The Kakun igneous cumulate magnetite deposit,SW Nigeria

Summary Field and mineralogical information concerning a Precambrian iron deposit in Kakun, southwestern Nigeria is presented and a genetic model is advanced. The Kakun deposit consists of titaniferous magnetite concentrated at the base of an amphibolite sheet within a large-scale banded metasediment-orthogneiss suite of Eburnean (ca. 2000 Ma) age. The suite is truncated by concordant and discordant Pan-African (ca. 600 Ma) granitic to dioritic intrusives including pegmatites and aplites. The ore-grade zone is marked by preferential concentration of early formed heavy minerals as well as cumulate and ophitic textures, while the host amphibolite exhibits preferred mafic mineral alignment and triple junction mosaic of its felsic mineral matrix. These relationships are here explained in a two-stage model comprising an Eburnean synkinematic magmatic phase and a Pan-African deformation-metamorphic phase. The first stage involved the intrusion of a gabbroic magma as a sill into pre-existing rocks under high grade metamorphic conditions; mineral segregation of the sill into an ore-rich base by crystal settling during consolidation; followed by a post-consolidation pulse of anorthosite melt injection into the ore zone. The second stage occurred under conditions of medium grade metamorphism and heterogeneous deformation. Competence, difference and localised access of water during this stage led to preferential shearing and amphibolitisation of the ore-poor upper part of the sill.

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Die magmatische Cumulat-Magnetit Lagerstatte von Kakun, SW Nigeria

Zusammenfassung Es werden feldgeologische und mineralogische Informationen von einer präkambrischen Eisenlagerstätte in Südwest-Nigeria präsentiert, und ein genetisches Model vorgestellt. Die Kakun Lagerstätte stellt eine Ti-Magnetit Mineralisation dar, die an der Basis einer Amphibolitabfolge konzentriert ist. Die Amphibolite gehören zu einer im Groß-Maßstab gebänderten Metasediment-Orthogneis Serie von Eburnean-Alter (ca. 2000 m.y.). Diese Serie wird von konkordanten und diskordanten granitischen bis granodioritischen Intrusionen und inkludierten Pegmatiten und Apliten (ca. 600 m.y.) durchbrochen. Die vererzte Zone ist durch Konzentration von frühgebildeten Schweremineralien, Cumulat- und ophitischen Texturen gekennzeichnet; die Amphibolitabfolge zeigt Einregelung der mafischen Mineralkomponenten, und ein tripple junction Mosaik der felsischen Gemengteile. Diese Charakteristika der Erz-Zone und der Amphibolite werden mit einem zweiphasigen Model erklärt: eine Eburnean synkinematische, magmatische Phase und eine Pan-Afrikanische, deformative metamorphe Phase. Die erste Phase umfaßt die Intrusion eines gabbroischen Sills unter hochgradigen Metamorphosebedingungen, Mineralsegregation innerhalb des Sills in eine erzreiche Basis, verursacht durch Kristall-Absinken während der Konsolidierung, und schließlich die Injektion eines anorthositischen Magmenpulses in die Erzzone im post-Konsolidierungsstadium. Die zweite Phase hat unter mittelgradigen Metamorphosebedingungen und heterogener Deformation stattgefunden. Kompentenz-Unterschiede und das stellenweise Eindringen von Wasser hat in dieser Phase zu Scherung und Amphibolitisierung des erzarmen, hangenden Anteils des Sills geführt.

     

Pyroxene chemistry and evolution of alkali basaltic rocks from Burgenland and Styria,Austria

Summary The mineral chemistry of several Pliocene alkali basaltic rocks from Burgenland and Styria (Eastern Austria) have been investigated in order to determine the evolution path of the basalt magmas prior to eruption. With their wide range of substitutions, clinopyroxenes provide the best records of the evolution history of rocks. Pyroxene phenocrysts of the investigated basalts show both concentric and sector zoning. The investigation of sector zoned crystals shows, that not only Ti, Al and Fe contents are different in different sectors but there can be significant differences also in their Cr content. This fact apparently suggests that the distribution of Cr between clinopyroxene and melt could be influenced by crystallization kinetics.The depth of crystallization and differentiation of the basalts can be estimated from Ti and Al contents of clinopyroxene phenocrysts. From a combination of data on clinopyroxene composition, compatible trace element contents and mg-values of the rocks, it is concluded, that the alkali basalts of Pauliberg and Steinberg underwent slight olivine and clinopyroxene fractionation in shallow magma chambers prior to eruption, while the nephelinite of Stradnerkogel evolved mainly through clinopyroxene fractionation under high pressure conditions, probably in the upper mantle.

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Chemie der Pyroxene und Entwicklung von Alkalibasalten aus dem Burgenland und der Steiermark, Österreich

Zusammenfassung Einige pliozäne alkalibasaltische Gesteine aus dem Burgenland und der Steiermark wurden mineral-chemisch untersucht, um Aufschluß über ihre Evolution vor der Eruption zu erhalten. Klinopyroxene mit ihren vielfältigen Subtitutionsmöglichkeiten erlauben am besten eine Abschätzung der Evolution der Basalte. Pyroxen-Einsprenglinge der Basalte zeigen sowohl konzentrischen als auch sektoralen Zonarbau. Die von uns untersuchten Einsprenglinge zeigen in den verschiedenen Sektoren nicht nur unterschiedliche Gehalte an Ti, Al und Fe, sondern vielfach auch unterschiedliche Cr-Gehalte. Dies macht es wahrscheinlich, daß die Verteilung von Cr zwischen Klinopyroxen und Schmelze von der Kristallisations-Kinetik beeinflußt wird.Die Tiefe in der die Basalte kristallisierten und differenzierten kann von den Ti- und Al-Gehalten der Klinopyroxen- Einsprenglinge abgeschätzt werden. Die Zusammensetzung der Klinopyroxene im Verein mit den Gehalten an kompatiblen Spurenelementen und denmg-Werten der Gesteine erlauben den Schluß, daß die Alkalibasalte von Pauliberg und Steinberg vor ihrer Eruption eine geringfügige Olivinund Klinopyroxen-Fraktionierung in einer seichten Magmakammer erlebten. Der Nephelinit vorn Stradnerkogel hingegen erfuhr hauptsächlich eine Klinopyroxen Fraktionierung unter Hochdruck-Bedingungen, möglicherweise im oberen Erdmantel.

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With 8 Figures     

Lattice vibration spectra. LXVIII. Single-crystal Raman spectra of marcasite-type iron chalcogenides and pnictides,FeX2 (X=S,Se, Te; P,As, Sb)

Single-crystal Raman spectra of marcasite-type FeS₂, FeSe₂, and FeTe₂ and loellingite-type FeP₂, FeAs₂, and FeSb₂ are presented and discussed with reference to the energies of the two X-X stretching modes v _x-x (A _g, B _1g) and the four X₂ librations Rx₂ (A _g, B _1g, B _2g, B _3g). The main results obtained are that (i) the intraionic X-X bonds of FeS₂ marcasite and FeS₂ pyrite are nearly equal in strengths (mean values of the S-S stretching modes 418 and 420 cm^-1, respectively) and (ii) the interactions of the metal ions and the dumbbell-like X₂ units increase on going from the chalcogenides to the respective pnictides and from FeS₂ marcasite to pyrite (as shown from the frequencies of the X₂ librations).     

Comparison of meteorological observations at Mt. Everest and K2: Examples of the 1984 and 1986 expedition

Summary Meteorological observations in the eastern part of the Himalayas (Tibetan side of Shisha Pangma and of Mt. Everest, 1984) are compared with observations of the Karakoram (north side of K2, 1986). Both sites are characterized by different weather patterns within the global circulation. This is evident also in the expedition data obtained during September and October 1984 and 1986. Temperature gradients and the development of the daily wind system of the two areas are shown.With 6 Figures     

Wind shear at tilted inversions

Vertical wind shear at a temperature inversion can be caused by baroclinicity associated with a tilt of the inversion. Four observational cases of tilted inversions are presented. The tilts on horizontal scales of 20–100 km range from 2–10 × 10^-3 and the vertical wind shear is between 1 and 25 m/s per 100 m. In general, there is remarkable agreement between observed and geostrophic wind shear.The observations show that the inversion tilt is particularly strong at the edges of mesoscale cloud fields. The Richardson number can reach subcritical values. Cloud fields may be surrounded by a cyclonically rotating wind field and cloud gaps by an anticyclonically rotating wind field.     

Application of a spectral correction method for measurements of covariances with fast-response sensors in the atmospheric boundary layer up to a height of 130 m and testing of the corrections

Measurement of meteorological quantities with fast-response sensors by means of the eddycorrelation technique allows calculation of turbulent transport without too severe demands with regard to stationarity and homogeneity. However, the measurement system causes frequency-dependent errors due to measuring height, averaging interval, sampling rate and mean wind speed. A correction method (Moore, 1986) was applied to measurements taken on a 130 m tower located at the German North Sea coast. The errors for 10-min averages, obtained at a height of 10 m were between 20 and 30%, mainly caused by the inertia of the sensors, increasing to between 30 and 60% at 130 m height due to a loss of low-frequency parts of the spectra because of the averaging interval. The corrections to the obtained data were validated by comparing the cospectra calculated from the data with the spectra used for the computation of the correction factors.     

Polish forest ecosystems: The influence of changes in the economic system on the carbon balance

In order to study the effects of a change in the economic system on carbon balance at a national scale, a balance of the carbon influx and effluxes was prepared for forest ecosystems in Poland for 1988, and was repeated for 1990. The year 1988 was the first year of drastic economic changes (and severe crises) in Poland. Two entirely different approaches were used to estimate the carbon influx into a forest ecosystem. The forest inventory approach was based on data from conventional measurements of merchantable timber in forests, whereas the carbon flux and allocation approach was based on the estimate of net photosynthetic productivity of forest ecosystems to calculate the carbon influx. Results from both approaches were within the range 1.17 to 5.77t C/ha/yr which most likely defined lower and upper limits for the carbon influx into forest ecosystems in Poland. On the national scale, the carbon influx into Polish forest ecosystems was estimated by the forest inventory approach to be about 12.8 Mt C/yr. This result was similar for both years. Efflux of carbon from Polish forest ecosystems resulted almost entirely from the decay of logging residues. The efflux in 1990 (3.82 Mt C/yr) was about 26% lower than that in 1988. Storage of the carbon in Polish forest ecosystems (including an estimate of the carbon pool in forest soils) was 1.8% greater in 1990 (2518 Mt C) than in 1988, when it equalled 2473 Mt C.