Computational study of tetrahedral Al–Si and octahedral Al–Mg ordering in phengite     

E. J. Palin  M. T. Dove  S. A. T. Redfern  C. I. Saniz-Díaz  W. T. Lee 《Physics and Chemistry of Minerals》2003,30(5):293-304

 As part of a wider study of the nature and origins of cation order–disorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K₂ ^[6](Al₃Mg)^[4](Si₇Al)O₂₀(OH)₄. Values of the atomic exchange interaction parameters J _n used to model the energies of order–disorder were calculated. Both tetrahedral Al–Si and octahedral Al–Mg ordering were studied and hence three types of interaction parameter were necessary: for T–T, O–O and T–O interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the T–T and O–O interactions were taken from results on other systems, whilst we calculated new values for the T–O interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole T–O–T layer, hence justifying the inclusion of the T–O interactions. Simulations of a whole T–O–T layer of phengite indicated the presence of short-range order, but no long-range order was observed. Received: 8 August 2002 / Accepted: 14 February 2003 Acknowledgements The authors are grateful to EPSRC (EJP) and the Royal Society (CIS) for financial support. Monte Carlo simulations were performed on the Mineral Physics Group's Beowulf cluster and the University of Cambridge's High Performance Computing Facility.  相似文献   

Spectral and Multiscale Signal-to-Noise Thresholding of Spherical Vector Fields   总被引：1，自引：0，他引：1  

W. Freeden  T. Maier 《Computational Geosciences》2003,7(3):215-250

The basic concepts of spectral and multiscale selective reconstruction of (geophysically relevant) vector fields on the sphere from error-affected data is outlined in detail. The reconstruction mechanism is formulated under the assumption that spectral as well as multiscale approximation is well-representable in terms of only a certain number of expansion coefficients at the various resolution levels. It is shown that spectral denoising by means of orthogonal expansions in terms of vector spherical harmonics reflects global a priori information of the noise (e.g., in form of a covariance tensor field), whereas multiscale signal-to-noise thresholding can be performed under locally dependent noise information within a multiresolution analysis in terms of spherical vector wavelets. An application of the multiscale formalism to Earth's magnetic field determination is presented.  相似文献   

Source of trace element variability in Great Barrier Reef corals affected by the Burdekin flood plumes     

Chantal Alibert  Les Kinsley  Malcolm T. McCulloch  Felicity McAllister 《Geochimica et cosmochimica acta》2003,67(2):231-246

Massive corals in the Great Barrier Reef, analyzed at high-resolution for Sr/Ca (thermal ionization mass spectrometry) and trace elements such as Ba and Mn (laser ablation inductively coupled plasma mass spectrometry), can provide continuous proxy records of dissolved seawater concentrations, as well as sea surface temperature (SST). A 10-yr record (1989 to 1998) from Pandora Reef, an inshore reef regularly impacted by the freshwater plumes of the Burdekin River, is compared with an overlapping record from a midshelf reef, away from runoff influences. Surface seawater samples, taken away from river plumes, show little variability for Sr/Ca (8484 ± 10 μmol/mol) and Ba (33.7 ± 0.7 nmol/kg). Discrete Ba/Ca peaks in the inshore coral coincide with flood events. The magnitude of this Ba/Ca enrichment is most likely controlled by the amount of suspended sediments delivered to the estuary, which remains difficult to monitor. The maximum flow rate at peak river discharge is used here as a proxy for the sediment load and is shown to be strongly correlated with coral Ba/Ca (r = 0.97). After the wet summer of 1991, the coral Ba/Ca flood peak is followed by a plateau that lingers for several months after dissipation of plume waters, signifying an additional flux of Ba that may originate from submarine groundwater seeps and/or mangrove reservoirs. Both Mn and Y are enriched by a factor of ∼5 in inshore relative to midshelf corals. Mn/Ca ratios show a seasonal cycle that follows SST (r = 0.7), not river discharge, with an additional high variability in summer suggesting a link with biological activity. P and Cd show no significant seasonal variation and are at a low level at both inshore and midreef locations. However, leaching experiments suggest that part of the coral P is not lattice bound.  相似文献   

Experimental determination of the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions to 400°C     

A. Stefánsson  T.M. Seward 《Geochimica et cosmochimica acta》2003,67(7):1395-1413

The solubility of silver sulphide (acanthite/argentite) has been measured in aqueous sulphide solutions between 25 and 400°C at saturated water vapour pressure and 500 bar to determine the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave, connected to a high-performance liquid chromatographic pump, titanium sampling loop, and a back-pressure regulator on line. Samples for silver determination were collected via the titanium sampling loop at experimental temperatures and pressures. The solubilities, measured as total dissolved silver, were in the range 1.0 × 10⁻⁷ to 1.30 × 10⁻⁴ mol kg⁻¹ (0.01 to 14.0 ppm), in solutions of total reduced sulphur between 0.007 and 0.176 mol kg⁻¹ and pH_T,p of 3.7 to 12.7. A nonlinear least squares treatment of the data demonstrates that the solubility of silver sulphide in aqueous sulphide solutions of acidic to alkaline pH is accurately described by the reactions0.5Ag₂S(s) + 0.5H₂S(aq) = AgHS(aq) K_s,1110.5Ag₂S(s) + 0.5H₂S(aq) + HS⁻ = Ag(HS)2− K_s,122Ag₂S(s) + 2HS⁻ = Ag₂S(HS)22− K_s,232where AgHS(aq) is the dominant species in acidic solutions, Ag(HS)2− under neutral pH conditions and Ag₂S(HS)22− in alkaline solutions. With increasing temperature the stability field of Ag(HS)2− increases and shifts to more alkaline pH in accordance with the change in the first ionisation constant of H₂S(aq). Consequently, Ag₂S(HS)22− is not an important species above 200°C. The solubility constant for the first reaction is independent of temperature to 300°C, with values in the range logK_s,111 = −5.79 (±0.07) to −5.59 (±0.09), and decreases to −5.92 (±0.16) at 400°C. The solubility constant for the second reaction increases almost linearly with inverse temperature from logK_s,122 = −3.97 (±0.04) at 25°C to −1.89 (±0.03) at 400°C. The solubility constant for the third reaction increases with temperature from logK_s,232 = −4.78 (±0.04) at 25°C to −4.57 (±0.18) at 200°C. All solubility constants were found to be independent of pressure within experimental uncertainties. The interaction between Ag⁺ and HS⁻ at 25°C and 1 bar to form AgHS(aq) has appreciable covalent character, as reflected in the exothermic enthalpy and small entropy of formation. With increasing temperature, the stepwise formation reactions become progressively more endothermic and are accompanied by large positive entropies, indicating greater electrostatic interaction. The aqueous speciation of silver is very sensitive to fluid composition and temperature. Below 100°C silver(I) sulphide complexes predominate in reduced sulphide solutions, whereas Ag⁺ and AgClOH⁻ are the dominant species in oxidised waters. In high-temperature hydrothermal solutions of seawater salinity, chloride complexes of silver(I) are most important, whereas in dilute hydrothermal fluids of meteoric origin typically found in active geothermal systems, sulphide complexes predominate. Adiabatic boiling of dilute and saline geothermal waters leads to precipitation of silver sulphide and removal of silver from solution. Conductive cooling has insignificant effects on silver mobility in dilute fluids, whereas it leads to quantitative loss of silver for geothermal fluids of seawater salinity.  相似文献   

Lead isotopic disequilibrium between sulfide and plagioclase in the bushveld complex and the chemical evolution of large layered intrusions     

E.A Mathez  T.E Waight 《Geochimica et cosmochimica acta》2003,67(10):1875-1888

The Pb isotopic compositions of coexisting plagioclase and sulfide from the Bushveld Complex were determined by laser ablation multi-collector ICPMS (LA MC-ICPMS). The samples are of the upper Critical Zone in the northeast corner of the Complex and were collected from drill core and underground mine exposures. All the rocks are fresh and exhibit no evidence for alteration, weathering, or disruption of the Pb isotope systematics subsequent to the initial cooling of the intrusion. Furthermore, individual plagioclase and sulfide crystals do not contain enough U to warrant correction for radiogenic in-growth. For these reasons, the measured Pb isotope ratios approximate the initial ones. For plagioclase, ²⁰⁷Pb/²⁰⁶Pb ranges from 0.98 to 1.02 and ²⁰⁸Pb/²⁰⁶Pb from 2.26 to 2.35. Low ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios characterize grain boundaries and partially annealed microcracks, some of which contain minute fragments of sulfide and other phases, and this accounts for most, if not all, the heterogeneity exhibited by individual samples. Real compositional differences exist, however, in plagioclase from different lithologic layers. For example, plagioclase ²⁰⁷Pb/²⁰⁶Pb values vary from 1.004 in norite beneath the Merensky pyroxenite to 1.009 in the mineralized pyroxenite, and 0.997 in overlying norite. In most samples in which sulfide and plagioclase coexist, the sulfide ²⁰⁷Pb/²⁰⁶Pb ratio is lower and ²⁰⁸Pb/²⁰⁶Pb ratio higher than the corresponding ones in plagioclase. For example, in a mineralized Merensky reef sample, average sulfide ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios are 0.993 and 2.313, respectively, while those in plagioclase are 1.000 and 2.292. In one sample, the sulfide is extremely heterogeneous, with ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios as low as 0.84 and 2.12. In this particular sample, the compositions must represent an isolated occurrence of addition of a young Pb component.The array of sulfide and plagioclase compositions requires multiple sources of Pb at the time of crystallization or soon thereafter. The disequilibrium between plagioclase and sulfide implies that some of the Pb originated from the isotopically distinct country rocks and was introduced at temperatures at which the composition of sulfide but not plagioclase could be modified. Thus, Bushveld sulfide, and to some extent plagioclase, do not reliably record the initial Pb isotopic composition(s) of the parent magma(s).  相似文献   

Isolation and water-level fluctuations of Lake Kachishayoot, Northern Québec, Canada     

Luc MiousseNajat Bhiry  Martin Lavoie 《Quaternary Research》2003,60(2):149-161

This study combines different methods, including grain size, macrofossil, and pollen analyses, to reconstruct paleogeographical and paleoclimatological conditions for periods before, during, and after the isolation of a small lake (Lake Kachishayoot) in northern Québec. After the retreat of the Laurentide Ice Sheet around 8000 ¹⁴C yr B.P., the area was submerged by the Tyrrell Sea. The transition from marine to lacustrine environment occurred about 5400 yr B.P. Two major periods of water-level fluctuations were inferred from organic and mineral sediments: a high water level that occurred after 3200 yr B.P. and a low water level that started before 2200 yr B.P. Our chronological data for the first period are consistent with those from nearby Lac des Pluviers and from other lakes in east central Canada and in the northeastern United States. During the low-water-level period, however, there is no evidence for minor fluctuations, whereas other lakes in northern Québec and east-central Canada underwent several brief lowerings. Long-term changes in atmospheric circulation caused by changing global boundary conditions likely explained long-term water-level fluctuations of Lake Kachishayoot.  相似文献   

Recent advances in accessibility research: Representation,methodology and applications   总被引：22，自引：0，他引：22  

Mei-Po Kwan  Alan T. Murray  Morton E. O'Kelly  Michael Tiefelsdorf 《Journal of Geographical Systems》2003,5(1):129-138

 In this article we examine recent advances in accessibility research and their implications for future studies. We base our discussion on three intersecting dimensions that are useful for evaluating the contribution of recent studies: representation, methodology and applications. Various examples are selected to show that research concerned with representation and methodological problem solving is often applied to issues of broad concern in policy and planning. It is, however, not clear that the simultaneous treatment of representation, methodological and application issues has ever been fully worked out. The questions raised in this article may serve as a foundation for addressing issues pertinent to accurate representation, improved model building, and more rigorous applications in accessibility research. Received: 9 December 2002 / Accepted: 10 February 2003  相似文献   

A coupled model study on ENSO, MJO and Indian summer monsoon rainfall relationships   总被引：1，自引：0，他引：1  

A. Chakraborty  T. N. Krishnamurti 《Meteorology and Atmospheric Physics》2003,84(3-4):243-254

Summary In this paper, we have tried to understand the ENSO, MJO and Indian summer monsoon rainfall relationships from observation as well as from coupled model results. It was the general feeling that El-Niño years are the deficient in Indian monsoon rainfall and converse being the case for the La-Niña years. Recent papers by several authors noted the failure of this relationship. We find that the model output does confirm a breakdown of this relationship. In this study we have seen that a statistically defined modified Indian summer monsoon rainfall (MISMR) index, a linearly regressed ISMR index and dynamical Webster index (WBSI), shows an inverse relationship with ENSO index during the entire period of integration (1987 to 1999). It is also seen from this study that the amplification of the MJO signals were large and the ENSO signals were less pronounced during the years of above normal ISMR. The MJO signal amplitudes were small and ENSO signals were strong during the years of deficient ISMR. It has been noted that here is a time lag between the MJO and ENSO signal in terms of their modulation aspect. If time lag is added with the ENSO signal then both signals maintain the amplitude modulation theory. A hypothesis is being proposed here to define a relationship between MJO and ENSO signals for the entire period between 1987 and 1999.Received September 18, 2002; revised November 22, 2002; accepted December 20, 2002 Published online: May 8, 2003  相似文献   

Evolution of the equatorial and dipole modes of the sea-surface temperature in the Tropical Atlantic at decadal scale     

R. V. Andreoli  M. T. Kayano 《Meteorology and Atmospheric Physics》2003,83(3-4):277-285

Summary ?One hundred and thirty six years (1856–1991) of monthly sea-surface temperature (SST) data in the Tropical Atlantic Ocean are used to investigate the propagating signals of the SST at a decadal (DD) time scale. The first and the third evolving modes show a relationship between the equatorial and the inter-hemispheric patterns, one evolving into the other mode and vice-versa. These modes describe two different evolutions of the SST at DD time-scale. The first EEOF features a 12-year period oscillatory regime with a strong 2-year duration inter-hemispheric pattern evolving into a weak 1-year duration equatorial pattern and vice-versa. This mode exhibits also a northward displacement of the anomalies in the band 15° S–15° N. The third EEOF also shows an oscillatory regime, but with a period of 10 years and with a relatively strong 2-year duration equatorial pattern evolving into a weak 1-year duration inter-hemispheric pattern and vice-versa. For this mode, the SST anomalies show a southward displacement in the band 15° S–15° N. These results have not yet been documented in previous works and explain some of the previous findings on the DD variability in the Tropical Atlantic. Received December 31, 2001; revised April 9, 2002; accepted September 4, 2002 Published online: March 20, 2003  相似文献   

Turbulence Statistics Inside and Over Forest: Influence on Footprint Prediction   总被引：2，自引：0，他引：2  

Ü. Rannik  T. Markkanen  J. Raittila  P. Hari  T. Vesala 《Boundary-Layer Meteorology》2003,109(2):163-189

Observations of wind statistics within and above a Scots pine forest are comparedwith those predicted from an analytical second-order closure model. The roughnesssublayer (RSL) effects, and the influence of stability on similarity functions, arestudied using observations. The commonly accepted forms of similarity functionsdescribe the influence of diabatic effects above the RSL well. According to earlierstudies they are expected also to apply within the RSL. As an exception, the averagewind speed normalised with friction velocity was found to be invariant with stabilityclose to the canopy top under unstable conditions. Lagrangian stochastic trajectorysimulations were used to evaluate the influence of canopy turbulence profiles onfootprint prediction. The main uncertainty was found to arise from parameterisationof the random forcing term in the Lagrangian velocity equation. The influence ofdiabatic conditions was studied, and it was found that thermal stability affectssignificantly the footprint function above the forest canopy, but significantuncertainty exists because of uncertainties in the formulation of stability functions.  相似文献