pyrolite as a possible mantle model. 相似文献
4.
The independent elastic constants of an upper mantle mineral, San Carlos olivine [(Mg(1.8)Fe(0.2))SiO(4)], were measured from 0 to 12.5 gigapascals. Evidence is offered in support of the proposition that the explicit temperature dependence of the bulk modulus is small over the range of temperatures and pressures thought to prevail above the 400-kilometer discontinuity, and thus the data can be extrapolated to estimate the properties of olivine under mantle conditions at a depth of 400 kilometers. In the absence of high-temperature data at high pressures, estimates are made of the properties of olivine under mantle conditions to a depth of 400 kilometers. In contrast with low-pressure laboratory data, the predicted covariance of shear and compressional velocities as a function of temperature nearly matches the seismically estimated value for the lower mantle. 相似文献
5.
Upper mantle electrical conductivities can be explained by hydrogen diffusivity in hydrous olivine. Diffusivity enhances the conductivity of olivine anisotropically, making the a axis the most conductive of the three axes. Therefore, the hypothesis that plate motion induces lattice-preferred orientation of olivine can be tested with the use of long-period electromagnetic array measurements. Here, we compared electrical anisotropies below the slow-moving Fennoscandian and fast-moving Australian plates. The degree of olivine alignment is greater in the mantle below the Fennoscandian plate than below the Australian plate. This finding may indicate that convection rather than plate motion is the dominant deformation mechanism. 相似文献
6.
Wood BJ 《Science (New York, N.Y.)》1995,268(5207):74-76
The 410-kilometer seismic discontinuity is generally considered to be caused by a phase transformation of the main constituent of the upper mantle, olivine, alpha-(Mg,Fe)(2)SiO(4), to beta-(Mg,Fe)(2)SiO(4). Recent data show that H(2)O dissolves in olivine and other nominally anhydrous mantle minerals and that the partitioning of H(2)O between olivine and beta-(Mg,Fe)(2)SiO(4) is about 1:10. Such behavior strongly affects the region over which the alpha to beta phase transformation occurs and hence the seismic discontinuity that results. The observed width of the discontinuity constrains the maximum H(2)O content of upper mantle olivine to about 200 parts per million by weight. 相似文献
7.
Water-induced fabric transitions in olivine 总被引：5，自引：0，他引：5
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. 相似文献
8.
Although enstatite is a major constituent of the Earth's upper mantle and subducting lithosphere, most kinetic studies of olivine phase transformations have typically involved single-phase polycrystalline aggregates. Transmission electron microscopy investigations of olivine to spinel and modified spinel (beta phase) reactions in the (Mg, Fe)(2)SiO(4)-(Mg,Fe)SiO(3) system show that transformation of olivine in the stability field of spinel plus phase begins with coherent nucleation of spinel on high-clinoenstatite grains. These observations demonstrate that high clinoenstatite can catalyze the transformation by enhancing nucleation kinetics and therefore imply that secondary phases can influence reaction kinetics during high-pressure mineral transformations. 相似文献
9.
Garnet has been identified for the first time as a meteorite mineral in the Coorara chondrite from Western Australia. It replaces olivine grains in a 1-millimeter veinlet traversing the body of the meteorite. The associated olivine has abnormally low birefringence, which suggests a highly shocked condition. Microprobe analyses do not distinguish the garnet from the associated olivine, which has the composition (Mg(.75)Fe(.25))(2) SiO(4); the garnet may have the composition Mg(3)Fe(2)Si(3)O(12) but be unresolvable from the accompanying olivine, or alternatively is nonstoichiometric. Transformation of olivine to garnet under high pressure could have significant implications for the phase composition of the Earth's mantle. 相似文献
10.
Seismic evidence for olivine phase changes at the 410- and 660-kilometer discontinuities 总被引：1，自引：0，他引：1
The view that the seismic discontinuities bounding the mantle transition zone at 410- and 660-kilometer depths are caused by isochemical phase transformations of the olivine structure is debated. Combining converted-wave measurements in East Asia and Australia with seismic velocities from regional tomography studies, we observe a correlation of the thickness of, and wavespeed variations within, the transition zone that is consistent with olivine structural transformations. Moreover, the seismologically inferred Clapeyron slopes are in agreement with the mineralogical Clapeyron slopes of the (Mg,Fe)2SiO4 spinel and postspinel transformations. 相似文献
11.
Basu AR Poreda RJ Renne PR Teichmann F Vasiliev YR Sobolev NV Turrin BD 《Science (New York, N.Y.)》1995,269(5225):822-825
An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a (40)Ar/(39)Ar plateau age of 253.3 +/- 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed (3)He/(4)He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-(3)He SFBP plume with a suboceanic-type upper mantle beneath Siberia. 相似文献
12.
We applied global waveform tomography to model radial anisotropy in the whole mantle. We found that in the last few hundred kilometers near the core-mantle boundary, horizontally polarized S-wave velocities (VSH) are, on average, faster (by approximately 1%) than vertically polarized S-wave velocities (VSV), suggesting a large-scale predominance of horizontal shear. This confirms that the D" region at the base of the mantle is also a mechanical boundary layer for mantle convection. A notable exception to this average signature can be found at the base of the two broad low-velocity regions under the Pacific Ocean and under Africa, often referred to as "superplumes," where the anisotropic pattern indicates the onset of vertical flow. 相似文献
13.
Farla RJ Jackson I Fitz Gerald JD Faul UH Zimmerman ME 《Science (New York, N.Y.)》2012,336(6079):332-335
Crystal defects form during tectonic deformation and are reactivated by the shear stress associated with passing seismic waves. Although these defects, known as dislocations, potentially contribute to the attenuation of seismic waves in Earth's upper mantle, evidence for dislocation damping from laboratory studies has been circumstantial. We experimentally determined the shear modulus and associated strain-energy dissipation in pre-deformed synthetic olivine aggregates under high pressures and temperatures. Enhanced high-temperature background dissipation occurred in specimens pre-deformed by dislocation creep in either compression or torsion, the enhancement being greater for prior deformation in torsion. These observations suggest the possibility of anisotropic attenuation in relatively coarse-grained rocks where olivine is or was deformed at relatively high stress by dislocation creep in Earth's upper mantle. 相似文献
14.
The lower mantle of the Earth is believed to be largely composed of (Mg,Fe)O (magnesiowustite) and (Mg,Fe)SiO3 (perovskite). Radiative temperatures of single-crystal olivine [(Mg0.9,Fe0.1)2SiO4] decreased abruptly from 7040 +/- 315 to 4300 +/- 270 kelvin upon shock compression above 80 gigapascals. The data indicate that an upper bound to the solidus of the magnesiowustite and perovskite assemblage at 4300 +/- 270 kelvin is 130 +/- 3 gigapascals. These conditions correspond to those for partial melting at the base of the mantle, as has been suggested occurs within the ultralow-velocity zone beneath the central Pacific. 相似文献
15.
Melt segregation and strain partitioning: implications for seismic anisotropy and mantle flow 总被引：2，自引：0，他引：2
Holtzman BK Kohlstedt DL Zimmerman ME Heidelbach F Hiraga T Hustoft J 《Science (New York, N.Y.)》2003,301(5637):1227-1230
One of the principal means of understanding upper mantle dynamics involves inferring mantle flow directions from seismic anisotropy under the assumption that the seismic fast direction (olivine a axis) parallels the regional flow direction. We demonstrate that (i) the presence of melt weakens the alignment of a axes and (ii) when melt segregates and forms networks of weak shear zones, strain partitions between weak and strong zones, resulting in an alignment of a axes 90 degrees from the shear direction in three-dimensional deformation. This orientation of a axes provides a new means of interpreting mantle flow from seismic anisotropy in partially molten deforming regions of Earth. 相似文献
16.
基于FVCOM数值模型,以宁波-舟山海域相关实测资料为支撑,选取环氧氯丙烷为典型溶解型危化品,模拟该海域突发溶解型危化品泄漏后的扩散行为。模拟结果表明,从不同事故等级来看,一般事故和较大事故危化品泄漏24 h后,环氧氯丙烷浓度基本扩散至安全浓度(0.02 mg/L)以下,重大事故泄漏24 h后仍存在较大(30~190 km2)超安全浓度污染水域;从不同扩散潮时来看,落潮时刻扩散快于涨潮时刻,落潮时刻扩散至安全浓度以下较涨潮时刻快2~6 h;从不同泄漏点来看,册子水道扩散快于镇海码头,其中在册子水道泄漏时,基本沿水道扩散,在镇海码头泄漏时,易在北仑区沿岸、北仑和大榭岛之间滞留。 相似文献
17.
Near the Mantle Electromagnetic and Tomography (MELT) Experiment, seamounts form and off-axis lava flows occur in a zone that extends farther to the west of the East Pacific Rise than to the east, indicating a broad, asymmetric region of melt production. More seamounts, slower subsidence, and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west. Variations in axial ridge shape, axial magma chamber continuity, off-axis volcanism, and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17 degrees15'S than beneath 15 degrees55'S. Recent volcanism occurs above mantle with the lowest seismic velocities. 相似文献
18.
The Hawaiian hotspot is often attributed to hot material rising from depth in the mantle, but efforts to detect a thermal plume seismically have been inconclusive. To investigate pertinent thermal anomalies, we imaged with inverse scattering of SS waves the depths to seismic discontinuities below the Central Pacific, which we explain with olivine and garnet transitions in a pyrolitic mantle. The presence of an 800- to 2000-kilometer-wide thermal anomaly (ΔT(max) ~300 to 400 kelvin) deep in the transition zone west of Hawaii suggests that hot material does not rise from the lower mantle through a narrow vertical plume but accumulates near the base of the transition zone before being entrained in flow toward Hawaii and, perhaps, other islands. This implies that geochemical trends in Hawaiian lavas cannot constrain lower mantle domains directly. 相似文献
19.
Fe-Mg interdiffusion coefficients for (Mg,Fe)SiO3 perovskite have been measured at pressures of 22 to 26 gigapascals and temperatures between 1973 and 2273 kelvin. Perovskite Fe-Mg interdiffusion is as slow as Si self-diffusion and is orders of magnitude slower than Fe-Mg diffusion in other mantle minerals. Length scales over which chemical heterogeneities can homogenize, throughout the depth range of the lower mantle, are limited to a few meters even on time scales equivalent to the age of Earth. Heterogeneities can therefore only equilibrate chemically when they are stretched and thinned by intense deformation. 相似文献
20.
Experimental determination of oxygen self-diffusion in CaTiO(3) perovskite, a structural analog of (Mg,Fe)SiO(3) perovskite, confirms a theoretical relation between diffusion constants and anion porosity. Oxygen diffusion rates in (Mg,Fe)SiO(3) perovskite calculated with this relation increase by about eight orders of magnitude through the lower mantle. Electrical conductivity values calculated from these diffusion rates are consistent with inferred conductivity values for the lower mantle. This result suggests that the dominant conductivity mechanism in the deep mantle is ionic. 相似文献