The phase assemblages of monticellite (CaMgSiO4) were investigated in the pressure range 80–300 kbar at about 1000°C in a diamond-anvil cell with laser heating. Incorporating earlier work, the following phase transformations are found: where the percentages give the decreases in zero-pressure volume for the new assemblage. If merwinite is a stable mantle mineral phase, even in very small quantities (～1 mole percent), the results imply that olivine might decrease substantially in amount in the lower part of the Earth's upper mantle. This study also suggests that the observed seismic discontinuities in the mantle should not be entirely attributed to high-pressure polymorphism or decompositions of individual mineral species, and that some discontinuities may result from chemical reactions between the individual phases. 相似文献
By incorporating the knowledge of the observed high-pressure phase transformations, the measured equilibrium phase boundaries for certain transitions, and the measured thermochemical data for certain phases in the system MgOSiO2, the equilibrium phase boundaries for all the phase transitions in this system at 1000°C have been calculated on the basis of an assumption that volume change across a phase boundary is independent of temperature. Three major seismic velocity discontinuities in the mantle (420, 570, and 650 km) have been chosen for comparison with the measured and calculated equilibrium phase boundaries for the high-pressure phases observed in the system MgOSiO2. Complications of phase changes due to the addition of FeO and Al2O3 to the system have also been accounted for. It is suggested by the results of this study that the 420- and 570-km discontinuities are probably related to phase transitions observed in the system MgOSiO2, but that the 650-km discontinuity is not likely to be associated with any of the equilibrium phase boundaries observed in olivine, pyroxene, and garnet, but may instead be a chemical change. 相似文献
Abstract Phase transformations in model mantle compositions and those in subducting slabs have been reviewed to a depth of 800 km on the basis of recent high-pressure experimental data. Seismic velocity and density profiles in these compositions have also been calculated using these and other mineral physics data. The nature of the seismic velocity and density profiles calculated for a pyrolite composition was found to generally agree with those determined by seismic observations (e.g. PREM). The locations of the seismic discontinuities at 400 and 670 km correspond almost exactly to the depths where the transformations of the olivine component to denser phases take place. Moreover, the steep gradients in the seismic velocity/density profiles observed between these depths are qualitatively consistent with those expected from the successive transformations in the complementary pyroxene-garnet component in the pyrolite composition. Further, the calculated seismic velocity and density values agree well with those observed in the upper mantle and mantle transition region within the uncertainties attached to these calculations and observations. Pyrolite or peridotite compositions are thus most likely to represent the composition of the mantle above 670 km depth, although some degrees of chemical heterogeneity may exist in the transition region. The observed sharp discontinuous increases of seismic velocities and density at this depth may be attributed either to the phase transformation to a perovskite-bearing assemblage in pyrolite or to chemical composition changes. Density profiles in subducted slabs have been calculated along adequate geotherms assuming that the slabs are composed of the former oceanic crust underlain by a thicker harzburgitic layer. It is shown that the former oceanic crust is substantially less dense than the surrounding pyrolite mantle at depths below 670 km, while it is denser than pyrolite in the upper mantle and the transition region. The subducted former oceanic crust may be trapped in this region, forming a geochemically enriched layer at the upper mantle-lower mantle boundary. Thick and cool slabs may penetrate into the lower mantle, but the chemically derived buoyancy may result in strong deformation and formation of megalith structures around the 670 km seismic discontinuity. These structures are consistent with those detected by recent seismic tomography studies for subduction zones. 相似文献
Fractional crystallization behaviour of a magma ocean extending to lower mantle depths was deduced from estimations of melting relations for the deep mantle and the density relationships between ultrabasic liquid and mantle minerals. The accretional growth of the Earth necessarily involves a molten zone (magma ocean) in the outer layer of the growing Earth. The fractionation by melting during accretion results in primary stratification composed of a molten ultrabasic upper mantle (magma ocean), a perovskite-rich lower mantle, and an iron core. A certain amount of Al2O3 and CaO was removed from the magma ocean and retained in the lower mantle due to eclogite fractionation in the early stage of accretion and the perovskite fractionation in the later stage of accretion. Models of the stratification of the upper mantle arising from fractional crystallization of the magma ocean and subsequent convective disturbance were deduced on the basis of estimations of melting relations for the deep mantle and the density relationships between the ultrabasic liquid and mantle minerals. The stratification of the mantle, which is consistent with geophysical constraints is as follows; the upper mantle is composed of two layers, the upper olivine-rich layer and the lower garnet-rich layer with a thickness around 200 km, and the lower mantle with a perovskite-rich composition. In this model, both the 400 and 650 km discontinuities are the chemical boundaries. 相似文献
This paper studied some properties of PdSwr phase related to 670 km discontinuities in detail, and theoretically processed a preliminary analysis to this phase. We discussed the relationships between the incident angle ih of PdSwr phase with its path, epicentral distance, travel-time and relative amplitude due to low velocity zone (LVZ) of upper mantle, and preliminarily pointed out the main characters of PdSwr phase recorded in seismogram. The PdSwr phase is concentrated in range of 13.5(～96.5(. When epicentral distance is greater than 33(, the start point of PdSwr phase is relatively well distinguishable and could thus be determined more easily. When the epicentral distance is between 13.5( to 33(, the triplication of PdSwr(s travel-time curve could be slightly distinguished due to the low velocity zone and 220 km seismic velocity discontinuity of upper mantle. The relevant observed PdSwr phase should be in a more complex pattern and it should be more difficult to determine its start point. 相似文献
We analyzed seismic waveforms recorded by the broadband ocean bottom seismographs deployed in the South Pacific superswell to determine the depths of the mantle discontinuities using a receiver function method. We estimated the thickness of the mantle transition zone (MTZ) to be 245 km on average beneath the superswell region, which is close to the global average. The MTZ is found to be thinned locally beneath the Society hot spot by 30 km. Temperature anomalies computed from the thinned transition zone and the Clapeyron slope of the olivine phase transformations are 150–200 K beneath the Society hot spot. Previous studies, using land-based data, suggested the presence of a hot MTZ beneath the Pitcairn hot spot. The locally hot transition zone beneath the hot spots is attributed to narrow mantle plumes probably rising from the lower mantle. The normal average thickness obtained from the present study indicates that there is no broad upwelling across the transition zone beneath the entire superswell region at present. 相似文献
Electrical conductivity structures of the Earth’s mantle estimated from the magnetotelluric and geomagnetic deep sounding
methods generally show increase of conductivity from 10−4–10−2 to 100 S/m with increasing depth to the top of the lower mantle. Although conductivity does not vary significantly in the lower
mantle, the possible existence of a highly conductive layer has been proposed at the base of the lower mantle from geophysical
modeling. The electrical properties of mantle rocks are controlled by thermodynamic parameters such as pressure, temperature
and chemistry of the main constituent minerals. Laboratory electrical conductivity measurements of mantle minerals have been
conducted under high pressure and high temperature conditions using solid medium high-pressure apparatus. To distinguish several
charge transport mechanisms in mantle minerals, it is necessary to measure the electrical conductivity in a wider temperature
range. Although the correspondence of data has not been yet established between each laboratory, an outline tendency of electrical
conductivity of the mantle minerals is almost the same. Most of mineral phases forming the Earth’s mantle exhibit semiconductive
behavior. Dominant conduction mechanism is small polaron conduction (electron hole hopping between ferrous and ferric iron),
if these minerals contain iron. The phase transition olivine to high-pressure phases enhances the conductivity due to structural
changes. As a result, electrical conductivity increases in order of olivine, wadsleyite and ringwoodite along the adiabat
geotherm. The phase transition to post-spinel at the 660 km discontinuity further can enhance the conductivity. In the lower
mantle, the conductivity once might decrease in the middle of the lower mantle due to the iron spin transition and then abruptly
increase at the condition of the D″ layer. The impurities in the mantle minerals strongly control the formation, number and
mobility of charge carriers. Hydrogen in nominally anhydrous minerals such as olivine and high-pressure polymorphs can enhance
the conductivity by the proton conduction. However, proton conduction has lower activation enthalpy compared with small polaron
conduction, a contribution of proton conduction becomes smaller at high temperatures, corresponding to the mantle condition.
Rather high iron content in mantle minerals largely enhances the conductivity of the mantle. This review focuses on a compilation
of fairly new advances in experimental laboratory work together with their explanation. 相似文献
It is demonstrated that in many cases the ratio of elastic velocities of polymorphs of a material depends primarily on the crystal structures involved, and only secondarily on the composition. Crucial to this demonstration is the use of estimates, from the Hashin-Shtrikman bounds, of the elastic properties of mixtures of simple oxides isochemical to particular compounds. Normalization to the oxide mixture properties displays the interrelationships between the densities and velocities of different crystal structures. A wide range of velocity-density trends through possible phase transitions is found. Transitions involving increases of Si coordination from 4 to 6 tend to have lower slopes. Velocity-density correlations are reasonably consistent with the average trends through phase transitions.Comparison of models of the transition zone with the characteristics of the olivine-spinel-oxides transitions indicates that in some studies the lower mantle density may have been underestimated or the mean atomic weight of the lower mantle overestimated. The transition zone of Model B1 of Jordan and Anderson, which was derived entirely from seismic data, is consistent, overall, with the olivine-oxides transition and uniform composition, but the individual discontinuities are not consistent with the olivine-spinel and spinel-oxides transitions. 相似文献
Cratons have a long history of evolution. In this paper, applications of the magnetotelluric method used in the study of craton lithosphere over the past 30 years were reviewed, examining case studies of cratons in North America, South America, Asia, Australia, and Africa. The nuclei of the Archean cratons, for example the Kalahari Craton and Rae Craton, are usually characterized by thick and highly resistive lithospheric roots. During or after the formation of the cratons, tectonothermal events, such as collision, mantle plume, and asthenosphere upwelling led to the formation of high-conductivity zones in the craton lithosphere, which could be attributed to the increased hydrogen content (of nominally anhydrous minerals), higher iron content, and formation of graphite films or sulfides along the grain boundary of minerals. These conductive zones are characterized by resistivity discontinuities in craton lithosphere. In particular, the conductive zones include (1) large-scale lithospheric mantle conductors beneath the Slave Craton, Gawler Craton, and central part of North China Craton(Trans-North China Orogen); (2) near-vertical high-conductivity zone associated with the fossil subduction zone beneath the Dharwar Craton and Slave Craton; and (3) regional lateral electrical discontinuities, such as a conductive anomaly under the Bushveld Complex of the Kaapvaal Craton. The eMoho refers to the electrical discontinuity in the crust-mantle boundary. In existing research, this has been detected under the condition of extremely high lithospheric resistivity with only a slight decrease in the lower crust, and in the case of a very thin conductive lower crust or the lack thereof. In the resistivity model, the unique “mushroom-like” lower crust-lithosphere mantle conductor and very thin lower crust layer of the North China Craton may represent lithosphere destruction and/or thinning. We also find that some of the cratons are still not well understood. Therefore, extensive three-dimensional inversion and joint interpretation of geochemical, geophysical, and geologic data are necessary to understand the tectonic evolutionary history of craton lithosphere.
High-pressure polymorphs of olivine and enstatite are major constituent minerals in the mantle transition zone(MTZ).The phase transformations of olivine and enstatite at pressure and temperature conditions corresponding to the lower part of the MTZ are import for understanding the nature of the 660 km seismic discontinuity.In this study,we determine phase transformations of olivine(MgSi2O4) and enstatite(MgSiO3) systematiclly at pressures between 21.3 and 24.4 GPa and at a constant temperature of 1600℃.The most profound discrepancy between olivine and enstatite phase transformation is the occurency of perovskite.In the olivine system,the post-spinel transformation occures at 23.8 GPa,corresponding to a depth of 660 km.In contrast,perovskite appears at 23 GPa(640 km) in the enstatite system.The ~1 GPa gap could explain the uplifting and/or splitting of the 660 km seismic discountinuity under eastern China. 相似文献
The phase behaviour of MSiO4 compounds (MHf, Zr, U and Th0 has been investigated at high pressures and temperatures in a diamond-anvil press coupled with laser heating. All of these compounds have been found to undergo two or perhaps three phase transformations at pressures below 300 kbar. The high-pressure phase transformations of these compounds differ from one another, with the exception of HfSiO4 and ZrSiO4, which undergo identical phase transformations. The ultimate phase assemblages of these compounds are of dense component dioxides (although this is yet to be confirmed in ThSiO4). It is suggested that the heat-producing elements U and Th would exist as dioxide solid solutions rather than silicates in the deep interior of the earth. Moreover, the densities of these dioxides are more than twice as great as mantle silicates and even slightly greater than pure iron under similar P, T conditions. Gravitational separation due to mandle convection may transport these dioxides to the deep interior of the earth to form deep heat sources. It is also suggested, however, that these deep heat sources are located in the inner-outer core boundary instead of in the lower mantle. 相似文献
The process of multiple self-nucleation and ascent of mantle plumes is studied in the numerical models of thermal convection.
The plumes are observed even in the simplest isoviscous models of thermal convection that leave aside the more complex rheology
of the material, thermochemical effects, phase transformations, etc., which, although controlling the features of plumes,
are not necessary for their formation. The origin of plumes is mainly due to the instability of the mantle flows at highly
intense (low-viscous) thermal convection. At high viscosity, convective flows form regular cells. As viscosity decreases,
the ascending and descending flows become narrower and unsteady. At a further decrease in viscosity, the ascending plumes
assume a mushroom-like shape and occasionally change their position in the mantle. The lifetime of each flow can attain 100
Ma. Using markers allows visualizing the evolution of the shape of the mantle plumes. 相似文献
Seismic data recorded in the upper mantle triplication distance range between 10° and 30° are generated by wave propagation through complex upper mantle structure. They can be used to place constraints on seismic velocity structures in the upper mantle, key seismic features near the major discontinuities, and anisotropic structure varying with depth. In this paper, we review wave propagation of the upper mantle triplicated phases, how different key seismic features can be studied using upper mantle triplicated data, and the importance of those seismic features to the understanding of mantle temperature and composition. We present two examples of using array triplicated phases to constrain upper mantle velocity structures and detailed features of a certain discontinuity, with one for a shallow event and the other for deep events. For the shallow event, we present examples of how the array triplication data can be used to constrain several key properties of the upper mantle: existence of a lithospheric lid, existence of a low velocity zone beneath the lithospheric lid, and P/S velocity ratio as a function of depth. For deep events, we show examples of how array triplication data can be used to constrain the detailed structures of a certain discontinuity: velocity gradients above and below the discontinuity, velocity jumps across the discontinuity and depth extents of different velocity gradients. We discuss challenges of the upper mantle triplication study, its connection to other approaches, and its potential for further studying some other important features of the mantle: the existence of double 660-km discontinuities, existence of low-velocity channels near major discontinuities and anisotropy varying with depth. 相似文献
Based on observations from a dense broadband seismic array located along the northeastern (NE) margin of the Tibetan Plateau in southeastern Gansu Province,we use receiver functions (RFs) to pick the arrival times of P-to-S converted waves and bin the traces in different grids according to the piercing points of the 410 and 660 km discontinuities in the upper mantle.The depths of the two discontinuities are estimated by the ray tracing method with the IASP91 velocity model and a 3-D tomography model.The results indicate the following:(1) The arrival times of the P410s and P660s converted phases are delayed by approximately 1 s than those predicted by the IASP91 model.The mantle transition zone (MTZ) is thicker than that in the global model.(2) The synchronous lags in the P410s and P660s arrival times are consistent with low-velocity anomalies in the upper mantle,which are believed to result mainly from the eastward migration of materials beneath the NE margin of the Tibetan Plateau.(3) Combined with previous tomography results,the depression of the'660'discontinuity and the thickened MTZ are somewhat consistent with the big mantle wedge (BMW) model.However,due to data limitations,more studies are required to explore the BMW in the future. 相似文献
Hydrous minerals are important water carriers in the crust and the mantle, especially in the subduction zone. With the recent development of the experimental technique, studies of the electrical conductivity of hydrous silicate minerals under controlled temperature, pressure and oxygen fugacity, have helped to constrain the water distribution in the Earth’s interior. This paper introduces high pressure and temperature experimental study of electrical conductivity measurement of hydrous minerals such as serpentine, talc, brucite, phase A, super hydrous phase B and phase D, and assesses the data quality of the above minerals. The dehydration effect and the pressure effect on the bulk conductivity of the hydrous minerals are specifically emphasized. The conduction mechanism of hydrous minerals and the electrical structure of the subduction zone are discussed based on the available conductivity data. Finally, the potential research fields of the electrical conductivity of hydrous minerals is presented. 相似文献
