GEOGRAPHIES OF STRUGGLE IN THE SAN LUIS VALLEY*     

KEITH W. LINDNER 《Geographical review》2012,102(3):372-381

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Effects of CO2-induoed climatic changes on soowpack and streamflow     

KEITH R. COOLEY 《水文科学杂志》2013,58(5):511-522

Abstract

Atmospheric concentrations of carbon dioxide (CO₂) may double during the next century, causing changes in the Earth's climate. Warming of up to 4°C, slight cooling, and 10% changes in precipitation have been projected. Researchers have studied the possible impacts these changes may have on various aspects of the hydrological cycle, but little emphasis has been placed on snow accumulation and melt. In this study, the effects of climatic change on streamflow from a snowmelt-dominated basin in southwestern Montana, USA, are investigated. The National Weather Service River Forecast System model (NWSRFS) was first calibrated using data for the 1973–1984 period. Daily temperature and precipitation values were then changed, and the model ran again to assess the effects on snowpack and streamflow of some possible climatic changes. Results indicate that streamflow may vary by from ?22 to +45% depending on the combination of climatic changes imposed.  相似文献   

Marsham Street blues     

KEITH MOSELEY 《Geology Today》1995,11(5):176-176

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Terranes vs Dewey-grams — or how best to understand the geological evolution of foldbelts     

KEITH A.W. CROOK 《Geology Today》1986,2(1):9-10

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Geological conservation – yes please!     

KEITH DUFF 《Geology Today》1985,1(4):103-104

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Tailor–made geology 2: Staple Edge, Forest of Dean     

KEITH DUFF 《Geology Today》1985,1(5):155-157

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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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