Über den Nachweis von echtem Leverrierit in Tonsteinen aus unterkarbonischen Steinkohlenflözen von Dobrilugk

Zusammenfassung Die Existenz des Leverrierit als selbständiges Mineral wird mineralogisch (A. Schüller) und chemisch (H. Grassmann) nachgewiesen. Zusammenfassend seien die Eigenschaften des Leverrierit und des mit ihm verwechselten Kaolinit gegenübergestellt (Tabelle 4).     

Die Tonsteine aus den Steinkohlenflözen von Dobrilugk und ihre Entstehung

Zusammenfassung Es werden Tonsteine beschrieben, die als biochemische Sedimente im Steinkohlenmoor gebildet wurden. Die bisherige Deutung, solche Tonsteine seien ehemalige vulkanische Aschentuffe, wird als zur Zeit petrographisch unbegründet abgelehnt.     

The Nigerian manganese-rich iron-formations and their host rocks—from sedimentation to metamorphism

This investigation deals with the Nigerian iron-formations and their host rocks and is based on about 560 mineral analyses (electron-microprobe) and 93 whole-rock analyses (64 iron-formations and 29 host rocks). The manganese-rich and Al-bearing iron-formations occurring in various schist belts of the northern and southern part of West-Nigeria consist of the magnetite-free silicate, the magnetite–silicate and the quartz-rich hematite facies.Iron-formations and host rocks originated from submarine-volcanogenic exhalations enriched in Fe, Mn and CO₂ and from Al₂O₃, SiO₂ and alkali (K₂O and Na₂O)-rich continental-derived pelitic to psammitic material. From these sources and their interaction and controlled by the volcanogenic activity, differently composed protoliths were deposited in the marine basin during the Birimian time. Subsequent metamorphism of greenschist to low amphibolite facies conditions during the Eburnian time led to the formation of the metaprotoliths of the magnetite–silicate (consisting of predominantly magnetite and quartz and subordinate of garnet and amphibole), the silicate facies (consisting of garnet, amphibole and rarely Mn-bearing ilmenite and quartz) and the metasediment phyllite. Garnets are predominantly almandine–spessartine solid solutions, whereas amphiboles are Mn and Ca-bearing grunerite–cummingtonite solid solutions. In the course of a second tectono-metamorphic event of Pan-African age, the magnetite–silicate facies iron-formation/phyllite association was transformed into the hematite facies and muscovite/biotite schists, whereas the silicate facies is characterized by extensive silicification features. The hematite facies and the silicified silicate facies are restricted to southern Nigeria where the second and heterogeneous tectono-metamorphic event is more pronounced (amphibolite facies conditions) than in northern Nigeria.The genesis, summarized as the metamorphic model, shows that the carbonate-rich (siderite, rhodochrosite and subordinate magnesite and calcite) protoliths were metamorphically transformed into the silicate and magnetite–silicate facies. The separation of Mn and Fe, leading to manganese-bearing iron-formations and iron-bearing manganese-formations was explained by varying pH-conditions, under which siderite (pH: 6.8–9.4) and rhodochrosite (pH: 9–11) precipitated.Similar to the Gunfit and Biwabik iron-formations of Minnesota, USA, the iron-formation of Bingi (Maru schist belt), now present in the form of the fayalite bearing silicate facies, was overprinted by contact metamorphism caused by a gabbro intrusion.     

The mechanisms of water diffusion in polymerized silicate melts

Diffusion of water was experimentally investigated for melts of albitic (Ab) and quartz-orthoclasic (Qz₂₉Or₇₁, in wt %) compositions with water contents in the range of 0 to 8.5 wt % at temperatures of 1100 to 1200 °C and at pressures of 1.0 and 5.0 kbar. Apparent chemical diffusion coefficients of water (D _water) were determined from concentration-distance profiles measured by FTIR microspectroscopy. Under the same P-T condition and water content the diffusivity of water in albitic, quartz-orthoclasic and haplogranitic (Qz₂₈Ab₃₈ Or₃₄, Nowak and Behrens, this issue) melts is identical within experimental error. Comparison to data published in literature indicates that anhydrous composition only has little influence on the mobility of water in polymerized melts but that the degree of polymerization has a large effect. For instance, D_water is almost identical for haplogranitic and rhyolitic melts with 0.5–3.5 wt % water at 850 °C but it is two orders of magnitude higher in basaltic than in haplogranitic melts with 0.2–0.5 wt % water at 1300 °C. Based on the new water diffusivity data, recently published in situ near-infrared spectroscopic data (Nowak 1995; Nowak and Behrens 1995), and viscosity data (Schulze et al. 1996) for hydrous haplogranitic melts current models for water diffusion in silicate melts are critically reviewed. The NIR spectroscopy has indicated isolated OH groups, pairs of OH groups and H₂O molecules as hydrous species in polymerized silicate melts. A significant contribution of isolated OH groups to the transport of water is excluded for water contents above 10 ppm by comparison of viscosity and water diffusion data and by inspection of concentration profiles from trace water diffusion. Spectroscopic measurements have indicated that the interconversion of H₂O molecules and OH pairs is relatively fast in silicate glasses and melts even at low temperature and it is inferred that this reaction is an active step for migration of water. However, direct jumps of H₂O molecules from one cavity within the silicate network to another one can not be excluded. Thus, we favour a model in which water migrates by the interconversion reaction and, possibly, small sequences of direct jumps of H₂O molecules. In this model, immobilization of water results from dissociation of the OH pairs. Assuming that the frequency of the interconversion reaction is faster than that of diffusive jumps, OH pairs and water molecules can be treated as a single diffusing species having an effective diffusion coefficient . The shape of curves of D_water versus water content implies that increases with water content. The change from linear to exponential dependence of D_water between 2 and 3 wt % water is attributed to the influence of the dissociation reaction at low water content and to the modification of the melt structure by incorporation of OH groups. Received: 26 March 1996 / Accepted: 23 August 1996     

Pi 2 pulsations and the eastward electrojet: A case study

We report the results of a case study of two Pi 2 pulsations observed near the eastward electrojet by the Scandinavian Magnetometer Array. The power of the two Pi 2 pulsations, calculated using a standard Fast Fourier Transform method, peaks near the centre of the eastward electrojet. For both events there is a strong latitudinal gradient in the power poleward of the equatorward border of the electrojet. The sense of polarisation is predominantly clockwise at the northern stations and anticlockwise at the southern stations although the reversal from clockwise to anticlockwise does not occur at a constant latitude. For the first event the polarisation reversal occurs at higher latitudes in the western half of the array; for the second the polarisation reversal occurs at higher latitudes at the edges of the array. The polarisation reversal does not appear to be related to the location of the eastward electrojet. Equivalent current vectors of the Pi 2 pulsations, obtained by rotating the band pass filtered data through 90°, exhibit clear vortex structures in both events. The vortices change sense of direction at half the period of the Pi 2 pulsation. A simple model for the ionospheric electric field in accord with the field line resonance theory reconstructs the basic features of the observed Pi 2 equivalent current system. We thus conclude that Pi 2 signatures in the region of the eastward electrojet and far away from the auroral break-up region are governed by the field line resonance mechanism.     

Comprehensive chemical analyses of natural cordierites: implications for exchange mechanisms

Cordierite samples from pegmatites and metamorphic rocks have been analysed for major [electron microprobe analysis (EMPA)] and trace elements [inductively coupled plasma mass spectrometry (ICP-MS), secondary ion mass spectrometry analyses (SIMS)] as well as for H₂O and CO₂ (coulometric titration), and the results evaluated in conjunction with published data in order to determine which exchange mechanisms are significant. Apart from the homovalent substitutions FeMg₋₁ and MnMg₋₁ on the octahedral site, some minor KNa₋₁ on the Ch0 channel site, and Fe³⁺Al₋₁ on the T₁1 tetrahedral site, the three most important substitution mechanisms are those for the incorporation of Li on the octahedral sites (NaLi□₋₁Mg₋₁), and of Be and other divalent cations on the tetrahedral T₁1 site (NaBe□₋₁Al₋₁ and Na(Mg,Fe²⁺)□₋₁Al₋₁). The dominant role of the last vector is clearly demonstrated. We propose a new generalized formula for cordierite: ^Ch(Na,K)_0–1 ^VI(Mg,Fe²⁺,Mn,Li)₂ ^IVSi₅ ^IVAl₃ ^IV(Al, Be, Mg, Fe²⁺, Fe³⁺)O₁₈ *x^Ch(H₂O, CO₂…). Our results show that the population of (Mg, Fe²⁺) on the T₁1-site is limited to about 0.08 a.p.f.u. Other exchange mechanisms that were encountered in experiments operate only under P–T conditions or in bulk compositions that are rarely realized in nature. Routine analyses by electron microprobe in which Li and Be are not determined can be plotted as (Mg+Fe+Mn) versus (Si+Al) to assess whether significant amounts of Li and Be could be present. These amounts can be calculated as Li (a.p.f.u.)=Al+Na–4 and Be (a.p.f.u.)=10–2Al–M²⁺–Na.     

Feeding of European cisco (Coregonus albula and C. lucinensis) on the glacial relict crustacean Mysis relicta in Lake Breiter Luzin (Germany)

Lake Breiter Luzin, located in north-eastern Germany, is known for the rare occurrence of sympatric populations of European cisco, Coregonus albula and C. lucinensis. Moreover, the lake is inhabited by the glacial relict crustacean Mysis relicta, currently found in only three other lakes in Germany. In the present study, the role of M. relicta in the diet of ciscoes was investigated. Between 2001 and 2002 ciscoes, mysids and zooplankton were caught in Lake Breiter Luzin and stomach contents of ciscoes were analysed. There were seasonal changes in the food items in the stomachs, largely reflecting changes in prey availability. The main food items were copepods (45–81%) and mysids (5–26%). Cladocerans had high amounts in the diet only in summer. Seasonal changes in selectivity were also noted for most prey groups. There were also some consistent patterns of prey preference, with an overall preference for mysids. In general, the prey composition in cisco stomachs did not show significant diet changes, but there were some significant differences between day and night feeding in single prey groups, such as cladocerans. Diet composition of ciscoes varied with the different depth strata in which the fish were caught. With increasing depth, the proportion of mysids in the diet significantly increased, whereas that of copepods significantly decreased. Between C. albula and C. lucinensis, no distinct differences in feeding were evident. Mysids provided an additional and important food resource to ciscoes, and were mainly consumed when the availability of other prey organisms decreased, as in autumn and in the deeper strata of the water column. However, mysids preyed on the same food organisms as ciscoes, indicating a strong competition for food between fish and mysids.     

Simulations of a Line W-based observing system for the Atlantic meridional overturning circulation

In a series of observing system simulations, we test whether the Atlantic meridional overturning circulation (AMOC) can be observed based on the existing Line W deep western boundary array. We simulate a Line W array, which is extended to the surface and to the east to cover the basin to the Bermuda Rise. In the analyzed ocean circulation model ORCA025, such an extended Line W array captures the main characteristics of the western boundary current. Potential trans-basin observing systems for the AMOC are tested by combining the extended Line W array with a mid-ocean transport estimate obtained from thermal wind “measurements” and Ekman transport to the total AMOC (similarly to Hirschi et al., Geophys Res Lett 30(7):1413, 2003). First, we close Line W zonally supplementing the western boundary array with several “moorings” in the basin (Line W-32°N). Second, we supplement the western boundary array with a combination of observations at Bermuda and the eastern part of the RAPID array at 26°N (Line W-B-RAPID). Both, a small number of density profiles across the basin and also only sampling the eastern and western boundary, capture the variability of the AMOC at Line W-32°N and Line W-B-RAPID. In the analyzed model, the AMOC variability at both Line W-32°N and Line W-B-RAPID is dominated by the western boundary current variability. Away from the western boundary, the mid-ocean transport (east of Bermuda) shows no significant relation between the two Line W-based sections and 26°N. Hence, a Line W-based AMOC estimate could yield an estimate of the meridional transport that is independent of the 26°N RAPID estimate. The model-based observing system simulations presented here provide support for the use of Line W as a cornerstone for a trans-basin AMOC observing system.     

An early warning system for wave-driven coastal flooding at Imperial Beach,CA

Natural Hazards - Waves overtop berms and seawalls along the shoreline of Imperial Beach (IB), CA when energetic winter swell and high tide coincide. These intermittent, few-hour long events flood...