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1.
The melting curves of the fluorides ZnF2 and NiF2 (rutile structure), CaF2, SrF2 and BaF2 (fluorite structure), and of the fluoroberyllates Na2BeF4 and Li2BeF4 have been studied at pressures ? 40 kbar by differential thermal analysis in a piston-cylinder high-pressure device. The initial slopes (dTm/dP)0 of these melting curves are respectively 7.2, 5.8, 16.7, 15.2, 15.7, 15.1 and <0°C/kbar. A new Li2BeF4 polymorph, apparently of the olivine structure type, is stable at pressures > 10 kbar and its melting curve has an average slope of ～6.7°C/kbar. These new data and those for SiO2, BeF2, GeO2, LiF and MgF2, recently studied by Jackson, are combined with existing data for elements, ionic compounds and silicates to assess the influence of crystal structure, molar volume and the nature of interatomic bonding on the initial slopes of melting curves. It is found that the entropy of fusion (ΔSm) is primarily a function of crystal structure while the volume change on fusion (ΔVm) is controlled by crystal molar volume within each isostructural series. Such systematics have recently facilitated estimation of the initial slopes of the melting curves of periclase and stishovite. New and existing melting data for silicates and their analogues have been analysed and a systematic dependence of (dTm/dP)0 on SiO2 concentration has been demonstrated. Possible implications of this trend for partial melting of upper-mantle garnet lherzolite are illustrated. Finally, the use of the traditional silicate-germanate and oxide-fluoride modelling schemes is reviewed in the light of information from this present study.  相似文献   

2.
Uranyl phases are found as the alteration products of uraninite under oxidizing condi- tions[1—3]. They are also expected to be the dominant alteration products of spent UO2 fuel in an oxidizing environment, such as at nuclear waste repositories[4—8]. …  相似文献   

3.
Mechanical weakening due to solid state transformation of mineral phases has long been proposed to be a significant mechanism for localization of deformation in the Earth’s lithosphere and the mantle transition zone. However, experimental observations confirming such a proposition are lacking. Here we present a novel approach to prove the existence of a minimum in the creep strength of quartz at the α–β transition by observing the deformation of fluid inclusions in a quartz crystal using a hydrothermal diamond-anvil cell. Pressure differences required for permanent deformation of the quartz around fluid inclusions were significantly lower at the phase transition than in either the stability fields of α- or β-quartz. These results indicate that transformation plasticity of silicates can indeed cause a considerable localized reduction in the strength of the Earth’s crust and mantle.  相似文献   

4.
Geophysical prospection employing magnetometry and electromagnetic measurements has been applied in and around two small sized pyramids of Hellenikon and Ligourio in Argolid, Greece. The magnetic anomalies appropriately assessed were interpreted as possible archaeological targets. Subsequent test excavations revealed the presence of room foundations and parts of walls, as well as a plethora of ceramic ware. Typological study of the ceramics classified them to as early as the proto-Helladic period and to as late as the first centuries A.D. The earlier periods have been also confirmed by a novel application of thermoluminescence (TL) dating of ceramics and the megalithic stones themselves. The present results question earlier attempts classifying these pyramids at the Classical period and favour much earlier periods.  相似文献   

5.
Mineralogical and crystallographic data and electron microprobe analyses of silicate minerals for Antarctic ureilites Y74123, Y74130, Y74659, Y790981, ALHA77257, ALHA78019 and ALH82106 extend the chemical and textural variations among ureilites beyond those known for non-Antarctic ureilites. Low-Ca pyroxenes, possibly orthopyroxene, poikilitically enclosed in olivine and coexisting with Mg-rich pigeonites, have been identified in Y74659 and ALHA77257. Coexisting low-Ca pyroxene, pigeonite, and augite, together with the very Fe-rich olivine in ureilites were observed in Y74130. Almost Fe-free pigeonite and diopside coexist in ALH82106. No systematic differentiation trend has been reconstructed from these data, but the pyroxene pairs yield formation temperatures above 1200°C. A weak anticorrelation between MnO and FeO delineated by these pyroxenes suggests that a reduction reaction is responsible for the chemical variations. A working hypothesis for ureilite formation involves high-temperature episoded and a planetesimal-scale collision, which might have facilitated compaction and recrystallization of mafic silicates. During the episodes, FeNiS eutectic melt and a partial melt rich in Ca, Al and Fe might have been lost from a carbonaceous-chondrite-like source material.  相似文献   

6.
We have investigated the low-temperature behavior of a suite of ‘grown’ synthetic and natural magnetites that span single-domain (SD) and multidomain (MD) behavior. Synthetic samples had been grown in the laboratory either in an aqueous medium or in glass. Natural samples included SD magnetites occurring in plagioclase and truly MD magnetites in the form of large octahedra. In all experiments a sample was first given a saturation remanence at room temperature; next, moment was measured continuously during cooling and warming between 230 K and 60 K. Similar to results reported earlier by other workers, magnetic memory is large in SD samples, whereas truly MD samples are almost completely demagnetized by cycling between room temperature and 60 K. Pseudo-single-domain samples exhibit behavior that is intermediate with respect to that of the SD and truly MD states. When data from this study are combined with data obtained by Hartstra [10] from sized, natural magnetites, it is found that the percentage of total remanence that survives cycling between room temperature and 60 K decreases linearly with the logarithm of grain size and, thus, with increasing number of domains. This relation suggests that memory can provide a reasonable estimate of grain size in those magnetite-bearing rocks for which these samples provide good analogues. Remarkably, some of the large natural octahedra provide a magnified view of MD response to low temperatures and thus reveal two surprising and intriguing types of behavior. First, below approximately 180 K these octahedra demagnetize through a series of large Barkhausen jumps. Second, near 117 K these same octahedra exhibit a ‘wild zone’, where magnetic moment executes large, random excursions. We interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and, possibly, the broadening of domain walls as temperatures drop toward the isotropic point. One implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by MD grains, even without passage through the isotropic point of magnetite.  相似文献   

7.
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8.
Impure reworked evaporitic sandstones, preserved on Meridiani Planum, Mars, are mixtures of roughly equal amounts of altered siliciclastic debris, of basaltic provenance (40 ± 10% by mass), and chemical constituents, dominated by evaporitic minerals (jarosite, Mg-, Ca-sulfates ± chlorides ± Fe-, Na-sulfates), hematite and possibly secondary silica (60 ± 10%). These chemical constituents and their relative abundances are not an equilibrium evaporite assemblage and to a substantial degree have been reworked by aeolian and subaqueous transport. Ultimately they formed by evaporation of acidic waters derived from interaction with olivine-bearing basalts and subsequent diagenetic alteration. The rocks experienced an extended diagenetic history, with at least two and up to four distinct episodes of cementation, including stratigraphically restricted zones of recrystallization and secondary porosity, non-randomly distributed, highly spherical millimeter-scale hematitic concretions, millimeter-scale crystal molds, interpreted to have resulted from dissolution of a highly soluble evaporite mineral, elongate to sheet-like vugs and evidence for minor synsedimentary deformation (convolute and contorted bedding, possible teepee structures or salt ridge features). Other features that may be diagenetic, but more likely are associated with relatively recent meteorite impact, are meter-scale fracture patterns, veins and polygonal fractures on rock surfaces that cut across bedding. Crystallization of minerals that originally filled the molds, early cement and sediment deformation occurred syndepositionally or during early diagenesis. All other diagenetic features are consistent with formation during later diagenesis in the phreatic (fluid saturated) zone or capillary fringe of a groundwater table under near isotropic hydrological conditions such as those expected during periodic groundwater recharge. Textural evidence suggests that rapidly formed hematitic concretions post-date the primary mineral now represented by crystal molds and early pore-filling cements but pre-date secondary moldic and vug porosity. The second generation of cements followed formation of secondary porosity. This paragenetic sequence is consistent with an extended history of syndepositional through post-depositional diagenesis in the presence of a slowly fluctuating, chemically evolving, but persistently high ionic strength groundwater system.  相似文献   

9.
This paper concerns the enrichment by nutrients and the oxygen utilization rates of an intermittently anoxic basin. The concentration of nutrients and dissolved oxygen have been determined during seasonal surveys of water characteristics. The oxygen utilization rates ranged between 1.3 and 1.8 ml l.−1 month−1 from March to June and were considerably higher than those reported for oceanic waters. The nutrients enrichment appears to be considerably influenced by eutrophication and human activity. During anoxic conditions in summer, the mineralization of organic matter was associated with consequent accumulation of enormous amounts of nitrogen (ammonia), phosphates and silicates. Nutrient ratios underwent great fluctuation mainly due to anoxic conditions.  相似文献   

10.
Since the first discovery of silicate perovskites at high pressures and high temperatures in the laboratory in 1974, silicate perovskites have probably become the most studied materials in the geophysical community during the past decade or so and it is nearly established that these silicates are the most abundant materials making up the bulk of the Earth. There are basically two groups of silicate perovskites. Ferromagnesian silicates with or without Al2O3 crystallizing in a common orthorhombic perovskite structure at high pressures and temperatures (HPT) are preservable at ambient conditions. Silicates of large cations such as Ca and Na crystallizing in an ideal cubic perovskite structure at HPT cannot be preserved at ambient conditions. Thus, the lattice parameters, crystal structure, thermal expansion and compressional data have been studied, both experimentally and theoretically, mainly for orthorhombic silicate perovskites, and for MgSiO3 in particular. For MgSiO3 perovskite, the recommended lattice parameters area=4.777±0.003,b=4.931±0.003 andc=6.899±0.004 Å; bulk modulusB 0=2.4±0.2 Mbar; and volume thermal expansivity =(3±1)×10–5 deg–1 at ambient conditions. Cubic CaSiO3 perovskite is probably less compressible than orthorhombic MgSiO3 perovskite. The lattice parameters of MgSiO3 perovskite increase linearly with increasing contents of both FeSiO3 and Al2O3, forming limited solid solutions. The degree of distortion of orthorhombic silicate perovskites does not appear to change at HPT.  相似文献   

11.
The adiabatic single-crystal elastic moduli of SmAlO3, GdAlO3 and ScAlO3, all with the orthorhombic perovskite structure, have been measured by Brillouin spectroscopy under ambient conditions. These 3 compounds display various degrees of crystallographic distortion from the ideal cubic perovskite structure. We find that longitudinal moduli in directions parallel to the axes of a pseudocubic subcell are nearly equal and insensitive to distortions of the crystal structure from cubic symmetry, whereas, the moduli C11 and C22, parallel to the orthorhombic axes, display pronounced anisotropy with the exception of ScAlO3. The shear moduli also correlate with distortion from cubic symmetry, as measured by rotation, or tilt angles, of the AlO6 octahedra. Our data support the observations of Liebermann et al. that perovskite-structure compounds define consistent elasticity trends relating bulk modulus and molar volume, and sound speed and mean atomic weight. These relationships have been used to estimate bulk and shear moduli for the high-pressure polymorphs of CaSiO3 and MgSiO3 with the perovskite structure.  相似文献   

12.
Variation in habitat structure provided by macrophytes is regarded as one of the determinants of macroinvertebrate species composition in lentic ecosystems, but mechanisms underlying this relationship appear to be confounded with site-specific factors, such as physicochemical factors, epiphyton and composition of the vegetation. To better understand the relationship between structural complexity of a macrophyte stand and its macroinvertebrate assemblage composition, it is essential to determine the ecological role of different components of habitat structure for the phytomacrofauna. Using artificial structures as macrophyte mimics, representing three growth forms (stems, broad-leaved, finely dissected) and three structure surface areas (0.1, 0.2, 0.3 m2), a full factorial field experiment was conducted in a series of drainage ditches. We investigated if macroinvertebrate assemblages colonizing the structures were affected by an increase in macrophyte structure surface area, structural complexity, or by a combination of both, and if the observed patterns were consistent among sites differing in physicochemical and habitat characteristics. Assemblages were characterized both in terms of taxonomic and functional composition, because we expected that non-taxonomic aggregation of species into functional categories would give a different insight in habitat complexity-macroinvertebrate relationships in comparison to approaches based on the taxonomic assemblage composition. Ditch intrinsic factors, in part reflected in the periphyton on the structures, explained the major proportion of the variance in both the taxonomical macroinvertebrate assemblages and functional groups among structures. Contrary to our expectation, patterns in the taxon-based and functional dataset resembled each other. Only a minor contribution of growth form to the explained variance was observed in the taxonomical dataset, whilst differences in functional composition were unrelated to habitat structure. In conclusion, processes operating on larger spatial scales overrode the micro-scale effects of habitat structural complexity and surface area on macroinvertebrates.  相似文献   

13.
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   

14.
We carried out viscosity measurements and sampling of a crystal suspension derived from alkali olivine basalt from the Matsuura district, SW Japan, at subliquidus temperatures from 1230 °C to 1140 °C under 1 atm with NNO oxygen buffered conditions. Viscosity increased from 31 to 1235 Pa s with a decrease in temperature from 1230 to 1140 °C. On cooling, olivine first appeared at 1210 °C, followed by plagioclase at 1170 °C. The crystal content of the sample attained 31 vol.% at 1140 °C (plagioclase 22%, olivine 9%). Non-Newtonian behaviors, including thixotropy and shear thinning, were pronounced in the presence of tabular plagioclase crystals. The cause of such behavior is discussed in relation to shear-induced changes in melt–crystal textures. Relative viscosities, ηr (= ηs / ηm, where ηs and ηm are the viscosities of the suspension and the melt, respectively), were obtained by calculating melt viscosities from the melt composition and temperature at 1 atm using the equation proposed by Giordano and Dingwell [Giordano, D., Dingwell, D.B., 2003. Non-Arrhenian multicomponent melt viscosity: a model. Earth and Planetary Science Letters, 208, 337–349.]. The obtained relative viscosities are generally consistent with the Einstein–Roscoe relation, which represents ηr for suspensions that contain equant and equigranular crystals, even though the crystal suspension analyzed in the present experiments contained tabular plagioclase and granular olivine of various grain sizes. This consistency is attributed to the fact that the effect of crystal shape was counterbalanced by the effect of the dispersion of crystal size. The applicability of the Einstein–Roscoe equation with respect to crystal shape is discussed on the basis of the present experimental results. Our experiments and those of Sato [Sato, H., 2005. Viscosity measurement of subliquidus magmas: 1707 basalt of Fuji volcano. Journal of Mineralogical and Petrological Sciences, 100, 133–142.] show that the relationship between relative viscosity and crystal fraction is consistent with the Einstein–Roscoe relationship for axial ratios that are smaller than the critical value of 4–6.5, but discrepancies occur for higher ratios.  相似文献   

15.
Post-aragonite phase of CaCO3, experimentally known to be stable above 40 GPa [S. Ono, T. Kikegawa, Y. Ohishi, J. Tsuchiya, Post-aragonite phase transformation in CaCO3 at 40 GPa, Am. Mineral. 90 (2005) 667–671], is believed to be a major carbon-containing mineral in the Earth's mantle. Crystal structure of this mineral phase could not be solved using experimental data or traditional theoretical simulation methods and remained a controversial issue. Using a combination of advanced ab initio simulation techniques and high-pressure experiment, we have been able to determine the crystal structure of CaCO3 post-aragonite. Here, we performed simulations with the USPEX code [C.W. Glass, A.R. Oganov, and N. Hansen, (in preparation). USPEX: a universal structure prediction program], which is based on an evolutionary algorithm using ab initio free energy as the fitness function. This novel methodology for crystal structure prediction, which uses only the chemical composition as input, is described in detail. For CaCO3, we identify a number of energetically competitive structures, the most stable of which closely matches the experimental powder diffraction pattern and, in agreement with experiment, becomes more stable than aragonite above 42 GPa. This structure belongs to a new structure type, which is also adopted by the high-pressure post-aragonite phases of SrCO3 and BaCO3. It has 2 formula units in the orthorhombic unit cell (space group Pmmn) and contains triangular CO32− ions and Ca2+ ions in the 12-fold coordination. Above 137 GPa, a pyroxene-type structure (space group C2221) with chains of CO44− tetrahedra becomes more stable than post-aragonite. For MgCO3, this structure becomes more stable than magnesite above 106 GPa and is a good candidate structure for MgCO3 post-magnesite.  相似文献   

16.
Electrical conductivities and dielectric constants have been measured over the temperature range 90–300°K on several carbonaceous chondrites and some terrestrial analogues. The conductivities of meteorites of different petrologic subtypes range over many orders of magnitude and the low-temperature activation energies are typically much smaller than those observed in terrestrial materials at higher temperatures. The electrical properties of carbonaceous chondrites vary systematically with chemical-mineralogical characteristics in that: (1) activation energy at low temperature is greater in the more volatile-rich meteorites containing hydrated silicates, and (2) conductivity is greater in the more reduced meteorites of higher petrologic subtype. These new data on the electrical properties of chondrites hold important implications for both the thermal and magnetic histories of small bodies in the early solar system.  相似文献   

17.
Peridotite xenoliths with a broad range of textures provides evidence for consistent microstructural evolution in a vertical transect of the shallow lithospheric mantle (35–55 km depth) beneath the Persani Mountains, SE Carpathians, Romania, due to ongoing plate convergence in the Carpathian Arc nearby. The recrystallized grain size, crystal preferred orientations strength, and resulting seismic anisotropy vary continuously and display a strong correlation to equilibrium temperatures, suggesting a continuous change in deformation conditions with depth. The shallowmost xenoliths have microstructures typical of high stress deformation, marked by strong recrystallization to fine grain sizes, which results in weak crystal preferred orientations and anisotropy. The deepest xenoliths have coarse-grained porphyroclastic microstructures and strong crystal preferred orientations. Replacive orthopyroxene structures, consuming olivine, and high H2O concentrations in the pyroxenes are observed in some xenoliths indicating limited percolation of fluids or volatile-rich melts. Despite the high stress deformation and high H2O contents in some of the studied xenoliths, analysis of olivine crystallographic orientations indicates that [100] slip systems, rather than “wet” [001] accommodate most of the deformation in all samples. Seismic anisotropy estimated from the measured olivine and pyroxene crystal preferred orientations suggests that the strike-parallel fast SKS polarization directions and ~ 1 s delay times measured in the SE Carpathians are likely the consequence of convergence-driven belt-parallel flow in the lithospheric mantle.  相似文献   

18.
An analysis of the textures of pallasites has been made using data concerning the kinetic and rheological properties of silicates and metals. Pallasites containing rounded olivines (e.g., the Springwater and Brehnam pallasites) have been heated to above the solidus temperature of the metallic iron phases, ～ 1270 K. The rounded olivines of grain size 0.5–1.0 cm observed in the Springwater pallasites were formed between 1270 and 1370 K. On the other hand, those of grain size 0.5–1.0 mm as found in the Brehnam pallasites may have been heated to above 1370 K; however, the duration of heating at such high temperatures must have been less than 5 × 103 y. Pallasites containing angular olivines with microscopically rounded corners (e.g., the Eagle Station, Dora pallasites) have suffered shock events fracturing the olivine grains, which may correspond to collisions during the accretional stage of the parent body, and experienced successive annealing during cooling from a temperature between 1150 and 1270 K.  相似文献   

19.
The solubility of silica in water has been determined by Vinogradov in 1966 and shown to be dependent on pH, but he did not investigate the alkaline side, and there is also the interesting problem of bicarbonates, which though mildly alkaline in pH, are potentially capable of reacting with silicates to form silica and a normal carbonate. Many natural waters exist which do contain appreciable amounts of silica and bicarbonate. It is therefore of interest to know just how close these may be to precipitating silica. This has been determined experimentally.  相似文献   

20.
Injection of Newtonian crystal-free magmas into a partially crystallised host which may exhibit non-Newtonian properties produces magmatic structures such as pipes, syn-plutonic dikes or dendritic structures. Field relationships between the structure and the host rock commonly indicate what the rheological contrasts during the injection were. The manner in which a magma deforms in response to injection is mainly linked to crystal content and strain rate (i.e., injection rate). Three kinds of behaviour can be distinguished: (1) Newtonian at low crystal contents; (2) Non-Newtonian at intermediate (40–60%) crystal contents, or at high crystal contents if the strain rate is small; and (3) brittle failure at high crystal content or strain rates.Petrologic observations indicate that injection can take place when the host magma still behaves as a fluid. To investigate the physics of the injection process we review the results of injection experiments in non-Newtonian fluids. These experiments were performed to study viscous fingering in 2-D Hele Shaw cells. They provide the first step to establishing the main non-Newtonian effects during the formation of interfacial instabilities arising when a Newtonian fluid is injected into a more viscous fluid or paste. The qualitative comparison of the morphological features of the interfaces between the fluids in the experiments with those in nature suggests that, in magmas, irregularities of the interfaces (dikes and dendrites) result from non-Newtonian properties of the host. We conclude that fluid-like deformation, rather than brittle behaviour of the host, during injection is likely to produce the general features observed on the field. Cooling effects might be responsible for the widespread phenomenon of fragmentation. We emphasise that the main effect of non-Newtonian properties in partially crystallised magmas is to generate strongly heterogeneous media producing discontinuities which could explain the main morphological features of syn-plutonic injection structures.  相似文献   

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