共查询到20条相似文献,搜索用时 0 毫秒
1.
Tetsuya Komabayashi Kei Hirose Emiko Sugimura Nagayoshi Sata Yasuo Ohishi Leonid S. Dubrovinsky 《Earth and Planetary Science Letters》2008,265(3-4):515-524
Simultaneous volume measurements of MgSiO3 post-perovskite (PPv) and perovskite (Pv) were performed in a diamond anvil cell (DAC) combined with synchrotron X-rays. An externally-heated DAC was used in addition to a laser-heated DAC for the volume measurement experiment at high temperatures. The volume data were collected in the stability field of post-perovskite from 115 to 130 GPa. The temperature generated in the externally-heated and the laser-heated DACs for the volume measurement were up to 832 and 2330 K, respectively. Using two different but complementary heating techniques, we collected the data at a wide temperature range from 300 to 2330 K. The obtained P-V-T data for PPv and Pv were fitted to a third-ordered Birch-Murnaghan equation of state (EOS). For a precise comparison of the volume between the two phases, the EOSs were constructed based on the same pressure scale of MgO. The simultaneous volume measurements and the volumes calculated from the determined EOSs demonstrate that the volume difference between PPv and Pv of about 1.5% is almost constant with increasing temperature to 4000 K at the transition. At the base of the mantle, this density difference corresponds to a temperature anomaly of 1300 K without the phase transition due to the very small thermal expansivity of minerals, which has a significant effect on mantle dynamics. The thermal expansivity contrast between the top and the bottom of the mantle is a factor of 3.6. From a mantle convection study, this value suggests that huge and hot plumes are formed at the core–mantle boundary. 相似文献
2.
Shigehiko Tateno Kei Hirose Nagayoshi Sata Yasuo Ohishi 《Physics of the Earth and Planetary Interiors》2007,160(3-4):319-325
Phase relations in Mg0.5Fe0.5SiO3 and Mg0.25Fe0.75SiO3 were investigated in a pressure range from 72 to 123 GPa on the basis of synchrotron X-ray diffraction measurements in situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC). Results demonstrate that Mg0.5Fe0.5SiO3 perovskite is formed as a single phase at 85–108 GPa and 1800–2330 K, indicating a high solubility of FeO in (Mg,Fe)SiO3 perovskite at high pressures. Post-perovskite appears coexisting with perovskite in Mg0.5Fe0.5SiO3 above 106 GPa at 1410 K, the condition very close to the post-perovskite phase transition boundary in pure MgSiO3. The coexistence of perovskite and post-perovskite was observed to 123 GPa. In addition, post-perovskite was formed coexisting with perovskite also in Mg0.25Fe0.75SiO3 bulk composition at 106–123 GPa. In contrast to earlier experimental and theoretical studies, these results show that incorporation of FeO stabilizes perovskite at higher pressures. This could be due to a larger ionic radius of Fe2+ ion, which is incompatible with a small Mg2+ site in the post-perovskite phase. 相似文献
3.
本文从质量和电荷守恒方程出发,分析了控制电离层等离子体R-T不稳定性线性增长的各种因素,重点研究了热层风和背景电场的空间梯度对R-T不稳定性线性增长的影响.结果表明,热层风场和背景电场两者的空间梯度对R-T不稳定性线性增长有不可忽视的促进或抑制作用;对R-T不稳定性线性增长起促进作用还是起抑制作用,依赖热层风场和背景电场及其空间梯度的方向;对R-T不稳定性线性增长影响的显著程度主要依赖于热层风场和背景电场两者空间梯度的大小.数值计算结果表明,对典型的背景电离层条件,磁力线顶点高度为330 km时,对线性增长率的影响最高达到120%. 相似文献
4.
Here, we explore the transition from symmetric instability to ageostrophic baroclinic instability in the Eady model; an idealised representation of a submesoscale mixed layer front. We revisit the linear stability problem considered by Stone (J Atmos Sci, 23, 390–400, (Stone 1966)), Stone (J Atmos Sci, 27, 721–726, (Stone 1970)), Stone (J Atmos Sci, 29, 419–426, (Stone 1972)) with a particular focus on three-dimensional ‘mixed modes’ (which are neither purely symmetric or baroclinic) and find that these modes can have growth rates within just a few percent of the corresponding two-dimensional growth rate maximum. In addition, we perform very high resolution numerical simulations allowing an exploration of the transition from symmetric to baroclinic instability. Three-dimensional mixed modes represent the largest contribution to the turbulent kinetic energy during the transition period between symmetric and baroclinic instability. In each simulation, we see the development of sharp fronts with associated high rms vertical velocities of up to 30 mm s?1. Furthermore, we see significant transfer of energy to small scales, demonstrated by time-integrated mixing and energy dissipation by small-scale three-dimensional turbulence totalling about 30 % of the initial kinetic energy in all cases. 相似文献
5.
The new post-perovskite phase near the core-mantle boundary has important ramifications on lower mantle dynamics. We have investigated the dynamical impact arising from the interaction of temperature- and depth-dependent viscosity with radiative thermal conductivity, up to a lateral viscosity contrast of 104, on both the ascending and descending flows in the presence of both the endothermic phase change at 670 km depth and an exothermic post-perovskite transition at 2650 km depth. The phase boundaries are approximated as localized zones. We have employed a two-dimensional Cartesian model, using a box with an aspect-ratio of 10, within the framework of the extended Boussinesq approximation. Our results for temperature- and depth-dependent viscosity corroborate the previous results for depth-dependent viscosity in that a sufficiently strong radiative thermal conductivity plays an important role for sustaining superplumes in the lower mantle, once the post-perovskite phase change is brought into play. This aspect is especially emphasized, when the radiative thermal conductivity is restricted only to the post-perovskite phase. These results revealed a greater degree of asymmetry is produced in the vertical flow structures of the mantle by the phase transitions. Mass and heat transfer between the upper and lower mantle will deviate substantially from the traditional whole-mantle convection model. Streamlines revealed that an overall complete communication between the top and lower mantle is difficult to be achieved. 相似文献
6.
7.
《Earth and Planetary Science Letters》1985,72(4):433-439
Both single-crystal and powdered specimens of zircon (ZrSiO4) were shocked to peak pressures between 30 and 94 GPa using the gun method, and specimens recovered were studied by means of X-ray diffraction analysis, transmission electron microscopy and infrared spectroscopy. Transformation to the scheelite structure started above 30 GPa, and was completed above 53 GPa in the case of single crystal specimens. Tetragonal unit cell parameters of the scheelite type ZrSiO4 at room condition are measured to bea = 4.7341(1)Å, c = 10.51(1)Å, c/a = 2.219(2) andV = 235.5(2)Å3, which is smaller than that of the zircon type by 9.9%. The recovered scheelite-type ZrSiO4 reverts to the zircon type after rapid heating to 1200°C at room pressure. This transformation from the zircon type to the scheelite type is unique in that it is fast, displacive-like, but does not reverse. Tetragonal ZrO2 was detected as decomposition product in the single-crystal specimen shocked to 94 GPa, and further confirmed in a powdered specimen shocked to 53 GPa where enhancement of temperature is expected because of high porosity. Decomposition behavior of zircon observed in natural shock events is discussed on the basis of present experimental results. 相似文献
8.
Krystle Catalli Sang-Heon Shim Vitali B. Prakapenka Jiyong Zhao Wolfgang Sturhahn Paul Chow Yuming Xiao Haozhe Liu Hyunchae Cynn William J. Evans 《Earth and Planetary Science Letters》2010,289(1-2):68-75
Recent studies have indicated that a significant amount of iron in MgSiO3 perovskite (Pv) is Fe3+ (Fe3+/ΣFe = 10–60%) due to crystal chemistry effects at high pressure (P) and that Fe3+ is more likely than Fe2+ to undergo a high-spin (HS) to low-spin (LS) transition in Pv in the mantle. We have measured synchrotron Mössbauer spectroscopy (SMS), X-ray emission spectroscopy (XES), and X-ray diffraction (XRD) of Pv with all iron in Fe3+ in the laser-heated diamond-anvil cell to over 100 GPa. Fe3+ increases the anisotropy of the Pv unit cell, whereas Fe2+ decreases it. In Pv synthesized above 50 GPa, Fe3+ enters into both the dodecahedral (A) and octahedral (B) sites approximately equally, suggesting charge coupled substitution. Combining SMS and XES, we found that the LS population in the B site gradually increases with pressure up to 50–60 GPa where all Fe3+ in the B site becomes LS, while Fe3+ in the A site remains HS to at least 136 GPa. Fe3+ makes Pv more compressible than Mg-endmember below 50 GPa because of the gradual spin transition in the B site together with lattice compression. The completion of the spin transition at 50–60 GPa increases bulk modulus with no associated change in density. This elasticity change can be a useful seismic probe for investigating compositional heterogeneities associated with Fe3+. 相似文献
9.
S. Demouchy D. Mainprice A. Tommasi H. Couvy F. Barou D.J. Frost P. Cordier 《Physics of the Earth and Planetary Interiors》2011,184(1-2):91-104
We have studied the phase transformation of forsterite to wadsleyite under shear stress at the Earth's transition zone pressure and temperature conditions. Two-step experiments were performed using a multi-anvil press. First, we hot pressed iron-free forsterite at 6 or 11 GPa and 1100 °C. Then we deformed a slab of this starting material using a direct simple shear assembly at 16 GPa and 1400 °C for 1, 15, 35, 40, or 60 min. Both the starting material and the deformed samples were characterized using optical and scanning electron microscopy including measurements of crystal preferred orientations (CPO) by electron back scattered diffraction (EBSD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). The phase transformation occurs very rapidly, in less than 1 min, and metastable forsterite relics are not observed after deformation. The grain size of wadsleyite is slightly smaller than the forsterite starting material. The water contents obtained from FTIR analyses in forsterite and wadsleyite are 65–124 wt ppm H2O and 114–736 wt ppm H2O, respectively, which are well below water solubility at similar conditions in the presence of free water. Wadsleyite aggregates display weak CPO patterns with [1 0 0] axes concentrated at low angle to the shear direction, [0 1 0] axes perpendicular to the shear plane and nearly random [0 0 1] axes. Only a few dislocations were observed in wadsleyite with TEM. This observation is consistent with the assumption that most dislocations formed during the initial high-stress stages of these stress-relaxation experiments, were consumed in the phase transformation, probably enhancing the transformation rate. CPO patterns vary as a function of the water content: with increasing water content the density of [1 0 0] axes parallel to the shear direction decreases, and the density of [0 0 1] axes increases. Viscoplastic self-consistent modeling of CPO evolution using previously reported glide systems for wadsleyite, i.e., [1 0 0]{0 k l} and 1/2 〈1 1 1〉{1 0 1}, cannot reproduce the measured CPO, unless the [0 0 1](0 1 0) system, for which dislocations have not been observed by TEM, is also activated. In addition, wadsleyite grain growth suggests the participation of diffusion-assisted processes in deformation. Calculated anisotropies for P and S-waves using measured CPO are always below 1%. This very low anisotropy is due to both the low finite strain achieved in the experiments, which leads to weak wadsleyite CPO, and to the diluting effect of added majorite. The present experiments emphasize the importance of stress, grain size evolution and water content in the forsterite to wadsleyite phase transformation and subsequent deformation in the transition zone. 相似文献
10.
V. P. Trubitsyn A. N. Evseev A. A. Baranov A. P. Trubitsyn 《Izvestiya Physics of the Solid Earth》2007,43(12):981-991
An endothermic phase transition at a depth of 660 km in the mantle partially slows down mantle flows. Many models considering the possibility of temporary layering of flows with separation of convection in the upper and lower mantle have been constructed over the past two decades. The slowing-down effect of the endothermic phase transition is very sensitive to the slope of the phase-equilibrium curve. However, laboratory measurements contain considerable uncertainties admitting both a partial convection layering and only an insignificant slowing down of a part of downgoing mantle flows. In this work, we present results of calculations of mantle flows within a wide range of phase-transition parameter values, determine ranges of one-and two-layer convection, and derive dependences of the amplitude and period of oscillations on phase-transition parameters. 相似文献
11.
双流系统产生的不稳定性过程是近地空间中常见的等离子体不稳定性现象.地球磁尾、等离子体片边界层、地球弓激波、太阳风、极隙区和极光加速区观测到的静电孤立波被认为是由双流不稳定性产生.地球空间环境并非均匀稳态, 因此对双流系统演化过程的研究往往需要考虑初始的扰动.利用所建立的动理论粒子云模型方法, 我们研究了初始大扰动下双流系统的不稳定性现象, 发现初始大扰动条件下粒子相空间产生振荡现象, 且振荡频率与初始扰动的空间波数有关, 电场与电场能量也存在该频率成分.通过线性理论, 求解出了粒子相空间振荡频率.它与粒子云模拟得到的频率一致.该频率为朗缪尔振荡频率的基础之上叠加一个与初始扰动的空间波数有关的频率偏移量.我们改变等离子体频率, 进一步验证了上述结论.该研究结果表明初始扰动能够对双流系统的不稳定性现象产生重要影响, 对于加深理解地球磁层中双流系统的演化过程有重要参考意义.
相似文献12.
A surprisingly simple and precise major element mass balance is consistent with derivation of average upper mantle peridotite from a partially molten chondritic Earth by subtraction of perovskite and addition of olivine. Majorite involvement is precluded unless some as yet unidentified components play a role. Perovskite subtraction during a primordial melting event is expected to occur by crystal fractionation at depth, while olivine addition is accomplished by a combination of buoyancy mechanisms: crystal flotation from a deep layer of melt buried by its own compressibility to the base of the solidifying upper mantle and subsequent solid state convection of this buoyant magnesian olivine upward. These processes are consistent with known density relations of crystals and liquid at very high pressure. Mass balance predicts that the residual magma body at depth after supplying olivine by flotation upward can be komatiitic. Distribution of originally C1 chrondritic bulk Earth material a few 100 m.y. after primordial differentiation is solid peridotite upper mantle, perovskite lower mantle, and a komatiitic liquid sandwich horizon. 相似文献
13.
《Physics of the Earth and Planetary Interiors》2002,129(1-2):1-11
Phase transformation between high-temperature clinoenstatite (HT-CEn) and high-pressure clinoenstatite (HP-CEn) was studied using molecular dynamics (MD) simulations. Starting from HT-CEn, the MD simulations of isothermal compression were carried out at different temperatures (at 2800, 2500, and 2000 K). At 2800 and 2500 K, the transformation into HP-CEn occurred directly at high pressure and the discontinuous change of the MD-simulated cell volume between HT-CEn and HP-CEn showing an isosymmetric or first-order transition was observed. At 2000 K, on the other hand, low-temperature clinoenstatite (LT-CEn) appears between HT-CEn and HP-CEn. The crystal structures of three clinoenstatites (LT-CEn, HT-CEn, and HP-CEn) were compared one another, and it was also confirmed that the switching among the bonds between Mg atoms in the M2 sites and the coordinated O3 atoms occurs during the transitions among the three CEn’s. A phase relation among these three CEn’s is also proposed, although the stability fields of protoenstatite and orthoenstatite are appeared at low pressure. 相似文献
14.
15.
Determination of phase transition pressures of ZnTe under quasihydrostatic conditions 总被引:1,自引:0,他引:1
Keiji Kusaba Laurence Galoisy Yanbin Wang Michael T. Vaughan Donald J. Weidner 《Pure and Applied Geophysics》1993,141(2-4):643-652
Pressure behavior of ZnTe at room temperature was studied using an X-ray energy dispersive method on a DIA type cubic anvil apparatus (SAM-85) at NSLS-X17B1. By using powdered polyethylene, the sample and NaCl for a pressure scale were held under quasihydrostatic conditions, which were confirmed by X-ray diffraction method. Two high-pressure phase transitions were confirmed using X-ray powder diffraction simultaneously with electrical resistance measurements. The phase transition pressures under quasihydrostatic conditions were determined to be 9.6 GPa, at which the resistance increased, and 12.0 GPa, which was the midpoint of a large resistance decrease. Errors in the pressure determinations were estimated to be less than 0.2 GPa. These pressure values may depend on grain size and anisotropic stress effects on the calibrant. From X-ray observation of ZnTe, the bulk modulus of the zinc blende structure was calculated to beK
0=51(3) GPa andK
0
=3.6(0.8), and the first transition at 9.6 GPa was found to have about 9% volume change. It was consistent with an anomaly in the pressure generating curves. 相似文献
16.
Caroline Beghein J. Arthur Snoke Matthew J. Fouch 《Earth and Planetary Science Letters》2010,289(3-4):467-478
We present fundamental-mode Rayleigh-wave azimuthally anisotropic phase velocity maps obtained for the Great Basin region at periods between 16 s and 102 s. These maps offer the first depth constraints on the origin of the semi-circular shear-wave splitting pattern observed in central Nevada, around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached. Our phase velocity study helps constrain the three-dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western United States. The dispersion measurements were made using data from the USArray Transportable Array. At periods of 16 s and 18 s, which mostly sample the crust, we find a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semi-circular pattern of fast seismic directions. Away from central Nevada the fast directions are ~ N–S in the eastern Great Basin, NW–SE in the Walker Lane region, and they transition from E–W to N–S in the northwestern Great Basin. Our short-period phase velocity maps, combined with recent crustal receiver function results, are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. At longer periods (28–102 s), which sample the uppermost mantle, isotropic phase velocities are significantly reduced across the study region, and fast directions are more uniform with an ~ E–W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere–asthenosphere boundary at depths of ~ 60 km. We interpret the fast seismic directions observed at longer periods in terms of present-day asthenospheric flow-driven deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere. Our results also provide context to regional SKS splitting observations. We find that our short-period phase velocity anisotropy can only explain ~ 30% of the SKS splitting times, despite similar patterns in fast directions. This implies that the origin of the regional shear-wave splitting signal is complex and must also have a significant sublithospheric component. 相似文献
17.
Influence of an endothermic phase transition on mass transfer between the upper and the lower mantle 总被引:1,自引:1,他引:0
V. P. Trubitsyn A. N. Evseev A. A. Baranov A. P. Trubitsyn 《Izvestiya Physics of the Solid Earth》2008,44(6):443-455
Geochemical data indicate that two major reservoirs 1–2 Ga in age are present in the mantle. The upper mantle, feeding mid-ocean ridges, is depleted in chemical elements carried away into the continental crust. The lower mantle, feeding hotspot plumes, is close in composition to primordial matter. The 660-km depth of an endothermic phase transition in olivine has been considered over the last two decades as a possible boundary between the reservoirs. In this period, many models of mantle convection were constructed that used values of the phase transition parameters, which led to temporal (up to 1 Gyr long) convection layerings and periodic avalanche-induced mantle intermixing events. However, laboratory measurements with new improved instrumentation give other values of the phase transition parameters that require a revision of the majority of the existence of large-scale avalanches in the Earth’s history becomes disputable. The paper is devoted to comprehensive study of the phase transition effect on the structure of mantle flows with different values of phase transition parameters and Rayleigh numbers; in particular, the mass transfer through the phase boundary is calculated for different regimes of steady-state convection. 相似文献
18.
Experimental study of spinel-garnet phase transition in upper mantle and its significance 总被引:3,自引:0,他引:3
Qicheng Fan Ruoxin Liu Hongsen Xie Yueming Zhang Ping Xu Zhuoran Lin 《中国科学D辑(英文版)》1997,40(4):383-389
Experimental study of spinel-garnet phase transition was carried out using natural mineral and rock specimens from xenolith
of mantle rocks in Cenozoic basalt as starting materials. From the result it was found that the condition of spinel Iherzolite-garnet
Ihenolite phase transition (T = 1 100°C andP = 1.8–2.0 GPa) is consistent with theP-T equilibrium condition of the five-phase assemblage spinel/garnet Iherzolite in eastern China, suggesting that there may exist
a spinel-garnet Iherzolite phase transition zone with the thickness of a few km to several ten km at the depth of 55–70 km
in the continental upper mantle of eastern China. The depth of phase transition from spinel pyroxenite to garnet pyroxenite
is found to be less than 55 km. Experiment results also show that water promotes metasomatism on one hand but suppresses phase
transition on the other. Zoning of mineral composition was also discussed.
Project supported by the National Natural Science Foundation of China. 相似文献
19.
The theory of relaxation of geomagnetic depression observed during magnetic storm has been proposed based on the knowledge of the interaction between the Dst index and the magnetotail current. It has been indicated that the disruption of the tail current that was enhanced during the storm main phase can be caused by the interchange instability, which develops in the boundary plasma sheet due to the sheet curvature in the near to the Earth region and due to a sharp, directed toward the curvature center, plasma pressure gradient at the boundary between the plasma sheet and the tail lobe. The dispersion equation and expression for the instability growth rate have been obtained. The theoretically predicted characteristic time of storm depression relaxation τ is ~10 h and is in good agreement with the experimental estimate. 相似文献
20.
《Physics of the Earth and Planetary Interiors》2002,129(1-2):131-143
The phase diagram for calcite (CaCO3) is re-evaluated in relation to dynamic compression and following release from shock. Available shock compression data on Hugoniot dynamic measurements, analysis of recovered samples, and observation at terrestrial impact sites are compared with theoretically derived equations of state (EOS) for CaCO3 and its decomposition products CaO and CO2. The study results in a refined phase diagram for CaCO3 in which the major change is the extension of the liquid field of CaCO3. A general outcome of this analysis is that release of CO2 from naturally shocked carbonates to the atmosphere is (grossly) overestimated if based on the calcite phase diagram constructed from thermodynamic equilibrium conditions. 相似文献