Liquid immiscibility between trachyte and carbonate in ash flow tuffs from Kenya     

R. Macdonald  B. A. Kjarsgaard  I. P. Skilling  G. R. Davies  D. L. Hamilton  S. Black 《Contributions to Mineralogy and Petrology》1993,114(2):276-287

Three thin, syn-caldera ash flow tuffs of the Suswa volcano, Kenya, contain pumiceous clasts and globules of trachytic glass, and clasts rich in carbonate globules, in a carbonate ash matrix. Petrographic and textural evidence indicates that the carbonate was magmatic. The trachyte is metaluminous to mildly peralkaline and varies from nepheline- to quartz-normative. The carbonate is calcium-rich, with high REE and F contents. The silicate and carbonate fractions have similar ¹⁴³Nd/¹⁴⁴Nd values, suggesting a common parental magma. Chondrite-normalized REE patterns are consistent with a carbonate liquid being exsolved from a silicate liquid after alkali feldspar fractionation. Sr isotopic and REE data show that the carbonate matrix of even the freshest tuffs interacted to some degree with hydrothermal and/or meteoric water. A liquid immiscibility relationship between the trachyte and carbonate is indicated by the presence of sharp, curved menisci between them, the presence of carbonate globules in silicate glass and of fiamme rich in carbonate globules separated by silicate glass, and by the fact that similar phenocryst phases occur in both melts. It is inferred that the carbonate liquid separated from a carbonated trachyte magma prior to, or during, caldera collapse. Viscosity differences segregated the magma into a fraction comprising silicate magma with scattered carbonate globules, and a fraction comprising carbonate globules in a silicate magmatic host.Explosive disruption of the magma generated silicate-and carbonate-rich clasts in a carbonate matrix. The silicate liquid was disaggregated by explosive disruption and texturally appears to have been budding-off into the carbonate matrix. After emplacement, the basal parts of the flows welded slightly and flattened. The Suswa rocks represent a rare and clear example of a liquid immiscibility relationship between trachyte and carbonate melts.  相似文献   

Factors affecting the canopy resistance of a Douglas-fir forest     

C. S. Tan  T. A. Black 《Boundary-Layer Meteorology》1976,10(4):475-488

The physiological nature of canopy resistance was studied by comparing the stomatal and canopy resistance of a 10-m high Douglas-fir forest. Stomatal resistance of the needles was measured using porometry, while the canopy resistance was calculated using energy balance/Bowen ratio measurements of evapotranspiration. A typical steady increase in the forest canopy resistance during daytime hours, even at high soil water potentials, was observed. A similar trend in the stomatal resistance indicated that increasing canopy resistance during the daytime was caused by gradually closing stomata. During a dry period when soil water potentials declined from 0 to –10.5 bars, the mean daytime value of canopy resistance increased in proportion to the mean daytime value of the stomatal resistance. Values of canopy resistance calculated from stomatal resistance and leaf area index measurements agreed well with those calculated from energy balance measurements. The dependences of stomatal resistance on light, vapour pressure deficit, twig and soil water potentials art summarized.  相似文献   

The Significance and Prediction of Observation Well Response Delay in Semiconfined Aquifer-Test Analysis     

J. H. Black  K. L. Kipp  Jr. 《Ground water》1977,15(6):446-451

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Permafrost terminology: R. J. E. Brown and W. O. Kupsch. 1974. National Research Council of Canada,Technical Memorandum No. 111, 62 pp., $3.00     

Robert F. Black 《Quaternary Research》1975,5(3):468

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Historical and pooled flood frequency analysis for the River Tay at Perth, Scotland   总被引：1，自引：0，他引：1  

N Macdonald  A Werritty  A R Black  L J McEwen† 《Area》2006,38(1):34-46

Improved estimates of UK flood risk during a period of increased climatic variability place challenges on existing methods that rely on short instrumental records. This paper examines the value of using historical data (both documentary and epigraphic) to augment existing gauged records for the River Tay at Perth as part of a multi-method approach to assessing flood risk. Single station and pooled methods are compared with flood risk estimates based on an augmented historical series (1815–2000) using the Generalized Logistic and Generalized Pareto distributions. The value of using an even longer, but less reliable, extended historical series (1210–2000) is also examined. It is recommended that modelling flood risk for return periods >100 years should incorporate historical data, where available, and that a multi-method approach using a high threshold Generalized Pareto distribution can also add confidence in flood risk estimates for return periods <100 years based on standard methods.  相似文献   

Open system alkaline magmatism in northern Kenya: evidence from U-series disequilibria and radiogenic isotopes   总被引：4，自引：0，他引：4  

Stuart Black  Ray Macdonald  Barbara A. Barreiro  Peter N. Dunkley  Martin Smith 《Contributions to Mineralogy and Petrology》1998,131(4):364-378

U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and are associated with small volumes of hawaiite. The mafic rocks are characterised by high (²³⁰Th/²³²Th) (≥1.06) with low (²³⁸U/²³⁰Th) ratios (≤0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly lower (²³⁰Th/²³²Th) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (²³⁸U/²³⁰Th) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar to single crystal laser fusion ⁴⁰Ar/³⁹Ar ages from sanidine phenocrysts (16–38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (²³⁰Th/²³²Th)₀ ratios (0.69–1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano. Received: 29 May 1996 / Accepted: 24 November 1997  相似文献   

Petrology and geochemistry of magmatic suites of rocks in the continental and oceanic crusts. A volume dedicated to Professor Jean Michot: Edited by D. Demaiffe Available from Prof. D. Demaiffe, Département des Sciences de la Terre, CP160/02, ULB, B-1050, Brussels, Belgium. 385p. 70$     

Russell Black 《Journal of African Earth Sciences》1998,26(4):649-650

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Forced migration,natural resource use and environmental change: the case of the Senegal River Valley     

Black R  Sessay M 《International journal of population geography : IJPG》1998,4(1):31-47

This study examined the environmental consequences of forced migration in the Matam and Podor regions of the Middle Senegal River Valley, Senegal. The analysis was based on a framework offered by Black (1994) and Leach (1992). This framework posits that: 1) refugees in a zone increase population/resource ratios and that resource accounting must consider the extent of renewable resources, the use of stocks of fixed capital, the extent to which resource use generates technological or socioeconomic changes which influence the ratios, and geographic area; 2) refugees tend to be "exceptional resource degraders"; and 3) refugees may ignore or be excluded from sustainable resource use regulations. The study area in the Senegal River Valley had Mauritanian refugees in 1989. The number of refugees was an estimated 67,800 in 1995, of which 47,000 were in the study area. The study region had experienced severe drought during the 1970s and early 1980s, had experienced acute pressure on the land and forest resources, and was experiencing conflicts between farmers and pastoralists. The Mauritanian migrants were not exceptional, but were a third wave of movements in the Middle Valley. The negative environmental impacts of forced migration were minimized by the scattered sites of settlement and the long history of contact between the two sides of the border. Refugees did not use resources in a more destructive or wasteful way than local populations and were not exceptional resource degraders. Environmental degradation is attributed to prior drought, actions by governmental and nongovernmental agencies, and actions by strangers from the south.  相似文献   

Coherent eddies and temperature structure functions for three contrasting surfaces. Part I: Ramp model with finite microfront time     

Wenjun Chen  Michael D. Novak  T. Andrew Black  Xuhui Lee 《Boundary-Layer Meteorology》1997,84(1):99-124

Air temperature time series within and above canopies reveal ramp patternsassociated with coherent eddies that are responsible for most of thevertical transport of sensible heat. Van Atta used a simple step-changeramp model to analyse the coherent part of air temperature structurefunctions. However, his ocean data, and our own measurements for aDouglas-fir forest, straw mulch, and bare soil, reveal that even withoutlinearization his model cannot account for the observed decrease of thecubic structure function for small time lag. We found that a ramp model inwhich the rapid change at the end of the ramp occurs in a finite microfronttime can describe this decrease very well, and predict at least relativemagnitudes of microfront times between different surfaces. Averagerecurrence time for ramps, determined by analysis of the cubic structurefunction with the new ramp model, agreed well with values determined usingthe Mexican Hat wavelet transform, except at lower levels within theforest. Ramp frequency above the forest and mulch scaled very well withwind speed at the canopy top divided by canopy height. Within the forest,ramp frequency did not vary systematically with height. This is inaccordance with the idea that large-scale canopy turbulence is mostlygenerated by instability of the mean canopy wind profile, similar to aplane mixing layer. The straw mulch and bare soil experiments uniquelyextend measurements of temperature structure functions and ramp frequencyto the smallest scales possible in the field.  相似文献   

Four zircon ages from one rock: the history of a 3930 Ma-old granulite from Mount Sones,Enderby Land,Antarctica   总被引：6，自引：1，他引：6  

L. P. Black  I. S. Williams  W. Compston 《Contributions to Mineralogy and Petrology》1986,94(4):427-437

Ion microprobe U-Th-Pb analyses of zircons from a granulite-grade orthogneiss from Mount Sones, Enderby Land, Antarctica, record the ages of four principal events in the history of the gneiss, three of which already have been recognized through previous isotopic dating of other samples. The structure of the zircons indicates at least four different stages of growth. The several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate U-Pb discordance patterns for each stage. The stratigraphically oldest zircon (rare discrete cores) is indistinguishable in age from the most common, euhedrally zoned zircon. Both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927±10 Ma ago, making the orthogneiss currently the oldest known terrestrial rock. The outer parts of most grains and some whole grains recrystallized at 2948±31/–17 Ma, during or immediately after possibly 100 Ma of high granulite grade metamorphism. The recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the Napier Complex at 1000 Ma. In the intervening time, at 2479±23 Ma, the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic Pb which left most of the crystals with radiogenic Pb deficiencies, but produced local radiogenic Pb excesses in others. A new generation of zircon, characterized by very high Th/U and low U, grew at that time. That event — deformation and possibly a minor rise in temperature — produced widespread perturbations of other isotopic systems throughout the Napier Complex.  相似文献