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91.
Stephan Lowitzer Dan J. Wilson Björn Winkler Victor Milman Julian D. Gale 《Physics and Chemistry of Minerals》2008,35(3):129-135
Knowledge of the defect properties of Lunar and Mercurian minerals has recently become important, with the advent of models
which attempt to explain the formation of the thin exosphere of these celestial bodies. Here, we have calculated the formation
energies of sodium and oxygen vacancies in the mineral albite (NaAlSi3O8), as well as the Schottky defect energy for the removal of a Na2O unit. We have employed both the supercell and Mott–Littleton approaches, using Kohn–Sham density functional theory and classical
interatomic potential methods. As well as reporting the defect energies and structures, we comment upon the relative merits
of the methods used. 相似文献
92.
The ultramafic-hosted Logatchev Hydrothermal Field (LHF) at 15°N on the Mid-Atlantic Ridge and the Arctic Gakkel Ridge (GR) feature carbonate precipitates (aragonite, calcite, and dolomite) in voids and fractures within different types of host rocks. We present chemical and Sr isotopic compositions of these different carbonates to examine the conditions that led to their formation. Our data reveal that different processes have led to the precipitation of carbonates in the various settings. Seawater-like 87Sr/86Sr ratios for aragonite in serpentinites (0.70909 to 0.70917) from the LHF are similar to those of aragonite from the GR (0.70912 to 0.70917) and indicate aragonite precipitation from seawater at ambient conditions at both sites. Aragonite veins in sulfide breccias from LHF also have seawater-like Sr isotope compositions (0.70909 to 0.70915), however, their rare earth element (REE) patterns show a clear positive europium (Eu) anomaly indicative of a small (< 1%) hydrothermal contribution. In contrast to aragonite, dolomite from the LHF has precipitated at much higher temperatures (~ 100 °C), and yet its 87Sr/86Sr ratios (0.70896 to 0.70907) are only slightly lower than those of aragonite. Even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine–talc fault schists form north of the LHF. These calcites show unradiogenic 87Sr/86Sr ratios (0.70460 to 0.70499) indicative of precipitation from evolved hydrothermal fluids. A simple mixing model based on Sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation. Dolomite precipitated from a mixture of 97% seawater and 3% hydrothermal fluid that should have had a temperature of approximately 14 °C assuming that no heat was transferred. The much higher apparent precipitation temperatures based on oxygen isotopes (~ 100 °C) may be indicative of conductive heating, probably of seawater prior to mixing. The hydrothermal calcite in the fault schist has precipitated from a mixture of 67% hydrothermal fluid and 33% seawater, which should have had an isenthalpic mixing temperature of ~ 250 °C. The significantly lower temperatures calculated from oxygen isotopes are likely due to conductive cooling of hydrothermal fluid discharging along faults. Rare earth element patterns corroborate the results of the mixing model, since the hydrothermal calcite, which formed from waters with the greatest hydrothermal contribution, has REE patterns that closely resemble those of vent fluids from the LHF. Our results demonstrate, for the first time, that (1) precipitation from pure seawater, (2) conductive heating of seawater, and (3) conductive cooling of hydrothermal fluids in the sub-seafloor all can lead to carbonate precipitation within a single ultramafic-hosted hydrothermal system. 相似文献
93.
Abandoned shorelines are an important archive used to constrain past fluctuations in the hydrological balance of lakes around the globe. Within Australia, the shorelines preserved at Lake George, NSW, form one of the few shoreline archives in the south-east of the continent that record palaeoenvironmental conditions throughout the late Quaternary. Here, we examined and tested the lake-level record for Lake George constructed in the 1970s by dating a well-preserved shoreline sequence at Luckdale, on the lake's eastern shore, using single-grain optically stimulated luminescence (OSL) dating. Ten stratigraphic units were identified, and these suggest a late Quaternary highstand for Lake George in MIS 3, with fluctuations superimposed upon an overall drying trend throughout MIS 2 and into the present. At Luckdale, the highest four shoreline-associated units were deposited ~13 to 19 m above lake base and date to between 39 ± 2 and 29 ± 1 ka ago. Our study pushes back the timing of maximum lake depth at Lake George to at least MIS 3, rather than MIS 2. The overall drying trend is supported by similar reductions in both Riverine Plain fluvial activity and other associated lake-level records from within the Murray basin. 相似文献
94.
We explore the construction of community meaning on an emerging amenity landscape in the western United States through in-depth interviews and ethnographic fieldwork with residents of rural Fremont County, Colorado. The meanings ascribed to community center on different interpretations of home, social interaction, and personal identity. Although in some cases these differences correspond to the conventional division between new and long-term residents, the more significant finding was a discourse of “co-opetition” that captures a broader countywide understanding of community as simultaneous cooperation and competition among local stakeholders. Ultimately, this conceptualization of rural community is a way of negotiating the form and function of the amenity landscape. The production of meaningful social interaction points to opportunities for engaging residents in land management as they interact with one another and the landscape. 相似文献
95.
96.
The development and recovery of a shallow and hypertrophic lake following a reduction in the external phosphorus load has been documented. In spite of this reduction, phosphorus concentrations and phytoplankton biomass in the lake water are still very high. The reason for this development can be explained by three main factors: (1): the sediment has been accumulating a large phosphorus pool which is now causing a high internal phosphorus load, (2) due to the shallow conditions, resuspension of the upper sediment often takes place because of wind action and thereby increases the phosphorus loading from the sediment, (3) the development of a large population of planktivorous fish decreases the zooplankton biomass resulting in the development of a large phytoplankton biomass. 相似文献
97.
Stochastic Structural Modeling 总被引:3,自引:0,他引:3
Lars Holden Petter Mostad Bjørn Fredrik Nielsen Jon Gjerde Chris Townsend Signe Ottesen 《Mathematical Geology》2003,35(8):899-914
A consistent stochastic model for faults and horizons is described. The faults are represented as a parametric invertible deformation operator. The faults may truncate each other. The horizons are modeled as correlated Gaussian fields and are represented in a grid. Petrophysical variables may be modeled in a reservoir before faulting in order to describe the juxtaposition effect of the faulting. It is possible to condition the realization on petrophysics, horizons, and fault plane observations in wells in addition to seismic data. The transmissibility in the fault plane may also be included in the model. Four different methods to integrate the fault and horizon models in a common model is described. The method is illustrated on an example from a real petroleum field with 18 interpreted faults that are handled stochastically. 相似文献
98.
Using sewage sludge as a sealing layer to remediate sulphidic mine tailings: a pilot-scale experiment, northern Sweden 总被引:1,自引:0,他引:1
At the Kristineberg mine, northern Sweden, sulphidic mine tailings were remediated in an 8-year pilot-scale experiment using sewage sludge to evaluate its applicability as a sealing layer in a composite dry cover. Sediment, leachate water, and pore gas geochemistry were collected in the aim of determining if the sludge was an effective barrier material to mitigate acid rock drainage (ARD) formation. The sludge was an effective barrier to oxygen influx as it formed both a physical obstruction and functioned as an organic reactive barrier to prevent oxygen to the underlying tailings. Sulphide oxidation and consequential ARD formation did not occur. Sludge-borne trace elements accumulated in a reductive, alkaline environment in the underlying tailings, resulting in an effluent drainage geochemistry of Cd, Cu, Pb and Zn below 10 μg/L, high alkalinity (810 mg/L) and low sulphate (38 mg/L). In contrast, the uncovered reference tailings received a 0.35-m deep oxidation front and typical ARD, with dissolved concentrations of Cd, Zn and sulphate, 20.8 μg/L, 16,100 μg/L and 1,390 mg/L, respectively. Organic matter degradation in the sludge may be a limiting factor to the function of the sealing layer over time as 85 % loss of the organic fraction occurred over the 8-year experimental period due to aerobic and anaerobic degradation. Though the cover may function in the short to medium term (100 years), it is unlikely to meet the demands of a long-term remedial solution. 相似文献
99.
Characteristics of Water Ingress in Norwegian Subsea Tunnels 总被引:2,自引:1,他引:2
Bjørn Nilsen 《Rock Mechanics and Rock Engineering》2014,47(3):933-945
100.
N. R. J. Poolton K. B. Ozanyan J. Wallinga A. S. Murray L. Bøtter-Jensen 《Physics and Chemistry of Minerals》2002,29(3):217-225
Most natural feldspars contain many charged impurities, and display a range of bond angles, distributed about the ideal.
These effects can lead to complications in the structure of the conduction band, giving rise to a tail of energy states (below
the high-mobility conduction band) through which electrons can travel, but with reduced mobility: transport through these
states is expected to be thermally activated. The purpose of this article is twofold. Firstly, we consider what kind of lattice
perturbations could give rise to both localized and extended conduction band-tail states. Secondly, we consider what influence
the band tails have on the luminescence properties of feldspar, where electrons travel through the sample prior to recombination.
The work highlights the dominant role that 0.04–0.05-eV phonons play in both the luminescence excitation and emission processes
of these materials. It also has relevance in the dating of feldspar sediments at elevated temperatures.
Received: 11 May 2001 / Accepted: 6 September 2001 相似文献