Strontium Isotopic Studies of Alkalic Rocks: The Potassium-rich Lavas of the Birunga and Toro--Ankole Regions, East and Central Equatorial Africa     

BELL  KEITH; POWELL  J. L. 《Journal of Petrology》1969,10(3):536-572

The present-day Sr⁸⁷/Sr⁸⁶ ratios of 117 representative samplesfrom the Birunga and Toro-Ankole regions vary significantlyand range between 0.7036 and 0.7111. The feldspar-bearing lavashave higher ratios (average = 0.707) than the melilite- andnepheline-bearing varieties (average = 0.705). Samples of carbonatedlavas have slightly lower Sr⁸⁷/Sr⁸⁶ ratios. The Sr⁸⁷/Sr⁸⁶ ratios show a highly significant, positive, linearcorrelation with Rb/Sr ratio, and a negative correlation withSr, Nb, and Zr abundances. Graphs of Sr⁸⁷/Sr⁸⁶ ratios againstelemental abundances in some cases give hyperbolic patterns.Such relationships are true, not only for the volcanic fieldas a whole, but also for lava flows from one extensively sampledvolcano. Hypotheses involving simple fractional crystallization or limestonesyntexis are inconsistent with the isotopic data. The elementaland isotopic abundance patterns are most easily explained bythe mixing of two end members of quite different Sr isotopicand chemical compositions. If mixing is assumed, approximatelimits can be set for the compositions of the two end members.These limits are consistent with the hypothesis of assimilationof sialic material by either a carbonatitic or nepheliniticparent magma.  相似文献   

The Source Regions of Ocean Island Basalts   总被引：33，自引：17，他引：16  

McKENZIE  DAN; O'NIONS  R. KEITH 《Journal of Petrology》1995,36(1):133-159

The geochemical modelling of many small-volume continental magmasshows that their source regions must have been depleted by basaltformation, and later enriched by the addition of a metasomaticmelt, formed by melting 03% of the MORB source. The presenceof such magmas throughout western Turkey and the Aegean, whereno plume is present, requires such magmas to be formed at temperaturesconsiderably below the dry solidus. Similar magmas elsewherebring up nodule suites, many of which have the same compositionas the source regions of the host magmas. Pressure and temperatureestimates from garnetbearing suites, and temperature estimatesfrom those without garnet, show that the nodules last equilibratedat pressures and temperatures close to those of the wet solidus.Magmas from the smaller oceanic islands and from some seamountsclosely resemble small-volume continental magmas, and also comefrom sources that have been metasomaticaUy enriched. However,no data sets from any of the oceanic islands that have yet beenmodelled require their source regions to have been depletedbefore being enriched The density of the sources of continentaland oceanic basalts can be obtained from their calculated modes.In the garnet peridotite stability field the sources of oceanisland basalts have densities that are slightly greater thanthat of the MORB source, whereas those of most small-volumecontinental magmas are lighter. Therefore ocean island sourcesalone are easily entrained into the thermal convection beneaththe plates. A numerical experiment shows that material in thehot and cold boundary layers of high Rayleigh number time-dependentconvection tends to remain in the boundary layers for severaloverturns, rather than moving into the interior of the circulation.A simple model that can account for the elemental and isotopiccomposition of ocean island basalts forms their sources by theaddition of metasomatic melt to the undcplcUd MORB source whileit forms the lower part of the mechanical boundary layer beneathcontinents. The isotopic differences between ocean island basaltand MORB are generated before the source becomes entrained inthe cold sinking plumes that fall to the base of the convectinglayer. At the base the material is heated and rises as partof a hot plume. Because the metasomatic melt contains waterand carbonates, the enriched regions start to melt and generatemore melt on decompression than does the MORB source. Such regionscan therefore generate islands and seamounts. Even when theenriched material moves into the interior of the circulationand acquires the mean potential temperature of the mantle, itwill still generate more melt on decompression than will theMORB source, and the isotopic and elemental composition willstill be distinctive. The model can therefore account for theobserved composition of magmas from seamounts that cannot beproduced from either the MORB or the primitive source. ^*Corresponding author  相似文献   

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton   总被引：6，自引：1，他引：6  

DOWNES  HILARY; MACDONALD  RAY; UPTON  BRIAN G. J.; COX  KEITH G.; BODINIER  JEAN-LOUIS; MASON  PAUL R. D.; JAMES  DODIE; HILL  PETER G.; HEARN  B. CARTER  JR 《Journal of Petrology》2004,45(8):1631-1662

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette  相似文献   

THE CLASSIFICATION OF CROSS-STRATIFIED UNITS, COMMENT ON A PAPER BY J. R. L. ALLEN     

KEITH A. W. CROOK 《Sedimentology》1965,5(3):249-254

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Flood management through LANDSAT TM and ERS SAR data: a case study     

GIULIANA PROFETI  HEATHER MACINTOSH 《水文研究》1997,11(10):1397-1408

This paper deals with the experimental activity carried out in the field of flood monitoring at the Civil Engineering Department of Florence University (Italy) in cooperation with EOSAT (USA) and Eurimage (Italy). The aim of the study is to research the possibilities of satellite data utilization to aid in modelling of the hydrological behaviour of a river basin and monitoring flood emergencies. The area selected for the study is the Fucecchio Marsh (Tuscany, Italy), in which flooding events are very frequent. This paper describes the results of the study, with particular reference to the use of Landsat TM data to estimate soil water content, and the use of ERS SAR data to analyse flash flood events. © 1997 John Wiley & Sons, Ltd.  相似文献   

WIND EROSION AND PM10 EMISSIONS FROM AGRICULTURAL FIELDS ON THE COLUMBIA PLATEAU     

LARRY D. STETLER  KEITH E. SAXTON 《地球表面变化过程与地形》1996,21(7):673-685

Research to investigate relations between simultaneous processes of soil erosion by wind and PM₁₀ emissions from dry land fields on the Columbia Plateau, Washington, has been initiated by the U.S. Department of Agriculture — Agricultural Research Service (ARS) and Washington State University. Dry land agriculture is the primary cropping system world-wide but is constantly threatened by erosive winds which reduce the soil resource and/or remove fines which are the most active soil portion for plant production. Soils on the Columbia Plateau are very fine-grained containing more than 4 per cent of freely occurring aggregates or particles less than 10 μm aerodynamic diameter. Analysis from eight wind events show that significant relations exist between total mass of soil in both horizontal (saltation) and vertical (suspension) transport. The data suggest that by limiting horizontal soil erosion, emitted dust can be simultaneously reduced. Aggressive conservation tillage can be an effective weapon in reducing soil erosion by maintaining surface residue and roughness.  相似文献   

NEW UNCERTAINTY CONCEPTS IN HYDROLOGY AND WATER RESOURCES edited by Z. W. Kundzewicz,Cambridge University Press/UNESCO/IASH International Hydrology Series, 1995. No. of pages: xiii + 322. Price: £75.00 (hb). ISBN 0-521-46118-9.     

KEITH BEVEN 《地球表面变化过程与地形》1996,21(10):982-983

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Tidal influence in Cretaceous fluvial strata from Utah, USA: a key to sequence stratigraphic interpretation   总被引：6，自引：0，他引：6  

KEITH W. SHANLEY  PETER J. McCABE†  ROBERT D. HETTINGER† 《Sedimentology》1992,39(5):905-930

Detailed models already exist that outline physical and temporal relationships in marine and marginal marine strata. Such models are still in their infancy in alluvial deposits. Recognition of tidal and estuarine influence in fluvial strata is critical to the development of high resolution sequence stratigraphic correlations between marine and non-marine strata. Strata that have previously been interpreted as low energy meandering river deposits contain sedimentary and biogenic structures that suggest a tidal influence. These structures include sigmoidal bedding, paired mud/silt drapes, wavy and lenticular bedding, shrinkage cracks, multiple reactivation surfaces, inclined heterolithic strata, complex compound cross-beds, bidirectional cross-beds, and trace fossils including Teredolites, Arenicolites and Skolithos. Although none of these structures is unique to tidal processes, the preponderance of data suggests that fluvial systems have been affected by tidal processes well inland of coeval shoreline deposits. These deposits rarely form a significant proportion of a depositional sequence; however, their occurrence allows time significant surfaces to be extended for tens or even hundreds of kilometres inland from coeval shoreline deposits. In Turonian through Campanian strata exposed in the Kaiparowits Plateau of southern Utah, tidally influenced facies are recognized within at least two distinct stratigraphic levels that were deposited during periods of relatively rapid base level rise. These strata form part of an alluvial transgressive systems tract. Landward of each of the marine transgressive maxima, tidal facies are present in fluvial channels that are completely encased in non-marine strata at distances up to 65 km inland from a coeval palaeoshoreline. Our work suggests that such deposits may have gone unrecognized in the past, but they form a significant component of alluvial strata in many depositional sequences. Although these tidally influenced fluvial deposits may be difficult to recognize, they are temporally equivalent to marine maximum flooding surfaces and provide a chronostratigraphic correlation between alluvial and nearshore marine deposits.  相似文献   

Triassic carbonate submarine fans along the Arabian platform margin, Sumeini Group, Oman     

KEITH F. WATTS 《Sedimentology》1988,35(1):43-71

ABSTRACT The Sumeini Group formed along the passive continental margin slope that bounded the northeastern edge of the Arabian carbonate platform. With the initial development of this passive continental margin in Oman during Early to Middle Triassic time (possibly Permian), small carbonate submarine fans of the C Member of the Maqam Formation developed along a distally steepened slope. The fan deposits occur as several discrete lenticular sequences of genetically related beds of coarsegrained redeposited carbonate (calciclastic) sediment within a thick interval of basinal lime mudstone and shale. Repeated pulses of calciclastic sediment were derived from ooid shoals on an adjacent carbonate platform and contain coarser intraclasts eroded from the surrounding slope deposits. Sediment gravity flows, primarily turbidites with lesser debris flows and grain flows, transported the coarse sediments to the relatively deep submarine fans. Channel erosion was a major source of intraformational calcirudite. Two small submarine fan systems were each recurrently supplied with calciclastic sediment derived from point sources, submarine canyons. The northern fan system retrogrades and dies out upsection. The southern fan system was apparently longer-lived; calciclastic sediments in it are more prevalent and occur throughout the section. The proximal portions of this fan system are dominated by channelized beds of calcirudite which represent inner- to mid-fan channel complexes. The distal portions include mostly lenticular, unchannelized beds of calcarenite, apparently mid- to outer-fan lobes. Carbonate submarine fans appear to be rare in the geological record in comparison with more laterally continuous slope aprons of coarse redeposited sediment. The carbonate submarine fans of the C Member apparently formed by the funnelling of coarse calciclastic sediment into small submarine canyons which may have developed due to rift and/or transform tectonics. The alternation of discrete sequences of calciclastic sediment with thick intervals of ‘background’ sediment resulted from either sea-level fluctuations or pulses of tectonic activity.  相似文献   

AN INTEGRATED APPROACH TO MODELLING HYDROLOGY AND WATER QUALITY IN GLACIERIZED CATCHMENTS     

KEITH RICHARDS  MARTIN SHARP  NEIL ARNOLD  ANGELA GURNELL  MICHAEL CLARK  MARTIN TRANTER  PETER NIENOW  GILES BROWN  IAN WILLIS  WENDY LAWSON 《水文研究》1996,10(4):479-508

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