High-pressure synthesis of ZnSiO3 ilmenite

High-pressure phase relations in ZnSiO₃ and Zn₂SiO₄ were investigated at about 1000°C and in the pressure range of 100–500 kbar, using a double-staged split-sphere type of high-pressure apparatus.Clinopyroxene-type ZnSiO₃ transforms directly into a polymorph with the ilmenite structure at 120 kbar. The hexagonal unit cell dimensions of the ZnSiO₃ ilmenite are determined to be $\text{a = 4.746 ± 0.001}\text{A}\text{?}\text{and}\text{c = 13.755 ± 0.002}\text{A}\text{?}$ under ambient conditions.The following reactions are also recognized at about 1000°C:

and:

The stabilities of silicate ilmenites, especially the absence of ilmenite of transition metal silicate composition, is discussed. It is pointed out that data on phase relations in zinc silicates may be informative for the consideration on those in magnesium silicates under very high pressures. It is suggested that the silicate ilmenite may be a major constituent in the lower mantle.     

Elasticity of MgSiO3 in the ilmenite phase

The single-crystal elastic moduli of the ilmenite phase of MgSiO₃ have been determined from Brillouin spectroscopy. They are: C₁₁ = 472, C₁₂ = 168, C₃₃ = 382, C₁₃ = 70, C₄₄ = 106, C₁₄ = ?27, C₆₆ = 152 and C₂₅ = ?24 in GPa. These elastic properties are consistent with a structural mechanical model where the silicon octahedra are very stiff under compression and shear. This latter property yields an unexpectedly high shear modulus for the magnesium silicate ilmenite as compared with analogue compounds. The further transformation to perovskite will probably be associated with a significant increase in elastic properties since the strong silicon polyhedra form a structural network in this phase. The transformation of spinel and stishovite to ilmenite is associated with a slight density increase and a slight decrease in acoustic velocities. This transformation will probably not produce a seismic discontinuity even if it does occur in the Earth's mantle.     

The Coprates trough assemblage: More evidence for martian polar wander

Martian surface features such as quasi-circular structures in polar regions have been previously cited as evidence of polar wander (i.e., large-scale relative motion between a body-fixed axis and the rotation pole). Another feature, the Coprates (Valles Marineris) trough assemblage, is proposed as further, plausible evidence of such wander. This trough assemblage is explained as a failure due to curvature changes in Mars' lithosphere, in the manner of certain hypotheses explaining terrestrial features (e.g. East African Rift System). In the Earth's case plate tectonics cause lithospheric curvature changes, whereas, in Mars' case polar wander is most probable. Membrane stresses which can reach 10 kbars are shown to most likely over-shadow bending stresses. Flattening changes are judged to be inappropriate. Simple wander scenarios are constructed to illustrate the hypothesis.     

Variation in copepod species assemblage distributions: The utility of the log-normal approach

Pelagic copepod species distributions were found to vary in relation to an environmental gradient. As suggested by Gray, systematic changes were noted in the skewness of the log-normal. However, no systematic changes in the standard of deviation of the log-normal were noted. The data indicate that such assemblages ill-fit Preston's ‘canonical’ expectation. Further, analysis of changes in the evenness function yielded similar conclusions.     

Elasticity of the ilmenite - perovskite phase transformation in CdTiO3

Ultrasonic data for the velocities of the ilmenite and perovskite polymorphs of CdTiO₃ have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 25 kbar. This transition is characterized by the following velocity (ν)-density (?) relationships: (1) the changes in compressional (ν_p) and bulk sound (ν_?) velocities are comparable in percentage magnitude to the density jump, while the shear (ν_s) velocity jump is three times greater than that for ?; (2) (ν_p/ν_s) decreases across the transition from the low- to high-pressure phase; and (3) low slopes (linear or logarithmic) on ν-? diagrams. The (ν_p/ν_s) behaviour for the ilmenite-perovskite transformation is unusual for the transitions studied in our laboratory. The observed relationships (1) and (2) are typical of the elasticity behaviour across phase transformations which involve increases in cation-anion co-ordination and in nearest-neighbour interatomic distances, such as those exhibited by CdTiO₃ in transforming from the ilmenite to the perovskite phase. Elasticity systematics for isostructural sequences are used to estimate the bulk moduli of the perovskite polymorphs of CaSiO₃ (2.7 Mbar) and MgSiO₃ (2.8 Mbar).     

Magnetism of natural pyrrhotite,haematite and ilmenite

Summary The paper deals with the magnetic properties of the natural minerals haematite, ilmenite and pyrrhotite. The natural remanent magnetization J_n, the volume susceptibility , the specific magnetization of saturation _so, the Curie temperature , the A.C. demagnetization of natural remanent magnetization, the thermal demagnetization J_n, the temperature dependence of the specific magnetization of saturation, and several other parameters were determined for the individual sets of minerals. Since natural minerals can and do contain various heterogeneous and isomorphic admixtures, their magnetic properties may change depending on the content and type of these admixtures. Therefore, all the investigations of the magnetic properties described in this paper were carried out with extensive sets of minerals, representing various Czechoslovak and world localities. A number of interesting results were obtained from the executed experiments, e.g., the different stability of the natural remanent magnetization of haematite with respect to A.C. demagnetization, the presence and type of heterogeneous inclusions in ilmenite, the phase changes connected with the -transition in natural pyrrhotites, etc. The types of distributions of the investigated values of the magnetic properties were also determined.     

Exsolution of ilmenite and Cr-Ti magnetite from olivine of garnet-wehrlite

Since coesite-bearing eclogite was found in the Dabie-Sulu area[1-3], the ultrahigh-pressure (UHP) me- tamorphic belt has become a hot topic studying by domestic and overseas scientists. The studying about the forming depth of UHP metamorphic rocks is ex- tremely important, because the peak-metamorphic depth of UHP metamorphic rocks exhumated back to the crust is the key to discuss the processes of magma formation, fluid activity and metamorphism at bottom of orogenic belt. The discovery o…     

High-pressure phase transformations and compressions of ilmenite and rutile, II. Geophysical implications

The hypothesis that the earth's core consists of metallized mantle substances has been recently reanimated. The large volume changes which accompany the phase transformations of rutile-cubic phase and perovskite—mixed oxides for TiO₂ and (Fe,Mg)TiO₃, respectively, contradict the shock data for similar phase transformations in SiO₂ and MgSiO₃. In view of the relatively low pressure at the mantle-core boundary, this does not favor the hypothesis of a silicate or oxide core for the earth.     

High-pressure phase transformations and compressions of ilmenite and rutile,I. Experimental results

Natural ilmenite (Fe,Mg)TiO₃ has been found to transform to the perovskite structure and then to disproportionate into its component oxides, (Fe,Mg)O plus a cubic phase of TiO₂, at loading pressures of 140 and 250 kbar respectively, and at temperatures of 1,400 to 1,800°C. Samples were compressed in a diamond-anvil press and heated by irradiation with a YAG laser. The lattice parameters of the perovskite phase of (Fe,Mg)TiO₃ at room temperature and 1 bar are $\text{a}{}_0\text{= 4.471 ± 0.004, b}{}_0\text{= 5.753 ± 0.005,}\text{and}\text{c}{}_0\text{= 7.429 ± 0.006}\text{A}\text{?}$ with 4 molecules per cell. The zero-pressure volume change is 8.0% for the ilmenite-perovskite transition, 13.3% for the perovskite-mixed-oxides transition, and 20.2% for the ilmenite-mixed-oxides transition. The cubic phase of TiO₂ can be indexed on the basis of space group Fm3m with $\text{Z = 4}\text{and}\text{a}{}_0\text{= 4.455 ± 0.008}\text{A}\text{?}$ at room temperature and 1 bar, which corresponds to a decrease in zero-pressure volume of 29.2% for the rutile-cubic-phase transition. An isentropic bulk modulus at zero pressure of 5.75 ± 0.30 Mbar and a pressure derivative greater than 8 were calculated for the high-pressure cubic phase. The calculated bulk modulus for the mixture of (Fe,Mg)O and cubic TiO₂ is 2.48 ± 0.25 Mbar. All the phase transformations, the calculated lattice parameters, and the bulk moduli observed in this study are in good agreement with published shock-Hugoniot data for ilmenite and rutile.     

The effect of cyclic operation of brittle and ductile deformation on the metamorphic assemblage in cataclasites and mylonites

Cataclasites and mylonites, and the brittle-ductile processes that produce them, were studied at exposures along the northern rim of the Grong culmination, a transverse basement antiform in the central Scandinavian Caledonides. The rock suite studied is composed of gneisses, mylonites, and cataclasites which have a granodioritic composition. The microstructure of the rocks appears to be the result of repeated alternations of brittle faulting (associated with hydrothermal mineral growth) and ductile deformation of the crystallization products. During brittle faulting K-feldspar-chlorite veins are formed, probably by incongruent pressure solution of micas. During plastic deformation of the rocks the mineral association is transformed to white mica and green biotite, according to the reaction. $$5 biotite + 3 white mica + 9 quartz + 4 H_2 O = 8 K - feldspar + 9 chlorite$$ During this reaction plagioclase is overgrown by mica and epidote, and K-feldspar crystals are replaced by albite. The reactions which involve K-feldspars are cyclic: K-feldspar that is generated in cracks tends to be removed by albitization during ductile deformation. It is concluded that the mylonites studied represent a movement zone in the Earth's crust in which seismic and aseismic slip alternated during a large part of the deformation history.     

Impacts of dredged material disposal on a tropical soft-bottom benthic assemblage

The soft-bottom benthic macrofauna in a spoil-ground of dredged material in Cleveland Bay, North Queensland, Australia, was studied to detect possible impacts of the disposal of sediments. The spatial distribution of the assemblage was studied in relation to the source of the impact at 28 stations on four occasions during 1998 and 1999. Additionally, environmental variables were measured on each occasion at each station. Macrobenthic assemblages inside the spoil-ground were different from assemblages outside the spoil ground only immediately after (15 days) the disposal of dredged material. Given the decrease in the abundance of organisms and number of species, it is suggested that this effect was due to direct burial of the macrobenthic assemblage inside the spoil-ground. Macrobenthic assemblages inside the spoil ground were not different from assemblages outside the spoil ground 3 months after dumping. These results suggest that the soft-bottom macrobenthic assemblages may respond quickly to the disturbance associated with the dumping of dredged material.     

固城湖晚全新世以来的孢粉组合及环境变迁

本文依据固城湖GD钻孔系统的孢粉分析资料,将井深6.3m岩心所做的孢粉图式,结合~(14)C测年,从下面上分为8个孢粉组合带,进而论述了4000年来该区的植被发展和气候的4次冷暖交替的变化。4次冷期约为3.0—2.5KaB.P.2.0—1.5KaB.P.、1.0—0.8KaB.P.和0.4Ka以来。此外,还根据沉积物中的硅藻分析、有机质δ~(13)C值、有机碳含量及历史记载等资料,侧重讨论了气候及人类活动对湖泊环境演变的影响。     

Environmental determinants of leech assemblage patterns in lotic and lenitic habitats

Leeches (Clitellata: Hirudinida) are abundant predators or ecto-parasites inhabiting various freshwater habitats; however many biotic and abiotic drivers of their assemblage patterns have been deduced rather than directly tested. To study species richness and composition changes in leech assemblages, 109 sites of running and stagnant water bodies were sampled in three regions of the Czech Republic in 2007–2010, together with several explanatory variables that are known or expected to be important predictors of leech distribution. In total, 17 species of leeches were recorded, varying between 0–7 and 0–9 species in lotic and lenitic sites, respectively. These differences in species richness of lotic and lenitic sites were highly significant, contrary to the abundances, which varied between 0–283 and 0–295 individuals. The main change in species composition was controlled by water temperature and morphological characteristics (e.g. substrate and cover of macrophytes), mostly reflecting the differences between lotic and lenitic habitats. We found the density of benthos (i.e. prey availability) to be the best predictor of species composition in both lotic and lenitic sites, together with the percentage of canopy cover. However, the other significant predictors (i.e. the substrate and water conductivity found to be significant in lotic sites, and the mean annual temperature and PO₄^3? in lenitic sites), differed between these habitats. Other than mean annual temperature and water temperature, which had different effects on species richness in lotic and lenitic sites, there were no other differences between lotic and lenitic sites in terms of how species richness and abundance responded to all other analyzed predictors. Our results stress the importance of prey availability and canopy for leech distribution patterns. Differences in the significant predictors of leech assemblage patterns between lotic and lenitic sites raise fundamental questions about the underlying mechanisms and ecological constraints to leech distribution in these main types of aquatic systems.     

Grain-growth kinetics in wadsleyite: Effects of chemical environment

Grain-growth kinetics in wadsleyite was investigated using a multianvil high-pressure apparatus. Fine-grained wadsleyite aggregates were synthesized by isostatic hot-pressing and were subsequently annealed under high pressure and temperature in a controlled chemical environment. Wadsleyite samples show normal grain-growth characterized by a log-normal grain-size distribution following the relation, where n is a constant, L the grain-size at time t, L₀ the grain-size at time t = 0 and k is a rate constant that depends on temperature T and chemical environments (fO₂: oxygen fugacity in Pa, C_OH: water content in H/10⁶Si) as:

     

The ichthyoplankton assemblage and the environmental variables off the NW and N Iberian Peninsula coasts,in early spring

Ichthyoplankton and mesozooplankton were sampled and fluorescence and physical environmental variables were measured off the NW and N Iberian Peninsula coasts, during April 2005. A total of 51 species of fish larvae, belonging to 26 families, were recorded. Sardina pilchardus, with 43.8% and 58.7% of the total fish egg and larval catches, respectively, dominated the ichthyoplankton assemblage. The study area was divided by a cross-shelf frontal structure into two hydrographic regions that coincided with the Atlantic and Cantabrian geographic regions. Ichthyoplankton abundance was higher in the Cantabrian region while larval diversity was higher in the Atlantic region. This was the main alongshore variability in the structure of the larval fish assemblage. Nevertheless, the stronger variability, related with the presence of a shelf-slope front, was found in the central-eastern Cantabrian region where two major larval fish assemblages, an “outer” and a “coastal”, were distinguished. The Atlantic region, where the shelf-slope front was not found, was inhabited by a single larval fish assemblage. Canonical correspondence analysis revealed that, off the NW and N Iberian Peninsula coasts, the horizontal distribution of larval fish species in early spring may be explained by a limited number of environmental variables. Of these, the most important were the physical variables depth and sea surface temperature.     

太湖流域直湖港支流龙延河水系鱼类集群现状

分别于2009年8月和2010年5月对直湖港支流龙延河水系的鱼类集群现状及其影响因素进行研究.结果表明,研究区域有多种鱼类存在,共采集到31种,隶属于5目9科22属;春、夏两季鱼类集群的种类数没有明显差异,但春季的鱼类多度和生物量均显著高于夏季;主干支流、次级支流和支流末端等不同区位的鱼类组成也有一定差异;水体透明度、...     

A conduit-related genesis of the Lengshuiqing intrusive assemblage (Sichuan,SW China)

Lengshuiqing is part of the late Proterozoic igneous province from the western margin of the Yangtze craton. The Lengshuiqing area comprises five ultramafic–mafic intrusions, emplaced in the late Proterozoic Yanbian Group. The intrusions from Lengshuiqing contain cumulate ultramafic zones (peridotite + olivine pyroxenite), with cumulus olivine and Cr-spinel, and intercumulus pyroxenes, hornblende, phlogopite and plagioclase. Ni–Cu ore (pyrrhotite + pentlandite + chalcopyrite) is hosted in the ultramafic zones. Olivine-free diorite–quartz diorite ± gabbro and granite zones commonly occur above the ultramafic rocks. The genesis of the intrusions (conduit-related accumulation or differentiation from stagnant magma) was investigated. The amount of sulphides in the intrusions from Lengshuiqing is one order of magnitude bigger than the sulphides that can be dissolved by a volume of mafic magma similar with the volume of the intrusions. Most intrusions from Lengshuiqing have bulk composition (peridotite ± diorite ± granite) more magnesian (MgO = 21–22%; Mg# > 78) than the deduced composition of their parental magma (MgO = 9–11%; Mg# = 64–67). This indicates the accumulation of sulphide and mafic silicates from a volume of magma much bigger than the volume of the intrusions, which can be explained by the fractionation from magma ascending through the intrusions to shallower depths. A continuous supply and vent of magma is consistent with the lack of chilled margins, the melting of the wall rocks and the generation of high-temperature mineral assemblages (K-feldspar, diopside, and sillimanite) in the Yanbian Group. The intrusions from Lengshuiqing are seen as microchambers on conduits draining olivine-, Cr-spinel-, and sulphide-bearing mafic magma from a larger staging chamber.