Na and K distribution in agpaitic pegmatites     

Daniel Müller-Lorch  Michael A.W. Marks  Gregor Markl   《Lithos》2007,95(3-4):315-330

Composition and zoning of amphibole in agpaitic pegmatites of the 1.16 Ga Ilímaussaq complex, South Greenland record the chemical evolution of the final stages of an already extremely fractionated melt. Our results show that the general differentiation trends found in the earlier rocks of the complex are continued in the pegmatites, albeit with some significant modifications: the dominating exchange mechanism of Na + Si Ca + Al in the amphiboles of the magmatic stage changes to K + Si Ca + Al and K Na in some pegmatitic samples. Na/K ratios in amphiboles, which generally increase in the course of the Ilímaussaq fractionation, partly display a reversal during the crystallization of the most differentiated amphiboles.

The alkali trends are probably related to the buffering of Na⁺and K⁺activity by the co-crystallization of albite and microcline. This buffering favors Na⁺in cooling fluids. This mechanism is lost when analcime replaces feldspar as a stable phase in the late stages of crystallization, e.g. due to locally elevated H₂O activity. Analcime does not incorporate significant amounts of K and accordingly, amphibole incorporates more K in analcime-bearing assemblages. The Na–K variation in amphiboles in the Ilímaussaq pegmatites allow a detailed view into the late-stage evolutionary trends of a textbook agpaitic complex. The transition from silicate melt to aqueous fluid is recorded by the change of the dominant alkali ion in the pegmatitic amphiboles from Na to K.

Only in the very latest stage, virtually K-free mineral assemblages in analcime–aegirine veins support the existence of a Na-dominated aqueous fluid.  相似文献   

Crustal contamination of mafic magmas: evidence from a petrological, geochemical and Sr–Nd–Os–O isotopic study of the Proterozoic Isortoq dike swarm, South Greenland     

Ralf Halama  Michael Marks  Gerhard Brügmann  Wolfgang Siebel  Thomas Wenzel  Gregor Markl   《Lithos》2004,74(3-4):199-232

The mid-Proterozoic Isortoq dike swarm in the Gardar Province, South Greenland, comprises a variety of alkaline rocks ranging from gabbroic to syenitic in composition. Major magmatic mineral phases are olivine, clinopyroxene, Fe–Ti oxides, amphibole, plagioclase and alkali feldspar. Quartz occurs in some samples as a late magmatic phase. Liquidus temperatures of olivine-bearing samples range between 1120 and 1145 °C and solidus temperatures are 850–930 °C. Calculated silica activities are highly variable between 0.53 and unity. Oxygen fugacities vary from −3 to +1 log units relative to the fayalite–magnetite–quartz buffer.

The rocks have MgO contents <6 wt.% with Mg# between 53 and 17. Primitive mantle-normalized trace element patterns show a relative enrichment of LIL elements with Ba peaks and Nb troughs. Clinopyroxenes show a general enrichment in REE relative to chondritic values with variable slightly positive to prominent negative Eu anomalies. Two of the dikes were dated with Sm–Nd three-point isochrons at 1190±44 and 1187±87 Ma, respectively. Initial ⁸⁷Sr/⁸⁶Sr ratios of mafic mineral separates range from 0.70289 to 0.70432 and initial _Nd values vary from +0.3 to −10.7. Whole-rock initial ¹⁸⁷Os/¹⁸⁸Os ratios are highly variable including very radiogenic values of up to 7.967. δ¹⁸O_v-smow values of separated clinopyroxene and amphibole range from +5.2‰ to +6.2‰ and fall within the range of typical mantle-derived rocks, although mixing with a lower crustal component is permitted by the data. Using energy-constrained assimilation-fractional crystallization (EC-AFC) modeling equations, the Sr–Nd isotope data of the more radiogenic samples can successfully be modeled by addition of up to 10% lower crustal granulite-facies Archean gneisses as contaminants. The Os isotopic data also suggest the involvement of old radiogenic crust. In accordance with seismic data, we conclude that a wedge of Archean crust extends from West Greenland further to the south below the present erosion level.  相似文献   

Dissolution rates of pure methane hydrate and carbon-dioxide hydrate in undersaturated seawater at 1000-m depth   总被引：2，自引：0，他引：2  

Gregor Rehder  Stephen H. Kirby  Laura A. Stern  John Pinkston  Peter G. Brewer 《Geochimica et cosmochimica acta》2004,68(2):285-292

To help constrain models involving the chemical stability and lifetime of gas clathrate hydrates exposed at the seafloor, dissolution rates of pure methane and carbon-dioxide hydrates were measured directly on the seafloor within the nominal pressure-temperature (P/T) range of the gas hydrate stability zone. Other natural boundary conditions included variable flow velocity and undersaturation of seawater with respect to the hydrate-forming species. Four cylindrical test specimens of pure, polycrystalline CH₄ and CO₂ hydrate were grown and fully compacted in the laboratory, then transferred by pressure vessel to the seafloor (1028 m depth), exposed to the deep ocean environment, and monitored for 27 hours using time-lapse and HDTV cameras. Video analysis showed diameter reductions at rates between 0.94 and 1.20 μm/s and between 9.0 and 10.6 · 10⁻² μm/s for the CO₂ and CH₄ hydrates, respectively, corresponding to dissolution rates of 4.15 ± 0.5 mmol CO₂/m²s and 0.37 ± 0.03 mmol CH₄/m²s. The ratio of the dissolution rates fits a diffusive boundary layer model that incorporates relative gas solubilities appropriate to the field site, which implies that the kinetics of the dissolution of both hydrates is diffusion-controlled. The observed dissolution of several mm (CH₄) or tens of mm (CO₂) of hydrate from the sample surfaces per day has major implications for estimating the longevity of natural gas hydrate outcrops as well as for the possible roles of CO₂ hydrates in marine carbon sequestration strategies.  相似文献   

Reviews     

Alois Gregor  Josef Pich 《Studia Geophysica et Geodaetica》1965,9(4):423-424

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The chemical evolution of the atmosphere and oceans: Heinrich D. Holland,Princeton University Press, 1984, 582 pp. cloth, $75.00; paper, $24.50     

Bryan Gregor 《Geochimica et cosmochimica acta》1985,49(8):1845

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Paleomagnetic investigations of the tertiary and quaternary rocks: IX The pliocene of southeast Sicily and some Cretaceous rocks from Capo Passero     

Prof. C. B. Gregor  Prof. Dr. Alan E. M. Nairn  Dr. Joerg F. W. Negendank 《International Journal of Earth Sciences》1975,64(1):948-958

A sampling of Plio-Pleistocene and Upper Cretaceous basalts in SE Sicily yielded the following informationAssuming that the Cretaceous direction is reversed, the confidence oval for the virtual pole overlaps that for the Cretaceous pole of Africa determined from the Mlanje Syenite byMcElhinny et al. If SE Sicily was attached to Africa when these Upper Cretaceous lavas erupted, this result implies that extensive relative movement between the two is not likely to have occurred subsequently.

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Zusammenfassung Die Aufsammlung und Untersuchung plio-pleistozäner und oberkretazischer Basalte in SE-Sizilien ergab die folgende paläomagnetische InformationUnter der Annahme einer inversen kretazischen Feld-Richtung überlappt der Konfidenzkreis des virtuellen Pols den kretazischen magnetischen Pol für Afrika, der vonMcElhinny et al. an den Gesteinen des Mlanje Syenites bestimmt wurde.Wenn SE-Sizilien zur Zeit der Extrusion dieser oberkretazischen Laven an Afrika angeheftet war, dann besagt dieses Ergebnis, da\ eine extensive relative Bewegung zwischen diesen beiden schwerlich nacheinander stattgefunden hat.

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Résumé La compilation et l'analyse de basaltes d'âge plio-pleistocène et crétacique supérieur dans la SE-Sicile fournit l'information paléomagnétique suivanteDans l'acceptation d'un champ inversé au Crétacique, le cercle de confidence du pÔle virtuel recouvre le pÔle magnétique crétacique pour l'Afrique, tel queMcElhinny et al. l'ont déduit des roches de la Syénite de Mlanje. Si l'on rattache le SE de la Sicile à l'Afrique au moment de l'éruption de ces laves du Crétacique supérieur, ce résultat signifie qu'il est peu probable qu'un mouvement relatif extensif ait eu lieu entre les deux.

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- - , , Elhinny . Mlanje Syenites. - «» , , - .

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A contribution to the theory of some storm phenomena     

Zdeněk Gregor  Vojtěch Vítek 《Studia Geophysica et Geodaetica》1960,4(3):256-269

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Trophic ecology of the dominant fishes in Elkhorn Slough,California, 1974–1980     

James P. Barry  Mary M. Yoklavich  Gregor M. Cailliet  David A. Ambrose  Brooke S. Antrim 《Estuaries and Coasts》1996,19(1):115-138

Food habits of the dominant fishes collected from 1974 to 1980 at eight locations in Elkhorn Slough, California, and the adjacent ocean were investigated. Epifaunal crustacea was the major prey group identified from stomach contents of more than 2,000 fishes, followed by epifaunal and infaunal worms, and molluscs. Overall, 18 fish species consumed 263 different prey taxa, ranging from 10 taxa to 125 taxa per fish species and including 99 crustacean, 56 polychaete, and 39 molluscan taxa. Mean prey richness was greatest at stations near the ocean and lowest at inshore stations. Detailed dietary data for all prey taxa were summarized as trophic spectra for each fish species. Trophic spectra represented functional groups of prey and were used for comparisons of dietary similarity. Cluster analyses, based on trophic spectra, resulted in four feeding guilds of fishes. Of 18 fish species, seven (Amphistichus argenteus, Leptocottus armatus, Embiotoca jacksoni, Clevelandia ios, Gillichthys mirabilis, Cymatogaster aggregata, andCitharichthys stimaeus) fed principally on epifaunal crustacea. Four species (Pleuronectes vetulus, Platichthys stellatus, Phanerodon furcatus, andMyliobatus californica) consumed mostly molluscs and infaunal worms. Two species (Psettichthys melanostictus andTriakis semifasciata) fed on mobile crustacea, and five species (Hyperprosopon anale, Engraulis mordax, Clupea pallasi, Atherinopsis californiensis, andAtherinops affinis) fed largely on zooplankton and plant material. Our results suggest that high food availability enhances the nursery function of imshore habitats, and emphasize the importance of invertebrate prey populations and the indirect linkage of plant production to the ichthyofaunal assemblarly marine immigrant species that are likely ‘estuarine dependent’.  相似文献   

Some evaluations of hail suppression system efficiency in Slovenia,Yugoslavia     

J. Rakovec  B. Gregorčič  A. Kranjc  T. Mekinda  L. Kajfež-Bogataj 《Theoretical and Applied Climatology》1990,41(3):157-171

Summary The efficiency of hail suppression is studied for the operative, non-randomized system established in 1971 on a territory of a 256.000 ha and enlarged in the eighties to almost 970.000 ha (Fig. 1). The system was built with thea priori assumption that it is successful and so no special effort was made to collect the data necessary for the testing of its efficiency. However, some data are still at disposal for this purpose: the daily and five minutes interval precipitation data, the radar cloud variables data used as seeding criteria, the data on days with thunderstorms and with hail, and (not abundant) set of data on the damages in agriculture as collected by an insurance company. A statistical evaluation of these data on the significance of the eventual differences between target and control area and/or before and during the project period do not confirm the hypothesis that the system is successful at some acceptable significance level (90 or 95%). The conclusion that the efficiency of the hail suppression system could not be proved holds for the system as a whole: for the theoretical part and the operational one, with all the accompanying advantages and malfunctions.With 7 Figures  相似文献   

Textural, chemical, and isotopic effects of late-magmatic carbonatitic fluids in the carbonatite–syenite Tamazeght complex, High Atlas Mountains, Morocco     

Michael A. W. Marks  Florian Neukirchen  Torsten Vennemann  Gregor Markl 《Mineralogy and Petrology》2009,97(1-2):23-42

Carbonatites of the Eocene Tamazeght complex, High Atlas Mountains, Morocco, consist of calciocarbonatites (alvikite and sövite dykes) and magnesiocarbonatites (diatreme breccias and dykes rocks). These are associated with ultramafic, shonkinitic, gabbroic to monzonitic and various foid syenitic silicate units. Stable and radiogenic isotope compositions for carbonatites and silicate rocks indicate that they share a common source in the mantle, although for some carbonatitic samples contamination with sedimentary rocks seems important. The observed isotopic heterogeneity is mainly attributed to source characteristics, fractional crystallization (accompanied by various degrees of assimilation), and late- to post-magmatic fluid–rock interaction. During the late fluid–rock interaction, Sr, Mn, and possibly also Fe were mobilized and redistributed to form secondary carbonate minerals in carbonatites. These fluids also penetrated into the adjacent syenitic rocks, causing enrichment in the same elements.  相似文献