首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 265 毫秒
1.
A phase transition in pure CaSiO3 perovskite was investigated at 27 to 72 GPa and 300 to 819 K by in-situ X-ray diffraction experiments in an externally-heated diamond-anvil cell. The results show that CaSiO3 perovskite takes a tetragonal form at 300 K and undergoes phase transition to a cubic structure above 490–580 K in a pressure range studied here. The transition boundary is strongly temperature-dependent with a slightly positive dT / dP slope of 1.1 (± 1.3) K/GPa. It is known that the transition temperature depends on Al2O3 content dissolved in CaSiO3 perovskite [Kurashina et al., Phys. Earth Planet. Inter. 145 (2004) 67–74]. The phase transition in CaSiO3(+ 3 wt.% Al2O3) perovskite therefore could occur in a cold subducted mid-oceanic ridge basalt (MORB) crust at about 1200 K in the upper- to mid-lower mantle. This phase transition is possibly ferroelastic-type and may cause large seismic anomalies in a wide depth range.  相似文献   

2.
The elastic moduli of ScAlO3 perovskite, a very close structural analogue for MgSiO3 perovskite, have been measured between 300 and 600 K using high precision ultrasonic interferometry in an internally heated gas-charged pressure vessel. This new capability for high temperature measurement of elastic wave speeds has been demonstrated on polycrystalline alumina. The temperature derivatives of elastic moduli of Al2O3 measured in this study agree within 15% with expectations based on published single-crystal data. For ScAlO3 perovskite, the value of (∂KS/∂T)P is −0.033 GPa K−1 and (∂G/∂T)P is −0.015 GPa K−1. The relative magnitudes of these derivatives agree with the observation in Duffy and Anderson [Duffy, T.S., Anderson, D.L., 1989. Seismic velocities in mantle minerals and the mineralogy of the upper mantle. J. Geophys. Res. 94, 1895–1912.] that |(∂KS/∂T)P| is typically about twice |(∂G/∂T)P|. The value of (∂KS/∂T)P for ScAlO3 is intermediate between those inferred less directly from V(P,T) studies of Fe-free and Fe- and Al-bearing MgSiO3 perovskites [Wang, Y., Weidner, D.J., Liebermann, R.C., Zhao, Y., 1994. P–V–T equation of state of (Mg,Fe)SiO3 perovskite: constraints on composition of the lower mantle. Phys. Earth Planet. Inter. 83, 13–40; Mao, H.K., Hemley, R.J., Shu, J., Chen, L., Jephcoat, A.P., Wu, Y., Bassett, W.A., 1991. Effect of pressure, temperature and composition on the lattice parameters and density of (Mg,Fe) SiO3 perovskite to 30 GPa. J. Geophys. Res. 91, 8069–8079; Zhang, Weidner, D., 1999. Thermal equation of state of aluminum-enriched silicate perovskite. Science 284, 782–784]. The value of |(∂G/∂T)|P for ScAlO3 is similar to those of most other mantle silicate phases but lower than the recent determination for MgSiO3 perovskite [Sinelnikov, Y., Chen, G., Neuville, D.R., Vaughan, M.T., Liebermann, R.C., 1998. Ultrasonic shear wave velocities of MgSiO3 perovskite at 8 GPa and 800K and lower mantle composition. Science 281, 677–679].

Combining the results from the previous studies and current measurements on ScAlO3 perovskite, we extracted the parameters (q and γ0) needed to fully specify its Mie–Grüneisen–Debye equation-of-state. In this study, we have demonstrated that acoustic measurements of KS(T), unlike V(P,T) data, tightly constrain the value of q. It is concluded that ScAlO3 has ‘normal’ γ0 (1.3) and high q (3.6). The high value of q indicates that ScAlO3 has very strong intrinsic temperature dependence of the bulk modulus; similar behaviour has been observed in measurements on Fe- and Al-bearing silicate perovskites (Mao et al., 1991; Zhang and Weidner, 1999).  相似文献   


3.
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   

4.
The pressure dependence of the three lattice parameters and unit cell volume of fayalite (Fe2SiO4 olivine) was determined by X-ray diffraction under hydrostatic pressures up to 70 kbar. In order to eliminate stress inhomogeneity within a composite material consisting of a specimen mixed with an internal-pressure standard, a liquid (1 : 1 mixture of ethanol and methanol) was used as a pressure-transmitting medium. The isothermal bulk modulus calculated on the basis of the second-order Birch-Murnaghan equation of state gives the values K0 = 1.19 ± 0.10 Mbar and K0′ = 7 ± 4, and if we assume K0′ = 5: K0 = 1.24 ± 0.02 Mbar. Three axes of fayalite were found to be compressible in the following order, b >c >a. Comparisons with the results obtained under non-hydrostatic compression are made.  相似文献   

5.
In this study, three receiver function stacking methods are used to study the detailed crust and upper mantle structure beneath south-central Alaska. We used teleseismic waveform data recorded by 36 stations in the Broadband Experiment Across the Alaska Range (BEAAR) and 4 permanent stations in Alaska. H − κ stacking method using P-to-S converted wave and its multiply reflected waves between the Earth's surface and the Moho discontinuity is adopted to estimate the crustal thickness (H) and average crustal VP/VS ratio (κ) in this region. The receiver function results for 24 stations show that the crustal thickness under Alaska ranges from 26.0 to 42.6 km with an average value of 33.8 km, and the VP/VS ratio varies from 1.66 to 1.94 with an average value of 1.81 which corresponds to an average Poisson's ratio of 0.277 with a range from 0.216 to 0.320. High Poisson's ratios under some stations are possibly caused by partial melting in the crust and the uppermost mantle. Common converted point (CCP) stacking results of receiver functions along three lines show clear Moho and slab images under this subduction zone. The depths of the slab from our CCP stacking images are consistent with those estimated from the Wadati–Benioff Zone (WBZ). In the area between two stations DH2 (147.8°W, 63.3°N) and DH3 (147.1°W, 63.0°N), a Moho depth offset of about 10 km is found by both the H − κ and CCP stacking techniques. Common depth point (CDP) stacking of receiver functions shows not only the 410-, 520- and 660-km discontinuities, but also significant variations (−30 to 15 km) in the transition zone thickness under the southwest and southeast parts of the study region. The transition zone becomes thinner by 20–30 km, indicating that the temperature there is 150–200 K higher than that of the normal mantle.  相似文献   

6.
Gehlenite (Ca2Al2SiO7) has been found to transform to a new phase at pressures greater than 100 kbar and at about 1000°C, using a diamond-anvil pressure cell coupled with laser heating. The atoms of the new phase appear to be arranged in a perovskite-related structure similar to that described for Na2Ti3O7. The structure probably consists of layers of (Al2SiO7)4−, which are built up from blocks of edge-sharing (Al, Si)O6 octahedra and these blocks are joined by common octahedra corners. A small cubic unit cell with a = 3.719 ± 0.004 Å indexes completely the strong lines of the powder diffraction pattern, and a superlattice with a = 14.88 ± 0.02 Å satisfies all the observed weak lines in addition to the strong ones. However, the cell may be pseudocubic. The small cell contains a half of the gehlenite formula while the large cell contains 32 gehlenite formulae. Hence the molar volume for the new phase of Ca2Al2SiO7 is calculated to be 61.96 ± 0.20 cm3 at atmospheric pressure and room temperature. The new sodium titanate-type structure is probably more closely packed than an ordinary perovskite-type structure in which all octahedral corners are shared. This view is strongly supported by the very great density of this new phase, which is about 8% denser than the equivalent mixture of CaAl2O4 (calcium ferrite type) plus CaSiO3 (cubic perovskite type). The new phase is probably the most closely packed silicate known. Mg2SiO4 (spinel) was found to transform to an assemblage containing MgSiO3 (perovskite) plus MgO (periclase) at P-T conditions equivalent to the upper part of the lower mantle. By reacting with MgO, the perovskite modification of both MgSiO3 and MgSiO3 · xAl2O3 may adopt the sodium titanate structure at the still greater depths of the lower mantle. If the sodium titanate structures of Mg2(Al2Si)O7 and Mg2(MgSi2)O7 are present in the deep part of the lower mantle, MgO does not exist as a separate phase at the mantle-core boundary. This might be an obstacle to the possibility of dissolving these oxides (specifically the FeO component) in the molten Fe in the outer core as suggested by geophysical and geochemical studies of the earth's interior. The mechanism for developing the chemical plumes in the deep mantle proposed by Anderson does not appear to be consistent with studies of phase transformations in Ca-Al-rich compounds as outlined in this paper.  相似文献   

7.
Experiments [T. Irifune (1994) Nature 370, 131–133; E. Ito et al. (1998) Geophys. Res. Lett. 25, 821–824; A. Kubo, M. Akaogi (2000) Phys. Earth Planet. Int. 121, 85–102] indicate that (Mg,Fe)SiO3 perovskite, commonly believed to be the most abundant mineral in the Earth, is the preferred host phase of Al2O3 in the Earth’s lower mantle. Aiming to better understand the effects of Al2O3 on the thermoelastic properties of the lower mantle, we use atomistic models to examine the chemistry and elasticity of solid solutions within the MgSiO3(perovskite)–Al2O3(corundum)–MgO(periclase) mineral assemblage under conditions pertinent to the lower mantle: low Al cation concentrations, P=25–100 GPa, and T=1000–2000 K. We assess the relative stabilities of two likely substitution mechanisms of Al into MgSiO3 perovskite in terms of reactions involving MgSiO3, MgO, and Al2O3, in a manner similar to the 0 Kelvin calculations of Brodholt [J.P. Brodholt (2000) Nature 407, 620–622] and Yamamoto et al. [T. Yamamoto et al. (2003) Earth Planet. Sci. Lett. 206, 617–625]. We determine the equilibrium composition of the assemblage by examining the chemical potentials of the Al2O3 and MgO components in solid solution with MgSiO3, as functions of concentration. We find that charge coupled substitution dominates at lower mantle pressures and temperatures. Oxygen vacancy-forming substitution accounts for 3–4% of Al substitution at shallow lower mantle conditions, and less than 1% in the deep mantle. For these two pressure regimes, the corresponding adiabatic bulk moduli of aluminous perovskite are 2% and 1% lower than that of pure MgSiO3 perovskite.  相似文献   

8.
The influence of a driving static shear stress on the liquefaction resistance of medium dense granular soils is investigated. A laboratory study of the behavior of five sands (percentage of silt varying between 2 and 42%, and D50 varying between 0.55 and 0.09 mm) was undertaken. These five sands were sampled from the foundations of two dams in Quebec (Canada). Sixty cyclic direct simple shear (DSS) tests were performed with and without a driving static shear stress. Relative densities Dr varied between 60 and 70%, and static shear stress ratios =τst/σ′vc varied between 0.2 and 0.6. The presence of static shear stress increased the cyclic strength of these soils. A relation between and K() factor giving the effect of preshearing on cyclic shear resistance is obtained. The mechanical competence is examined as a function of D50 and percentage of fines. The observed dilatancy yielded rather cyclic mobility than liquefaction.  相似文献   

9.
Analysis of the temporal fluctuations, described by a power spectral density S(f) decaying as 1/f with exponent >0, in the sequence of aftershocks of the Bovec (Slovenia) earthquake of April 12, 1998 has been performed, using the Allan Factor (AF) method. The sequence of the occurrence times of the events with threshold magnitude Mth≥2.0 is characterised by a scale-invariant behaviour from the time scale τ2·104 s with a scaling coefficient 0.9, evaluated by a least-square method. By gradually increasing the threshold magnitude up to Mth=2.9, the AF curves, associated, respectively with the processes of selected events with magnitude M≥Mth, indicate a monotonic decrease of the value of the scaling exponent , in the range of time scales stretching from 102 to 5·105 s. For all the thresholds considered, the AF curves increase monotonically with the counting time and are well fitted by an increasing power-law function for counting times greater than approximately 2·104 s. This monotonic power-law increase indicates the presence of fluctuations on many time scales and therefore of fractal clustering.  相似文献   

10.
地磁活动K指数自动计算FMI方法的改进   总被引：1，自引：1，他引：0       下载免费PDF全文
将Kp指数与国际地磁学与高空物理学协会(International Association of Geomagnetism and Aeronomy,下文简称IAGA)推荐的FMI方法(V1.0)计算的K值对比分析后发现,二者的差值在中国区域内出现"不同台站之间K1~K8两头离散、K值整体高出K_p约0.5个单位、个别日期S_R计算错误、相邻日期S_R频繁出现跨天台阶"现象.针对这些现象,分析了可能产生的原因,并对原FMI算法进行了四处改进.用改进后的FMI算法计算了全球8个台站、中国5个台站在2003-2005年和2009年共4年的K指数数据,并将计算结果分别与Kp指数以及这些台站用其他方法计算的K值进行一致性比较,发现改进后的计算结果与二者更为接近,说明本文对FMI方法的改进是有效的.  相似文献   

11.
We determined the partition coefficients of 19 elements between metallic liquid and silicate liquid at 20 GPa and 2500°C, and between metallic liquid and silicate perovskite at 27 GPa and 2200°C. Remarkable differences were observed in the partitioning behaviors of Si, P, W, Re, and Pb among the silicate liquid, perovskite, and magnesiowüstite coexisting with metallic liquid, reflecting incompatibility of the elements in the silicate or oxide phase. We could not observe any significant difference in the partitioning behaviors of V, Cr, Mn, Co, Ni, and Cu among the phases coexisting with metallic liquid.

Comparison of the present partitioning data with those obtained previously at lower pressure and temperature suggests that the exchange partition coefficients, Kmet/sil, of Co, Ni, Mo, and W decrease, whereas those of V, Cr, and Mn increase and tend to approach unity with increasing pressure and temperature. We also made preliminary experiments to clarify the effect of sulfur on the partitioning behaviors. Sulfur lowers the exchange partition coefficients, Kmet/sil, of Mo and W between metallic liquid and silicate liquid significantly at 20 GPa and 2300°C.

The mantle abundances of Co, Ni, Cu, Mo, and W calculated for the metal-silicate equilibrium model are lower than those of the real mantle, whereas P, K, and Mn are overabundant in the calculated mantle. The discrepancies in the abundances of Co and Ni could be explained by the chemical equilibrium at higher pressure and temperature. Large discrepancies in Mo and W between the calculated and real mantles could be accounted for by the effect of sulfur combined with the effects of pressure and temperature on the chemical equilibrium. The mantle abundances of P, K, and Cu could be accounted for by volatile loss in the nebula, perhaps before accretion of the Earth, combined with the chemical equilibrium at higher pressure and temperature. Thus the observed mantle abundances of P, K, Co, Ni, Cu, Mo, and W may be consistent with a model of sulfur-bearing metal-silicate equilibrium in lower-mantle conditions.  相似文献   


12.
Values of bulk modulus (K), shear modulus (G) and mechanical quality factor (Q) have been determined for polycrystalline samples across the CaTiO3 (CST0)–SrTiO3 (CST100) solid solution by resonant ultrasound spectroscopy. Because of similarities with low frequency elastic and anelastic anomalies due to twin wall motion reported in previous studies, a working hypothesis is developed in which dissipation processes are interpreted in terms of twin wall displacements. At high temperatures in CST50 the stability field of the I4/mcm structure is marked by the disappearance of all resonance peaks (superattenuation). This is attributed to anelastic domain wall sliding. At room temperature the I4/mcm phase of CST70 and CST80 has values of G which are lower than those of cubic or orthorhombic phases, and a concomitant drop in Q is interpreted as implying that the domain wall pinning process reported elsewhere to occur below 400–450 K is only partial. A similar drop in G and Q was found in CST95 below the transition at 238 K. The I4/mcm ↔ Pbcm transition in CST70 at 230 K is marked by an abrupt increase in Q, suggesting that mobile twins in crystals with the I4/mcm structure become effectively immobile in antiferroelectric crystals with the Pbcm structure. The I4/mcm ↔ Pnma transition in CST50 is marked by a similarly abrupt increase in Q, consistent with twin walls becoming effectively immobile also in crystals with the Pnma structure. A fall in Q below 800 K in CST0, however, could imply that a degree of twin wall mobility might develop in Pnma crystals if the tetragonal spontaneous strain departs significantly from zero. The remarkable attenuation behaviour of crystals with the I4/mcm structure at the relatively low stress conditions which apply during resonances of a parallelepiped with edge dimensions of 2–4 mm, is consistent with the view that a characteristic signature for tetragonal CaSiO3 in the Earth's lower mantle should be a marked attenuation of seismic waves.  相似文献   

13.
Melting relations of β-quartz were experimentally determined at 1.0 GPa (1900±20 °C), 1.5 GPa (2033±20 °C), and 2.0 GPa (2145±20 °C) using a new high-pressure assembly in a piston–cylinder apparatus and substantial differences were found with data previously reported. The new melting data of β-quartz were combined and optimized with all available thermodynamic, volumetric, and phase equilibria data for β-cristobalite, β-quartz and coesite to produce a P–T liquidus diagram for silica valid up to 6.0 GPa. Using the new optimized thermodynamic parameters, the invariant point β-cristobalite+β-quartz+liquid and β-quartz+coesite+liquid were determined to lie at 1687±17 °C and 0.457 GPa, and 2425±25 °C and 5.00 GPa, respectively.  相似文献   

14.
We report new metal-silicate partition coefficients for Ni, Co and P at 7.0 GPa (1650–1750°C), and Ni, Co, Mo, W and P at 0.8, 1.0 and 1.5 GPa (1300–1400°C). Guided by thermodynamics, all available metal-silicate partition coefficients, D(i), where i is Ni, Co, P, Mo and W, are regressed against 1/T, P/T, lnf(O2), ln(1 − Xs) (XS is mole fraction of S in metallic liquid) and nbo/t (non-bridging oxygen/tetrahedral cation ratio, a silicate melt compositional-structural parameter) to derive equations of the following form: ln D(i) = aln f(O2) + (b/T) + (cP/T) + d(nbo/t) + eln(1 − XS) + f. Expressions for solid metal-liquid silicate and liquid metal-liquid silicate partition coefficients are derived for S-free and S-bearing systems.

We investigate whether Earth's upper-mantle siderophile element abundances can be reconciled with simple metal-silicate equilibrium. Sulfur-free metallic compositions do not allow a good fit. However, Ni, Co, Mo, W and P abundances in the upper mantle are consistent with simple metal-silicate equilibrium at mantle pressures and temperatures (27 GPa, 2200 K, ΔIW(iron-wüstite) = −0.15, nbo/t = 2.7; XS = 0.15). Although these conditions are near the anhydrous peridotite solidus, they are well above the hydrous solidus and probably closer to the liquidus. A hydrous magma ocean and early mantle are consistent with predicted planetary accretion models. These results suggest that siderophile element abundances in Earth's upper mantle were established by liquid metal-liquid silicate equilibrium near the upper-mantle-lower-mantle boundary.  相似文献   


15.
In situ X-ray diffraction measurements on a calcium aluminosilicate (CAS) phase have been carried out using a laser-heated diamond anvil cell up to a pressure of 44 GPa, employing a synchrotron radiation source. CAS is the major mineral formed from sediments subducted into the Earth's mantle. The sample was heated using a YAG laser after each pressure increment to relax the deviatoric stress in the sample. X-ray diffraction measurements were carried out at T = 300 K using an angle-dispersive technique. The pressure was calculated using an internal platinum metal pressure calibrant. The Birch–Murnaghan equation of state for the CAS phase obtained from the experimental unit cell parameters showed a density of ρ0 = 3.888 g/cm3 and a bulk modulus of K0 = 229 ± 9 GPa for K′0 = 4.7 ± 0.7. When the first pressure derivative of the bulk modulus was fixed at K′0 = 4, then the value of K0 = 239 ± 2 GPa. From the experimental compressibility, the density of the CAS phase was observed to be lower than the density of co-existing Al-bearing stishovite, calcium perovskite, calcium ferrite-type phases, and (Fe,Al)-bearing Mg-perovskite in subducted sediments in the lower mantle. Therefore, the density of subducted sediments in the lower mantle decreases with increasing mineral proportion of the CAS phase.  相似文献   

16.
Shape analysis of Pacific seamounts   总被引：2，自引：0，他引：2  
Shape statistics have been compiled from 85 profiles of well-surveyed Pacific seamounts in the height range 140–3800 m. A flat-topped cone was fit to each seamount's cross-sectional profile maintaining the slopes of the sides as closely as possible. On each profile a basal widthdb, a summit widthdt, and a maximum heighth, were measured. The height-to-basal-radius ratio isξh is estimated by the ratio2hdb and flatnessf by the ratiodtdb. Slope angleφ = arctan(ε) is estimated fromε =2h(db − dt). Summit height and basal radius are found to be highly correlated (r = 0.93). The 85-point sample mean of the height-to-basal-radius ratio isξh = 0.21 ± 0.08 implying that a seamount's summit height is typically one fifth its basal radius. Despite the high correlation, individual points show some scatter, and there may be groupings into different morphological types. For example, all but one of the seamounts with summit heights above 1000 m have values ofξh that are larger than the sample mean. The 85-point sample mean of flatness isf = 0.31 ± 0.18. Data points show a large scatter with values off varying between 0 (a pointy cone) and 0.69 (a flat-topped cone). A histogram representation of flatness, however, indicates that certain values off may be more common than others: the histogram shows a bimodal distribution with maxima occurring at values off in the ranges 0.10–0.20 and 0.35–0.50. Moreover, there is some evidence that the mean flatness decreases with summit height so that the preferred shape of a large-sized seamount may be a pointy cone. Slope angle has an 85-point sample mean ofφ = 18 ± 6°; individual values ofφ vary between 5° and 36°. In addition to having a lower than average mean flatness seamounts with heights above 2600 m also have a lower than average mean slope angle (15°). To determine which variables account for most of the observed variation in the seamount shapes, a multivariate principal component analysis was performed on the data using five shape variables (summit height, basal radius, summit radius, flatness, and slope). The analysis indicates that most of the variation is described by two variables: flatness and summit height.  相似文献   

17.
New measurements of high field magnetisation (Is), remanence (Is), and coercive force (Hc) are presented between 4 and 300 K for x = 0.4, 0.5, 0.6 and 0.8 (Fe3−xTixO4). For x = 0.4 a pronounced minimum is found at T100 K and for X = 0.5 and 0.6 broad minima occur around T200 K, apparently coinciding with the temperature for K10. The magnetic properties below T60 K ar complex and were found to be significantly changed by cooling in the presence of a high magnetic field. With no applied field during cooling, a distinct decrease in Is is observed for T60 K, at which temperature there is a peak in the value of Irs. The effe cooling is to eliminate the sharp decrease in Is, reduce Hc and to increase Irs below 60 K to a value the peak value, giving essentially a square hysteresis loop. The results are interpreted in terms of a form of crystallographic phase transition coupled to the magnetisation direction, possibly by the magnetostriction. Square hysteresis loops in ferrites have been explained by the presence of Jahn-Teller ions and, in the present case, the low temperature of the observed effect may be a consequence of the weak Fe2+ Jahn-Teller ion coupled to other effects such as spin-lattice coupling.

Details of this work can be found in Schmidbauer, E. and Readman, P.W., 1982. Low temperature magnetic properties of Ti-rich Fe---Ti spinels. J. Magn. Magn. Mat., 27: 114–118. A paper reporting further work on Fe2.4Ti0.6O0.4 is in preparation.  相似文献   


18.
We investigated the influence of the composition of the vertical particle flux on the removal of particle reactive natural radionuclides (230Th and 231Pa) from the water column to the sediments. Radionuclide concentrations determined in sediment traps moored in the western, central and eastern Arabian Sea were related to the major components (carbonate, particulate organic matter (POC), opal, lithogenic material) of the particle flux. These data were combined with sediment trap data previously published from the Southern Ocean, Equatorial Pacific and North Atlantic [Z. Chase, R.F. Anderson, M.Q. Fleisher, P.W. Kubik, The influence of particle composition and particle flux on scavenging of Th, Pa and Be in the ocean, Earth Planet. Sci. Lett. 204 (2002) 215–229; J.C. Scholten, F. Fietzke, S. Vogler, M. Rutgers van der Loeff, A. Mangini, W. Koeve, J. Waniek, P. Stoffers, A. Antia, J. Kuss, Trapping efficiencies of sediment traps from the deep eastern North Atlantic: The 230Th calibration, Deep Sea Research II 48 (2001) 2383–2408]. The correlations observed between the particle-dissolved distribution coefficients (Kd) of 230Th and 231Pa and the concentrations of the particle types depend on the sediment trap data set used. This result suggests that scavenging affinities of the nuclides differ between oceanic regions. Several factors (Kd values, reactive surface areas of particles, inter-correlations in closed data set) can, however, influence the observed relationships and thus hamper the interpretation of these correlation coefficients as a measure of relative scavenging affinities of the nuclides to the particle types investigated. The mean fractionation factor (F(Pa/Th)=Kd(Pa)/Kd(Th)) from the Equatorial Pacific (F=0.11±0.03) is similar to that from the North Atlantic (F(Pa/Th)=0.077±0.026), and both are lower than the factors from the Arabian Sea (F(Pa/Th)=0.35±0.12) and from the Southern Ocean (F(Pa/Th)=0.87±0.4). For opal concentrations exceeding 60%, an increase in the fractionation factors is observed causing a higher mean fractionation factor for the Southern Ocean trap data set. For the other areas investigated, differences in the mean fractionation factors cannot be related to the particles types considered. In the Arabian Sea, seasonally variable 231Paex/230Thex ratios observed in the sediment traps as well as differences of the ratios between recently deposited phytodetritus (fluff) and normal surface sediments indicate seasonal changes in scavenging processes which the generally accepted reversible scavenging models do not envisage. We assume that variable sinking rates of particles, and/or particles not considered in this study (e.g. colloids, manganese oxides, transparent exopolymer particles) may play an important but as yet unexplored role in deep-water scavenging processes.  相似文献   

19.
Results are presented of laboratory resonant column and cyclic triaxial tests on specimens of two compacted soils (a sandy–silty clay and a sand–gravel mixture), planned to be used in the core and the shells, respectively, of a proposed earthfill dam. The values of low-amplitude shear modulus of the clayey material were found to increase with increasing confining pressure and decreasing water content, with deviations of ±20% from the predictions of the “Hardin equation”. On the other hand, the low-amplitude damping ratio was found to be at least four times higher than the values corresponding to natural undisturbed cohesive soils. The proposed G/G0−γc curve for the compacted cohesive soil was found to be independent of confining pressure and small variations of the water content on either side of the optimum value and showed a remarkable agreement with recently published similar curves for natural cohesive soils. For the case of the sand–gravel mixture, normalized G/G0–γc and D–γc curves are proposed, based on recently published results for gravelly soils and the limited data of the present study. The liquefaction resistance of the saturated sand–gravel mixture was found to be strongly dependent on its relative density, especially for high values of cyclic stress ratio. A relative density of at least 55% was found to be necessary to assure safety against earthquake-induced liquefaction of the material. The results presented herein may be used (directly or as guide) in the seismic analysis of (new or existing) earth dams constructed from similar soil materials and in addition they provide insight into the dynamic behavior of compacted soils.  相似文献   

20.
  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号