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91.
Merlin Méheut Michele Lazzeri Francesco Mauri 《Geochimica et cosmochimica acta》2007,71(13):3170-3181
Isotopic fractionation factors for oxygen, hydrogen and silicon have been calculated using first-principles methods for the kaolinite, quartz, water (ice and gas water) system. Good agreement between theory and experiment is obtained for mineral-water oxygen isotope fractionation. This approach gives reliable results on isotopic fractionation factors as a function of temperature, within a relative precision of typically 5%. These calculations provide independent quantitative constraints on the internal fractionation of oxygen in kaolinite, the fractionation of silicon isotopes at equilibrium, or hydrogen fractionation between kaolinite and water. Calculated fractionation factors at 300 K are 12.5‰ for the kaolinite internal-fractionation of oxygen, and 1.6‰ for silicon fractionation between quartz and kaolinite. 相似文献
92.
Halogen diffusion in a basaltic melt 总被引:2,自引:0,他引:2
The diffusion of the halogens fluorine, chlorine and bromine was measured in a hawaiitic melt from Mt. Etna at 500 MPa and 1.0 GPa, 1250 to 1450 °C at anhydrous conditions; the diffusion of F and Cl in the melt was also studied with about 3 wt% of dissolved water. Experiments were performed using the diffusion-couple technique in a piston cylinder. Most experiments were performed with only one halogen diffusing between the halogen-enriched and halogen-poor halves of the diffusion couple, but a few experiments with a mixture of halogens (F, Cl and Br) were also performed in order to investigate the possibility of interactions between the halogens during diffusion. Fluorine and chlorine diffusivity show a very similar behavior, slightly diverging at low temperature. Bromine diffusion is a factor of about 2-5 lower than the other halogens in this study. Diffusion coefficients for fluorine range between 2.3 × 10−11 and 1.4 × 10−10 m2 s−1, for chlorine between 1.1 × 10−11 and 1.3 × 10−10 and for bromine between 9.4 × 10−12 and 6.8 × 10−11 m2 s−1. No pressure effect was detected at the conditions investigated. In experiments involving mixed halogens, the diffusivities appear to decrease slightly (by a factor of ∼3), and are more uniform among the three elements. However, activation energies for diffusion do not appear to differ between experiments with individual halogens or when they are all mixed together. The effect of water increases the diffusion coefficients of F and Cl by no more than a factor of 3 compared to the anhydrous melt (DF = 4.0 × 10−11 to 1.6 × 10−10 m2 s−1; DCl = 3.0 × 10−11 to 1.9 × 10−10 m2 s−1). Comparing our results to the diffusion coefficients of other volatiles in nominally dry basaltic melts, halogen diffusivities are about one order of magnitude lower than H2O, similar to CO2, and a factor of ∼5 higher than S. The contrasting volatile diffusivities may affect the variable extent of volatile degassing upon melt depressurization and vesiculation, and can help our understanding of the compositions of rapidly grown magmatic bubbles. 相似文献
93.
We compare eruptive dynamics, effects and deposits of the Bezymianny 1956 (BZ), Mount St Helens 1980 (MSH), and Soufrière
Hills volcano, Montserrat 1997 (SHV) eruptions, the key events of which included powerful directed blasts. Each blast subsequently
generated a high-energy stratified pyroclastic density current (PDC) with a high speed at onset. The blasts were triggered
by rapid unloading of an extruding or intruding shallow magma body (lava dome and/or cryptodome) of andesitic or dacitic composition.
The unloading was caused by sector failures of the volcanic edifices, with respective volumes for BZ, MSH, and SHV c. 0.5,
2.5, and 0.05 km3. The blasts devastated approximately elliptical areas, axial directions of which coincided with the directions of sector
failures. We separate the transient directed blast phenomenon into three main parts, the burst phase, the collapse phase,
and the PDC phase. In the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into
the atmosphere, with much of the expansion having an initially lateral component. The erupted material fails to mix with sufficient
air to form a buoyant column, but in the collapse phase, falls beyond the source as an inclined fountain, and thereafter generates
a PDC moving parallel to the ground surface. It is possible for the burst phase to comprise an overpressured jet, which requires
injection of momentum from an orifice; however some exploding sources may have different geometry and a jet is not necessarily
formed. A major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be
attributed to shock waves within an overpressured jet, or alternatively to dynamic pressures and shocks within the energetic
collapse and PDC phases. Internal shock structures related to unsteady flow and compressibility effects can occur in each
phase. We withhold judgment about published shock models as a primary explanation for the damage sustained at MSH until modern
3D numerical modeling is accomplished, but argue that much of the damage observed in directed blasts can be reasonably interpreted
to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified PDC.
This view is reinforced by recent modeling cited for SHV. In distal and peripheral regions, solids concentration, maximum
particle size, current speed, and dynamic pressure are diminished, resulting in lesser damage and enhanced influence by local
topography on the PDC. Despite the different scales of the blasts (devastated areas were respectively 500, 600, and >10 km2 for BZ, MSH, and SHV), and some complexity involving retrogressive slide blocks and clusters of explosions, their pyroclastic
deposits demonstrate strong similarity. Juvenile material composes >50% of the deposits, implying for the blasts a dominantly
magmatic mechanism although hydrothermal explosions also occurred. The character of the magma fragmented by explosions (highly
viscous, phenocryst-rich, variable microlite content) determined the bimodal distributions of juvenile clast density and vesicularity.
Thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater, and laterally
from the axial region. The proximal stratigraphy of the blast deposits comprises four layers named A, B, C, D from bottom
to top. Layer A is represented by very poorly sorted debris with admixtures of vegetation and soil, with a strongly erosive
ground contact; its appearance varies at different sites due to different ground conditions at the time of the blasts. The
layer reflects intense turbulent boundary shear between the basal part of the energetic head of the PDC and the substrate.
Layer B exhibits relatively well-sorted fines-depleted debris with some charred plant fragments; its deposition occurred by
rapid suspension sedimentation in rapidly waning, high-concentration conditions. Layer C is mainly a poorly sorted massive
layer enriched by fines with its uppermost part laminated, created by rapid sedimentation under moderate-concentration, weakly
tractive conditions, with the uppermost laminated part reflecting a dilute depositional regime with grain-by-grain traction
deposition. By analogy to laboratory experiments, mixing at the flow head of the PDC created a turbulent dilute wake above
the body of a gravity current, with layer B deposited by the flow body and layer C by the wake. The uppermost layer D of fines
and accretionary lapilli is an ash fallout deposit of the finest particles from the high-rising buoyant thermal plume derived
from the sediment-depleted pyroclastic density current. The strong similarity among these eruptions and their deposits suggests
that these cases represent similar source, transport and depositional phenomena. 相似文献
94.
Marina A. Ivanova Michail I. Petaev Glenn J. Macpherson Michail A. Nazarov Lawrence A. Taylor John A. Wood 《Meteoritics & planetary science》2002,37(10):1337-1344
Abstract— Natural calcium monoaluminate, CaAl2O4, has been found in a grossite‐rich calcium‐aluminum‐rich inclusion (CAI) from the CH chondrite Northwest Africa 470. The calcium monoaluminate occurs as colorless ~10 μm subhedral grains intergrown with grossite, perovskite, and melilite. Nebular condensation is the most likely origin for the precursor materials of this CAI, but calculations suggest that dust/gas ratios substantially enhanced over solar are required to stabilize CaAl2O4. 相似文献
95.
Marina Manca Claudio Ramoni Patrizia Comoli 《Aquatic Sciences - Research Across Boundaries》2000,62(2):142-153
We reconstruct the impact of different environmental cues on Daphnia seasonal dynamics by means of an analysis of demographic and reproductive parameters, as well as of the size structure of the population. Data from 1996, indicative of the most recent productivity of the lake, with those from 1973, when the lake was meso-eutrophic, allow a discussion of the relevance of the observed changes for the structure of the pelagic food web of Lago Maggiore. Daphnia summer dynamics is mainly controlled by Bythotrephes longimanus. This is quite a different situation from that of the past, when Leptodora kindtii was the prevalent predatory cladocera. The size structure of Daphnia population revealed a depletion of small ovigerous females with increases in the predator. Our data allow a discussion of the idea, recently proposed, of Bythotrephes longimanus as a visual sit- and -wait predator. 相似文献
96.
97.
This study presents results of the first comprehensive research on ecotoxic trace metals (Cd, Pb, Cu and Zn) in aquatic anchialine ecosystems. Data show the influence of hydrological and geological characteristics on trace metals in highly stratified anchialine water columns. 相似文献
98.
Marina Gibilisco 《Astrophysics and Space Science》1997,249(2):189-210
In this paper, I investigate a local effect of polarization of the Cosmic Microwave Background (CMB) in clusters of galaxies,
induced by the Thomson scattering of an anisotropic radiation. A local anisotropy of the CMB is produced by some scattering
and gravitational effects, as, for instance, the Sunyaev Zel‘dovich effect, the Doppler shift due to the cluster motion and
the gravitational lensing. The resulting anisotropy ΔI/I depends on the physical properties of the clusters, in particular
on their emissivity in the X band on their size, on their gravitational potential and on the peculiar conditions characterizing
the gas they contain. By solving the Boltzmann radiative transfer equation in presence of such anisotropies I calculate the
average polarization at the centre of some clusters, namelyA2218, A576 and A2163, whose properties are quite well known. I
prove that the gravitational effects due to the contraction or to the expansion have some importance, particularly for high
density structures; moreover, the peculiar motion of the cluster, considered as a gravitational lens, influences the propagation
of the CMB photons by introducing a particular angular dependence in the gravitational anisotropy and in the scattering integrals.
Thus, the gravitational and the scattering effects overally produce an appreciable local average polarization of the CMB,
may be observable through a careful polarization measurements towards the centres of the galaxy clusters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
99.
Previous numerical simulations have shown that under certain conditions rotating gas clouds break up in two equal parts. We explore the consequences, if this process continues and a planar, equal mass four-body system forms. The results of the four-body calculations are used in Monte Carlo simulation of fragmentation of a rotating gas cloud in a galactic nucleus. Then it appears possible that clusters of large numbers of objects form; the dynamical evolution of these clusters have been calculated. The results were applied to ejection of supermassive objects from galactic nuclei. We find that several statistical properties of double radio sources may be understood as resulting from the fragmentation process. 相似文献
100.
Alessandro B. Romeo Gustaf Rydbeck Mauri J. Valtonen 《Astrophysics and Space Science》1994,216(1-2):359-359
Work is in progress for constructing physically consistentN-body models capable of supporting a long-lived spiral structure. Such models would represent a missing link between theory and observations of disc galaxies. 相似文献