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HUGH W. CALKINS NANCY J. OBERMEYER 《International journal of geographical information science》2013,27(3):341-351
Abstract Understanding the use and value of geographical information in decision making has been identified as crucial to the further development of geographical information systems and related spatial analysis techniques. Research into the use of geographical information will be through surveys and case studies. A taxonomy for investigating the use of geographical information and its associated value is needed to structure the surveys and case studies. This paper presents an initial attempt at such a taxonomy, intended to support continuing research into the use and value of geographical information. The survey methodology to be supported will be mostly telephone and/or mail questionnaires, and this has influenced the structure of the taxonomy. The goal of this work is to create a taxonomy suitable for use in surveys that will enhance the understanding of the use and value of geographical information in decision making. 相似文献
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GEOF H. BURBIDGE HUGH M. FRENCH BRAIN R. RUST 《Boreas: An International Journal of Quaternary Research》1988,17(1):33-40
Post-depositional structures in late Quaternary subaqueous outwash near St. Lazare, Québec resemble icewedge casts but are interpreted as water escape fissures. Cryogenic origin is discounted because, in contrast with ice-wedge casts, the fissures have a higher depth to width ratio, do not form an intersecting network, and do not exhibit adjacent upward turning of strata. In addition, their truncation by the sub-littoral unconformitydemonstrates formation before regression of the post-glacial Champlain Sea, under conditions in which ground ice development was highly unlikely. The fissures probably formed in response to elevated pore pressures caused by melting of remnant glacier ice or by liquefaction of deeper units. Excess pore water pressure initiated upward flow of dilute sediment-water mixtures that became concentrated in planar zones (fissures) along which they entrained and removed sediment. Slumping of the fissure walls followed, either during or after water escape. Minor faulting over cavities created by melting ice or water escape along fractures in underlying strata may have controlled the morphology of the fissures. 相似文献
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A Metamorphosed, Early Archaean Chromitite from West Greenland: Implications for the Genesis of Archaean Anorthositic Chromitites 总被引：2，自引：0，他引：2
An early Archaean (>3·81 Ga) chromitite–ultramaficlayered body from the Ujaragssuit nunât area, west Greenland,may represent the Earth’s oldest chromitite. The layeredbody occurs as a large xenolith (800 m x 100 m) entrained withintonalitic gneisses and preserves primary igneous layering andtextures. New Re–Os and Pb–Pb isotope results supportthe view that it has been metamorphosed twice, in the earlyand late Archaean at 3·75 Ga and 2·8 Ga. Mineralchemistry and textures indicate that the chromite compositionspreserve two different evolutionary trends. There is a mainmagmatic trend in which Cr/(Cr + Al) ratios remain relativelyconstant but in which there is strong enrichment in Fe3+, Fe2+and Ti with progressive differentiation. This trend is a compositeof magmatic-liquidus, magmatic-cooling and subsolidus re-equilibrationprocesses. A second trend is defined by chromites from harzburgitesin the upper part of the layered body. These chromites showmagmatic replacement textures in which Fe-rich chromites arealtered to aluminous chromites. Chromites showing magmatic replacementtextures are thought to have formed by reaction with a late,interstitial melt during the solidification of the layered body.The close association between the Fe3+–Cr-chromites ofthe main trend and Al-rich chromites of the type found in otherArchaean megacrystic anorthosites suggest a magmatic-geneticrelationship between the two types of chromite. We propose thatanorthositic chromites form in an Fe-rich basaltic melt derivedfrom a komatiitic, boninitic or basaltic parent magma throughreaction between the melt and early-formed Fe-rich chromite. KEY WORDS: chromite; anorthosite; Archaean; Greenland; Re–Os 相似文献
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Chromium as Cr3+ substitutes for octahedrally coordinated Alin upper-mantle minerals, thereby reducing the activity of Al2O3in the system and hence the concentration of Al2O3 in partialmelts. The effect of Cr2O3 on melt compositions multiply saturatedwith the spinel lherzolite phase assemblage has been quantifiedin the system CaO–MgO–Al2O3–SiO2–Cr2O3at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel(Cr#sp). The decrease of Al2O3 in the melt with increasing Cr#spis accompanied by increasing MgO and SiO2, whereas CaO remainsalmost constant. Consequently, the CaO/Al2O3 ratio of the meltincreases with Cr#sp, and the melt becomes richer in normativediopside, hypersthene and quartz. The effect may explain certainmantle melts with unusually high CaO/Al2O3 ratios. The concentrationof Cr2O3 in the melt remains low even at high Cr#sp, which meansthat the strong effect of Cr2O3 on partial melting equilibriais not readily apparent from its concentration in the melt itself.The existence of a highly refractory major component such asCr2O3 nullifies simplified conclusions from the ‘inverseapproach’ in the experimental study of basalt petrogenesis,as there is insufficient information in the composition of thepartial melt to reconstruct the conditions of melting. KEY WORDS: basalt petrogenesis; partial melting; reversal experiment; spinel lherzolite; system CMAS–Cr2O3; CaO/Al2O3 of melt; effect of Cr2O3 相似文献
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NICHOLSON HUGH; CONDOMINES MICHEL; FITTON J. GODFREY; FALLICK ANTHONY E.; GR?NVOLD KARL; ROGERS GRAEME 《Journal of Petrology》1991,32(5):1005-1020
The Krafla volcanic system consists of a central volcano andassociated fissure swarm in the NE axial rift zone of Iceland.Lavas spanning the whole of Krafla's exposed volcanic history(estimated to be 0-> 300 ka) have been analysed and rangein composition from olivine tholeiite to rhyolite. Major-elementcompositions suggest that fractional crystallization exertsthe main control over the differentiation process. However,K2O and the very incompatible trace elements, Rb, Th, and U,are all enriched beyond the extent expected by closed-systemfractional crystallization. Fractionation coupled with periodicreplenishment and tapping of the reservoir is unlikely to beresponsible for this enrichment, despite the geophysical evidencesuggesting a large number of inflations and deflations of ashallow magma reservoir (Tryggvason, 1986). Th- and O-isotope results confirm the work of previous authorsthat crustal assimilation is operating on a local scale beneathKrafla. A model is suggested, fitting both the Th- and O-isotopicdata, which involves the partial melting and incorporation ofa hydrothermally altered wall-rock contaminant during fractionalcrystallization (i. e., AFC processes). This process of partialmelting is likely to enhance the most highly incompatible elementconcentrations (e. g., increasing Rb/Zr) more than expectedby closed-system fractional crystallization. 相似文献
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A carbonate island barrier from the Great Oolite (Middle Jurassic) of central England 总被引：1，自引：0，他引：1
Over part of south-central England the widespread shallow-water carbonate sedimentation represented by the White Limestone Formation (Great Oolite Group: Middle Jurassic), culminated locally with the formation of a sun-cracked, stromatolitic micrite indicating a supratidal environment. The emergent area can be mapped as a broad strip running at an angle to, and offshore from, the coast-line of the ancient Anglo-Belgian landmass. As a result of the emergence of this carbonate area a polarization of environments occurred. To the north-east a series of clay-rich sediments containing lignite and a brackish and fresh-water fauna were laid down: a lagoonal environment is indicated. To the south-west fully marine shelf carbonates continued to be deposited. We suggest that isolation of the northerly area of reduced salinity from the open sea was due directly to emergence of this carbonate area, which can thus be specifically interpreted as an island barrier or spit. 相似文献
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Pluriactivity in (and beyond?) a Regulationist Crisis 总被引：1，自引：0，他引：1
The study of rural pluriactivity in New Zealand has been based on the assumption that it is a temporary phenomenon: a survival strategy in times of crisis. This paper seeks to position both pluriactivity and ‘rural crisis’ in a more sound theoretical framework by referring to recent work in regulation theory. Case studies from the Mid-Canterbury and Catlins areas are used to suggest the role that pluriactivity may fulfil in any future mode of growth in rural New Zealand. In doing so, questions are raised regarding rural ideology and agency in the process of economic change. 相似文献
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Subsolidus phase relations have been determined in the systemsSiO2-Cr-0 and MgO-SiO2-Cr-O in equilibrium with metallic Cr,at 1100–1500C and 0–288 kbar. There are no ternaryphases in the SiO2-Cr-O system at these conditions, ie. onlythe assemblage eskolaite + Cr-metal + SiO2 is found. However,in the MgO-containing system, extensive substitution of Cr2+for Mg is observed in (Mg,Cr2+)2SiO4 olivine, (Mg,Cr2+)2Si2O6pyroxene, and (Mg,Cr2+)Cr3+204 spinel. Cr 3+ levels in olivineand pyroxene are below detection limits. The pyroxene is orthorhombicat monoclinic at higher . The structure of the spinels becomestetragonally distorted at is limited by the breakdown of olivine to pyroxene + spinel+ metal. This maximum amount increases strongly with increasingtemperature, reaching >0.2 at 1500C and 48 kbar. Increasing pressure reduces the maximum. Increasing temperature also increases the maximum amounts of Cr2+ which substitute intopyroxene and spinel, indicating that end-member Cr2Si206 andCr3O4 may become stable above 1650C if melting does not intervene.Powder X-ray diffraction analysis of selected runs has beenused to extract molar volumes of the Mg-Cr2+ solid solutionsas a function of composition, which may be extrapolated to predictmolar volumes for Cr2SiO4 (olivine), Cr2Si2O6 (ortho- and clino-pyroxene)and Cr2O4 (cubic spinel) of 477, 680 and 449 cm3, respectively.The experimental data have been fitted to a thermodynamic model,including free energies of formation for end member Cr2SiO4,Cr2Si2O6 and Cr3O4. This model is then used to predict the amountsof Cr2+ which can be expected in olivine in equilibrium withCr-bearing spinel as a function of T, P and fo2. This amountincreases strongly with temperature along standard T-fo2 buffercurves, and is sufficient to explain the observed high Cr contentsof olivine from komatiites and diamond inclusions at reasonableterrestrial fo, values. The lower fo2 of the lunar environmentresults in significant Cr2+ in olivine being stable to muchlower temperatures. The tendency for the oxidation state ofCr, and hence its geochemical properties, to change with temperaturerelative to other redox reactions makes it a potentially usefulmonitor of the temperatures of uppermantle processes, and isa significant factor in the differing styles of igneous differentiationin the Earth and Moon.
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