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1.
Motohiko Murakami Yasuo Ohishi Naohisa Hirao Kei Hirose 《Earth and Planetary Science Letters》2009,277(1-2):123-129
The aggregate shear wave velocities of MgO (periclase) have been determined throughout Earth's lower mantle pressure regime approaching 130 GPa using Brillouin spectroscopy in conjunction with synchrotron X-ray diffraction technique in a diamond anvil cell apparatus. We found that the extrapolations of the high-pressure shear wave velocities and shear moduli to ambient pressure are highly consistent with earlier studies. However, the measurements over a wide pressure range revealed that the pressure derivative of the shear modulus (dG/dP = G0′) of MgO is 1.92(2), which is distinctly lower than that of previous lower-pressure experiments. Compared with the previous results on (Mg,Fe)O ferropericlase, there is no clear correlation between iron content and G0′. We calculate that the shear wave velocity profile of lower mantle along the adiabatic geotherm applied by the lower G0′ value of periclase can remarkably well reproduce the global seismological 1-D velocity profile model with uniform composition model. The best-fitting result indicates the possibility of a lower mantle mineralogy with ~ 92 vol.% silicate perovskite phase, implying that the bulk composition of lower mantle is likely not to be pyrolitic but more chondritic. The present acoustic measurements performed over the large pressure range have thus led us to a better understanding of compositional model of the Earth's lower mantle. 相似文献
2.
Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A SmNd mineral isochron for a plagioclase lherzolite yields an age,T = 427 ± 32 Ma and initialεNd = + 10.4 ? 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzoliteP-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The SmNd model age of the plagioclase lherzolite totalrock isTCHURNd = 3.4 AE. This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a SmNd mineral isochron age ofT = 435 ± 21 Ma andεNd = + 6.7 ? 0.3. A pyroxenite dike yields an initialεNd = + 7.3 ± 0.4. The initialεNd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. RbSr data do not give any coherent pattern. However, some bounds can be put on initial Sr values ofεSr ? ?21 for the plagioclase lherzolite andεSr ? ?8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and SmNd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ～ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (～ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ～ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude. 相似文献
3.
Attenuation of compressional waves in peridotite measured as a function of temperature at 200 MPa 总被引：3，自引：0，他引：3
Hiroki Sato I. Selwyn Sacks Tsutomu Murase Gregory Muncill Hiroyuki Fukuyama 《Pure and Applied Geophysics》1988,128(1-2):433-447
A technique has been developed to determine attenuation in rocks at high temperature using a gas-media, high-pressure apparatus. A pulse transmission technique and a spectral ratio method are used to study compressional seismic properties of rocks. Seismic waves are transmitted to and from the sample through buffer rods of mullite. The effect of seismic wave reflections within the sample assembly are cancelled out by taking ratios of the spectra measured at different temperatures. In order to obtain good signal-to-noise ratio for resolving the attenuation at high pressure and temperature, special care is taken in the sample assembly and the ultrasonic coupling between the sample, buffer rods and transducers. A very tight connection of the sample-buffer rod-transducer is essential for obtaining high frequency signals (>300 kHz) at high temperature. A small mass is attached to each outside end of the transducer to drive low frequency signals (<250 kHz) into the sample. Before attenuation measurements, the sample and the buffer rods are tightly compacted in a platinum tube at high pressure and room temperature to ensure pressure seal of the sample assembly. The frequency range of measurement covers 50 to 450 kHz for the sample. Attenuation is very small in the buffer rod compared to the sample for the entire temperature range of the study. Because of the small attenuation, a wide frequency band of 50 kHz to 3.2 MHz can be covered for investigating the attenuation in the buffer rod. The technique has been used to measure attenuation at high confining pressure, and temperatures including sub- and hyper-solidus of upper mantle rocks. Therefore, effects of partial melting on attenuation can be studied.The method is applied to the attenuation measurement in a peridotite as a function of temperature to 1225°C at 200 MPa confining pressure. At high temperature, signal amplitude decays more rapidly at high frequency than at low frequency, from which attenuation (andQ) can be determined using a spectral ratio method. No frequency dependence ofQ is resolved for both the sample and the buffer rod over the entire temperature and frequency ranges of the measurement. The results show thatQ decreases rapidly with increasing temperature even in the temperature range below the solidus of peridotites. Such temperature sensitivity ofQ is probably more useful to probe thermal structure in the upper mantle than that of conductivity at temperatures below the solidus. The results in this study are compared with available seismic velocity, electrical conductivity and solidus data for peridotites, suggesting that there is no discontinuous change in both mechanical and electrical properties of peridotites at the solidus temperature. Even at hypersolidus temperatures, it appears that velocity drops and conductivity increases continuously (not abruptly) with increasing melt fraction. This implies that mechanical and electrical properties of the upper mantle will gradually change at the boundary where the geotherm crosses the solidus. 相似文献
4.
—The Tien-Shan orogene is a region in which the earth’s crust undergoes considerable thickening and tangential compression. Under these conditions the lithosphere heterogeneities (composi tion, rheological) create the prerequisites for the development of various phenomena of tectonic layering (lateral shearing, different deformation of layers). To study the distribution of velocity, density and other elastic parameters, the results from a seismic tomography study on P-wave as well as S-wave velocities were used. Using empirical as well as theoretical formulas on the relationship between velocity, density and silica content in rocks, their distribution in the Tien-Shan’s lithosphere has been calculated. In addition, other elastic parameters, such as Young’s modulus, shear modulus, Poisson’s ratio and coefficient of general compressions have been determined. Zoning of different types of crust was carried out for the region investigated. The characteristics of the "crust-mantle" transition have been investi gated. Large blocks with different types of the earth’s crust were distinguished. Layers with inverse values of velocity, density and shear and Young modulus are revealed in the Tien-Shan lithosphere. All of the above described features open new ways to solve geodynamics problems. 相似文献
5.
Claude T. Herzberg 《Physics of the Earth and Planetary Interiors》1983,32(2):193-202
A review of experimental data for systems, pertaining to anhydrous fertile garnet-lherzolite shows strong convergence in the liquidus and solidus temperatures for the range 6.5–15 GPa. These can converge either to a common temperature or to temperatures which differ by only ～ 100°C. The major-element composition of magmas generated by even minor degrees of partial melting may be similar to the primordial bulk silicate Earth composition in an upper-mantle stratigraphic column extending over 160 km in depth.The convergence of the solidus and liquidus temperatures is a consequence of the highly variable of the fusion curves for minerals which crystallize in peridotite systems. In particular, for the forsterite fusion curve is much less than that for diopside and garnet. Whether or not the solidus and liquidus intersect, the liquidus mineralogy for undepleted garnet-lherzolite compositions changes from olivine at low pressures to pyroxene, garnet, or a complex pyroxene-garnet solid solution at pressures in excess of 10–15 GPa. Geochemical data for the earliest Archean komatiites are consistent with an upper-mantle phase diagram having garnet as a liquidus phase for garnet-lherzolite compositions at high pressures. All estimates of the anhydrous solidus and liquidus for the range 10–15 GPa are consistent with silicate liquid compressibility data, which indicate that olivine may be neutrally buoyant in ultramafic magmas at these pressures. 相似文献
6.
深部岩石的弹性波各向异性是人们了解地壳深部构造特征,分析其成因,探讨其动力学意义的重要岩石物理参数。实验结果表明由矿物晶格定向分布(LPO)所引起的地壳岩石平均各向异性强度通常不超过5%,远不足以解释在青藏高原地壳中所观测到的弹性波各向异性之强度。模拟结果显示,熔体的定向分布(MPO)能够引起强烈的弹性波各向异性。例如,当熔体的形态因子(α)值介于0.1～0.5之间,熔融程度为5%～10%时,由定向分布的酸性熔体囊所产生的各向异性强度可以达到2%～10%(P波)、2.2～40%(S波)。众多研究资料显示,青藏高原—川滇西部具有加厚的中、上地壳和高地热梯度,低度部分熔融作用在其深部地壳中广泛存在。低度熔体在构造应力作用下的定向分布可能是造成该地区深部地壳存在异常强的各向异性层的重要原因。这暗示目前在青藏高原—川滇西部探测到的异常强的区域性各向异性层是具有部分熔融成因的强烈构造变形带。该构造变形带具有潜在的"解耦"功能,并作为地壳浅部刚性层(块体)的底界协调着块体与其下伏地壳或岩石圈地幔的差异运动。 相似文献
7.
Increasing amount of crystals tends to reduce the mobility of magmas and modifies its elastic characteristics (e.g. [Caricchi, L. et al., 2007. Non-Newtonian rheology of crystal-bearing magmas and implications for magma ascent dynamics. Earth and Planetary Science Letters, 264: 402–419.; Bagdassarov, N., Dingwell, D.B. and Webb, S.L., 1994. Viscoelasticity of crystal- and bubble-bearing rhyolite melts. Physics of the Earth and Planetary Interior, 83: 83–99.]). To quantify the effect of crystals on the elastic properties of magmas the propagation speed of shear and compressional waves have been measured at pressure and temperatures relevant for natural magmatic reservoirs. The measurements have been performed in aggregates at variable particle fractions (? = 0–0.7). The measurements were carried out at 200 MPa confining pressure and temperatures between 300 K and 1273 K (i.e. across the glass transition temperature (Tg) from glass to melt). The specimens were mixtures of a haplogranitic melt containing 5.25 wt.% H2O and variable amounts of sub-spherical alumina particles. Additional experiments were carried out on a sample containing both, crystals and air bubbles. The temperature derivatives of the shear (dVs/dT) and compressional wave (dVp/dT) velocities for pure glass and samples with a crystal fraction of 0.5 are different below and above the glass transition temperature. For a crystal fraction 0.7, only dVp/dT changed above the Tg. In the presence of gas bubbles, Vp and Vs decrease constantly with increasing temperature. The bubble-bearing material yields a lower bulk modulus relative to its shear modulus. The propagation velocities of compressional and shear waves increase non-linearly with increasing crystal fraction with a prominent raise in the range 0.5 < ? < 0.7. The speed variations are only marginally related to the density increase due to the presence of crystals, but are dominantly related to the achievement of a continuous crystal framework. The experimental data set presented here can be utilized to estimate the relative proportions of crystals and melt present in a magmatic reservoir, which, in turn, is one of the fundamental parameters determining the mobility of magma and, consequently, exerting a prime control on the likelihood of an eruption from a sub-surficial magma reservoir. 相似文献
8.
H. Berckhemer W. Kampfmann E. Aulbach H. Schmeling 《Physics of the Earth and Planetary Interiors》1982,29(1):30-41
An apparatus designed to determine the complex shear modulus of rock samples by forced torsion oscillations at high temperature and in the seismic frequency band 0.003–30 Hz is briefly described. Measurements were performed on natural dunite from Åheim (Norway) up to 1400°C and on polycrystalline forsterite up to 1500°C at 1 atm pressure. The two materials were chosen to study, by comparison, the effect of melt on the elasticity and anelasticity of mantle rocks.Between 1000 and 1200°C the absolute values of the shear modulus G are almost equal for both materials. Above 1200°C G for natural dunite decreases progressively with temperature and at 1400°C and 1 Hz reaches of its value at 1100°C. In contrast, G of pure forsterite depends little on temperature. For petrological reasons, supported by simultaneous measurements of the electric resistivity, there is strong evidence that the decrease of G in dunite above 1200°C is due to melt from the lower melting components of the dunite. Based on different models estimates of the melt fraction are made.At high temperature, in both materials Q?1 is characterized by a monotonic decrease with frequency according to ω?α, with α ≈ 0.25. An apparent activation energy of 38±5 kcal mol?1 for forsterite and 48±8 kcal mol?1 for dunite was found with no significant change in the regime of partial melting. From this it is concluded that Q?1, even at partial melting, is dominated by solid state high temperature background absorption. There is no indication from these experiments for a constant-Q-band at low seismic frequencies or an increase of Q proportional to frequency as suggested by some seismologists. The present results are in good qualitative agreement with those for Young's modulus obtained previously by strain retardation experiments. 相似文献
9.
Ultrasonic compressional wave velocity Vp and quality factor Qp have been measured in alkali basalt, olivine basalt and basic andesite melts in the frequency range of 3.4–22 MHz and in the temperature range of 1100–1400°C. Velocity and attenuation of the melts depend on frequency and temperature, showing that there are relaxation mechanisms in the melts. Complex moduli are calculated from the ultrasonic data. The results fit well a complex modulus of Arrhenius temperature dependence with log-normal Gaussian distribution in relaxation times of attenuation. The analysis yields average relaxation time, its activation energy, relaxed modulus, unrelaxed modulus and width of Gaussian distribution in relaxation times. Relaxed modulus is smaller (17.5 GPa) for basic andesite melt of high silica and high alumina contents than for the other two basalt melts (18.1–18.4 GPa). The most probable relaxation times decrease from ～ 3 × 10?10 s for basic andesite to ～ 10?11 s for alkali basalt at 1400°C. Activation energies of attenuation, ranging from 270 to 340 kJ mol?1 in the three melts, are highest in basic andesite. Longitudinal viscosity values and their temperature dependences are also calculated from Vp and Qp data. The volume viscosity values are estimated from the data using the shear viscosity values. Longitudinal, volume and shear viscosities and their activation energies are highest in the basic andesite melt of the most polymerized structure. 相似文献
10.
High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions(δ26 Mg = –0.60‰ to –0.30‰) and relatively low87 Sr/86 Sr ratios(0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300-360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom(180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower Si O2 and Al2 O3 contents, and higher Ca O/Al2 O3 values, similar to those of nephelinites and basanites, and have higher εNdvalues(2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the asthenosphere and it could be easily replaced by convective mantle. The newly formed carbonated silicate melts will migrate upward and metasomatize the overlying lithospheric mantle. Similarly, such metasomatism and partial melting processes repeat, and as a result the cratonic lithosphere in North China would be thinning and the carbonated silicate partial melts will be transformed to high-Si O2 alkali basalts with lower εNdvalues(to-2). As the lithospheric thinning goes on,initial melting depth of carbonated peridotite must decrease from 130 km to close 70 km, because the craton geotherm changed to approach oceanic lithosphere geotherm along with lithospheric thinning of the North China craton. Consequently, the interaction between carbonated silicate melt and cratonic lithosphere is a possible mechanism for lithosphere thinning of the North China craton during the late Cretaceous and Cenozoic. Based on the age statistics of low δ26 Mg basalts in eastern China, the lithospheric thinning processes caused by carbonated metasomatism and partial melting in eastern China are limited in a timespan from 106 to25 Ma, but increased quickly after 25 Ma. Therefore, there are two peak times for the lithospheric thinning of the North China craton: the first peak in 135-115 Ma simultaneously with the cratonic destruction, and the second peak caused by interaction between carbonated silicate melt and lithosphere mainly after 25 Ma. The later decreased the lithospheric thickness to about70 km in the eastern part of North China craton. 相似文献
11.
Several researches published comprehensive reports on dynamic soil properties of cohesive soils, in which many of them outlined, i.e., key factors affecting the dynamic shear modulus. For cohesive soils, the modulus at small strains (g < 10t-3 %) is, first of all, a function of void ratio and effective confining stress. For clays, however, secondary time effects and clay mineralogy (fabric and structure) also appear to be important. The influence of confinement of laboratory-prepared as well as naturally deposited clays consists in an increase of shear modulus logarithmically as a function of time. In this paper, the effect of duration of the various confining pressures on dynamic shear modulus (G) of selected cohesive soils from Warsaw area was evaluated. Shear modulus was determined on the basis of resonant column tests, at low and high shearing strain amplitudes. It is shown that the calculated shear modulus is time-dependent; during approximately first 1000 minutes of consolidation, the moduli increased by almost 50%. Moreover, it is characterized by two phases: an initial one results from primary consolidation and a second one, which occurs after the end of primary consolidation, herein about 16-17 hours, and is called “long-term time effect”. This effect was found also for modulus at higher shearing strains (γ > 103%, e.g., 3 × 103%, 5 × 103%, 8 × 103%, 2 × 103%). 相似文献
12.
We investigate the origin of diversity of eruption styles in silicic volcanoes on the basis of a 1-dimensional steady conduit flow model that considers vertical relative motion between gas and liquid (i.e., vertical gas escape). The relationship between the assemblage of steady-state solutions in the conduit flow model and magma properties or geological conditions is expressed by a regime map in the parameter space of the ratio of liquid-wall friction force to liquid–gas interaction force (non-dimensional number ε), and a normalized conduit length Λ. The regime map developed in the companion paper shows that when ε is smaller than a critical value εcr, a solution of explosive eruption exists for a wide range of Λ, whereas an effusive solution exists only when Λ ~ 1. On the other hand, when ε > εcr, an effusive solution exists for a wide range of Λ. Diversity of eruption styles observed in nature is explained by the change in ε accompanied by the change in magma viscosity during magma ascent. As magma ascends, the magma viscosity increases because of gas exsolution and crystallization, leading to the increase in ε. For the viscosity of hydrous silicic magma at magma chamber, ε is estimated to be smaller than εcr, indicating that an explosive solution exists for wide ranges of geological parameters. When magma flow rate is small, the viscosity of silicic magma drastically increases because of extensive crystallization at a shallow level in the conduit. In this case, ε can be greater than εcr; as a result, a stable effusive solution co-exists with an explosive solution. 相似文献
13.
Nickel partitioning between forsterite and aluminosilicate melt of fixed bulk composition has been determined at 1300°C to 20 kbar pressure. The value of the forsterite-liquid nickel partition coefficient is lowered from >20 at pressures equal to or less than 15 kbar to <10 at pressures above 15 kbar.Published data indicate that melts on the join Na2O-Al2O3-SiO2 become depolymerized in the pressure range 10–20 kbar as a result of Al shifting from four-coordination at low pressure to higher coordination as the pressure is increased. This coordination shift results in a decreasing number of bridging oxygens in the melt. It is suggested that the activity coefficient of nickel decreases with this decrease in the number of bridging oxygens. As a result, the nickel partition coefficient for olivine and liquid is lowered.Magma genesis in the upper mantle occurs in the pressure range where the suggested change in aluminum coordination occurs in silicate melts. It is suggested, therefore, that data on nickel partitioning obtained at low pressure are not applicable to calculation of the nickel distribution between crystals and melts during partial melting in the upper mantle. Application of high-pressure experimental data determined here for Al-rich melts to the partial melting process indicates that the melts would contain about twice as much nickel as indicated by the data for the low-pressure experiments. If, as suggested here, the polymerization with pressure is related to the Al content of the melt, the difference in the crystal-liquid partition coefficient for nickel at low and high pressure is reduced with decreasing Al content of the melt. Consequently, the change ofDNiol-andesite melt is greater than that ofDNiol-basalt melt, for example. 相似文献
14.
《Earth and Planetary Science Letters》2006,241(3-4):530-542
New, high-precision W isotope data on iron meteorites are presented that provide important constraints on the timing of silicate–metal segregation in planetesimals. Magmatic iron meteorites all have ε182W within error or less radiogenic than initial ε182W estimated by studies of chondritic meteorites. At face value this implies that iron meteorites are as old and older than refractory calcium–aluminium rich inclusions (CAI), which are widely thought to be the oldest solar system objects. Moreover, different meteorites from the same magmatic groups, believed to be derived from the same planetissimal core, display a range of ε182W. We suggest that the paradoxical ε182W values more negative than initial Solar System Initial (SSI) are most readily explained as a result of secondary, spallation reactions with cosmic rays during transit between parent body and the earth. This is supported by the most negative ε182W being found in meteorites with the oldest exposure ages and the magnitude of the effect is shown to be consistent with known nuclear reactions. On the other hand, it is also striking that none of the magmatic iron group meteorites have ε182W analyses, outside error, more radiogenic than the estimated solar system initial ratio. This suggests that core formation in parent bodies of magmatic iron meteorites occurred ≤ 1.5 Myr after the formation age of CAI [Y. Amelin, A.N. Krot, I.D. Hutcheon, and A.A. Ulyanov, Lead isotopic ages of chondrules and calcium-aluminum inclusions, Science 297, 1678–1683, 2002]. This extremely early metal–silicate differentiation is coeval with the first chondrules [M. Bizzarro, J.A. Baker, and H. Haack, Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions, Nature 431, 275–278, 2004, A.N. Krot, Y. Amelin, P. Cassen, and A. Meibom, Young chondrules in CB chondrites from a giant impact in the early Solar System, Nature 436, 989–992, 2005]. Formation of later chondrules, and hence the parent bodies of some chondritic meteorites, must therefore have occurred in the presence of planetesimals large enough to possess iron cores. We conclude that early planetary accretion and differentiation was sufficiently fast for 26Al-decay to be an important heat source. Non-magmatic iron meteorites, however, display more radiogenic and varied W isotope signatures. This is in keeping with them being generated later, by impact melting during which the metal (partially) re-equilibrated with the then more radiogenic silicate fraction. 相似文献
15.
Geology of the saucer-shaped sill near Mahad, western Deccan Traps, India, and its significance to the Flood Basalt Model 总被引：1，自引：0，他引：1
An ～22-m-thick saucer-shaped sill occurs near Mahad and is exposed as a curvilinear, miniature ridge within the Deccan Traps. The sill has variable dips (42–55°). It has a 7.1-km long axis and 5.3 km short axis (aspect ratio of 1.4) and is larger than the MV sill of the Golden Valley sill complex, South Africa and the Panton sill, Australia. The sill has distinct glassy upper and lower chilled margins with a coarse-grained highly jointed core. The samples from the margin are invariably fractured and iron stained because of deuteric alteration. The rock from the sill is plagioclase-phyric basalt. At least three thick sill-like apophyses emanate from the base of the main sill. The apophyses change direction because of bending and thinning from a horizontal concordant sheet at the top to a discordant inclined form that bends again to pass into a lower horizontal concordant sheet. We interpret such features as ‘nascent saucer-shaped sills’ that did not inflate to form nested sills. Geochemically, the sill consists of poorly differentiated tholeiitic basalt that has a restricted geochemical range. Critical trace element ratios and primitive mantle normalised trace and REE patterns indicate that the sills have geochemical affinities to the Poladpur chemical type and that the pahoehoe flow they intrude belongs to the Bushe Formation. Calculated magmatic overpressures during sill emplacement range from 8.4 to 11.3 MPa (for Young’s modulus E?=?5 GPa) and 16.7 to 22.5 MPa (for E=10 GPa) and depth to magma chamber ranges from 8.5 to 11.5 km (E?=?5 GPa) and 17.1 to 22.9 km (E?=?10 GPa), consistent with petrological and gravity modelling. The volume of the Mahad sill is approximately 276 km3 and is constant irrespective of the variations in the values of host-rock Young’s modulus. In 1980, Cox (J Petrol 21:629–650, 1980) proposed a conceptual model of the crust–mantle section beneath the Karoo CFB which is considered as the fundamental model for flood basalt volcanism. Our paper confirms the presence of a sill plus the inferred substructure beneath Mahad that are compatible with predictions of that model. In LIPS, saucer-shaped sills are formed in areas experiencing extensional tectonics where processes such as the Cook–Gordon delamination and Dundurs elastic extensional mismatch between layered sedimentary rocks or lava flows are responsible for the deflection of dykes into sills. A similar process is envisaged for the formation of the Mahad sill. 相似文献
16.
Magnetic grains isolated from magnetococcoid bacterial cells were studied by means of transmission electron microscopy, electron diffraction and electron microprobe analysis. Observed in situ the magnetic grains are each surrounded by an organic membrane and are usually found in a random array although “chains” are also seen. Electron diffraction confirms the magnetite mineralogy and provides additional evidence in favor of vacancies in the structure. Electron microprobe analysis shows the magnetite to be slightly titaniferous. Electron microscopy indicates that the grains, rather than being flake shaped, are parallelepiped crystals with a mean length of99.3 ± 8.7nm, a mean width of62.3 ± 6.1nm yielding a width-to-length ratio of 0.63. These data support the contention that the magnetic bacterial grains are single-domain crystals capable of producing a natural remanent magnetization in sediments. 相似文献
17.
Harro Schmeling 《Physics of the Earth and Planetary Interiors》1985,41(1):34-57
With the aim of a simultaneous interpretation of elastic, anelastic and electric in situ data from the asthenosphere a comprehensive set of numerical models is developed for partial melt in different geometrical configurations. For the elastic and anelastic modulus use is made throughout of the melt squirt mechanism. Frequency dependence is not treated in detail but estimated from the limiting cases of the relaxed and unrelaxed modulus. This has the advantage that quantitative values of viscocity and flow path dimensions are not required. In the models melt can be assumed to occur in the form of tubes, films, and triaxial ellipsoidal inclusions of arbitrary aspect ratio. The conditions in which the solutions for triaxial ellipsoidal inclusions can be approximated by simpler ones for spheroidal inclusions are discussed. It is then shown up to which aspect ratio a published model on melt films is applicable. The problem of interconnection of inclusions is treated with a statistical numerical approach. It is found that a reduced degree of interconnection may have a significant influence on anelastic relaxation at melt fractions corresponding to a moderate modulus decrease. A useful representation of the anelastic melt models is introduced by plotting the relaxation strength against the effective modulus, both of which depend on the state of melting. Such diagrams allow a clear distinction between the different melt geometries and may be used for the interpretation of observed data. Finally, different melt geometries are superimposed and it is found that under certain conditions bulk dissipation may reach the order of that for shear. 相似文献
18.
D.C. Presnall 《Physics of the Earth and Planetary Interiors》1980,23(2):103-111
For a lherzolite mantle with about 0.1 wt.-percent CO2 or less, and a CO2/H2O mole ratio greater than about one, the mantle solidus curve in P-T space will have two important low-temperature regions, one centered at about 9 kbar (30 km depth) and another beginning at about 28 kbar (90 km depth). It is argued that the depth of generation of primary tholeiitic magmas beneath ridge crests is about 9 kbar, and that the geotherm changes from an adiabatic gradient at greater pressures to a strongly superadiabatic gradient at lesser pressures. Such a ridge geotherm would intersect the solidus at two separate depth intervals corresponding to the two low-temperature regions on the solidus. With increasing age and cooling of the lithosphere, the shallow partial melt zone would pinch out and the thickness of the deep partial melt zone would decrease. With increasing depth in a mature oceanic lithosphere, the rock types would consist of depleted harzburgite from directly beneath the crust to about 30 km depth, fertile spinel lherzolite from about 30 km to 50–60 km, and fertile garnet lherzolite from about 50–60 km to the top of the deep partial melt zone at about 90 km. 相似文献
19.
Eiji Ohtani 《Physics of the Earth and Planetary Interiors》1983,33(1):12-25
The melting curve of perovskite MgSiO3 and the liquidus and solidus curves of the lower mantle were estimated from thermodynamic data and the results of experiments on phase changes and melting in silicates.The initial slope of the melting curve of perovskite MgSiO3 was obtained as at 23 GPa. The melting curve of perovskite was expressed by the Kraut-Kennedy equation as , where Tm?2900 K and P?23 GPa; and by the Simon equation, .The liquidus curve of the lower mantle was estimated as (perovskite) and this gives the liquidus temperature Tliq=7000 ±500 K at the mantle-core boundary. The solidus curve of the lower mantle was also estimated by extrapolating the solidus curve of dry peridotite using the slope of the solidus curve of magnesiowüstite at high pressures. The solidus temperature is ～ 5000 K at the base of the lower mantle. If the temperature distribution of the mantle was 1.5 times higher than that given by the present geotherm in the early stage of the Earth's history, partial melting would have proceeded into the deep interior of the lower mantle.Estimation of the density of melts in the MgOFeOSiO2 system for lower mantle conditions indicates that the initial melt formed by partial fusion of the lower mantle would be denser than the residual solid because of high concentration of iron into the melt. Thus, the melt generated in the lower mantle would tend to move downward toward the mantle-core boundary. This downward transportation of the melt in the lower mantle might have affected the chemistry of the lower mantle, such as in the D″ layer, and the distribution of the radioactive elements between mantle and core. 相似文献
20.
Nd isotopic characteristics of S- and I-type granites 总被引：1，自引：0，他引：1
The initial Nd and Sr isotopic composition has been determined in S- and I-type granites from the Paleozoic Berridale and Kosciusko Batholiths of southeast Australia. The Nd and Sr isotopic variations form a strongly covariant array with S-types granites having a relatively restricted range inεNd values from ?6.1 to ?9.8 but a large range in initial87Sr86Sr of from 0.7094 to 0.7184. These characteristics are indicative of an～1400-m.y. sedimentary or metasedimentary source for S-types. I-types have variable initial Nd ranging from +0.4 to ?8.9, and a more limited range in initial87Sr86Sr of from 0.70453 to 0.7119. These isotopic characteristics are consistent with a two-component mixing model whereby a depleted mantle-like component (DMC) withεNd = +6 and87Sr86Sr= 0.703, is mixed with a crustal component (CC) havingεNd = ?9 and87Sr86Sr= 0.720. Although this two-component mixing model satisfies the isotopic constraints the source rock chemistry of the I-types is not compatible with the large proportion (up to 50%) of sedimentary material implied by the isotopic data. This indicates that more than two components are required to account for both the isotopic and chemical data. Both the chemical and isotopic data are consistent with I-type granites having been formed from source rocks of predominantly mantle derivation and obtained progressively from the mantle over a period of 1000 m.y. prior to granite formation. 相似文献