Geochemistry of magnesites and dolomites at the Oberdorf/Laming (Austria) deposit and implications for their origin

The major and trace element geochemistry of magnesite and dolomite samples from alternating rock masses at the Oberdorf/Laming (Styria, Austria) mineralization has been studied. The rare earth elements (REE) are considered to be of importance for the discussion of the origin of sparry magnesite deposits. The Al and REE contents are positively correlated, indicating a clay component as the main source for REE in the magnesites and their precursor rocks. Our data support the view that the Oberdorf and probably other magnesite mineralizations in the Eastern Alps have been formed by metasomatism. Through this process calcite (which was deposited by sedimentation in a marine environment) underwent dolomitization. Subsequently, Mg-rich solutions circulating through the rock masses formed the magnesite bodies.During the formation of the dolomites and the conversion to magnesites, the REE patterns of the original carbonates remained unchanged. The magnesites exhibit patterns that are very similar to sedimentary carbonates (which contain a REE-bearing clay component). Magnesites of purely sedimentary origin, formed in a predominantly evaporitic environment, should have lower total REE contents and different patterns due to the short residence times of these elements in seawater. Analyses of talc pseduomorphs after magnesite indicate as well that the REE patterns are not significantly altered during the talcification. The Mg-rich solutions that led to the formation of the magnesites have probably been derived from serpentinites below the gneiss complexes of eastern Styria and the South Penninic units of the Tauern window.

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Zusammenfassung Die Haupt- und Spurenelementgeochemie von Magnesit- und Dolomitproben, die aus alternierenden Gesteinsstöcken in der Magnesitmine von Oberdorf/Laming (Steiermark, Österreich) stammen, wurde untersucht. Besonderes Augenmerk wurde dabei auf die Geochemie der Seltenen Erdelemente (REE) gelegt, da diese für Aussagen über die Entstehung der Magnesitlagerstätten von besonderer Bedeutung sind. Eine positive Korrelation zwischen den Al und REE-Gehalten deutet darauf hin, daß der Hauptteil der REE einer Tonkomponente entstammt. Die Daten unterstützen eine metasomatische Entstehung der Spatmagnesite der Oberdorfer Lagerstätte und anderer Ostalpiner Lagerstätten. Sedimentär angelegter Kalzit wird dabei über Dolomit mit Mg-reichen Lösungen in Magnesit umgewandelt.Während der Metasomatose bleiben die chondritnormierten REE-Verteilungen der Ausgangskarbonate relativ unverändert erhalten. Die so entstandenen Spatmagnesite weisen Verteilungsmuster auf, die jenen der sedimentären Karbonate, aus denen sie entstanden sind, entsprechen. Rein sedimentär entstandene Magnesite werden praktisch nur in evaporitischem Milieu gebildet und sollten infolge der kurzen Verweilzeit dieser Elemente im Meerwasser wesentlich niedrigere REE-Gehalte und andere Verteilungsmuster aufweisen. Analysen einer Talkpseudomorphose zeigen, daß sich die REE-Verteilungsmuster auch bei der metasomatischen Talk-bildung nicht wesentlich ändern. Die Mg-reichen Lösungen, die zur Bildung der Magnesite notwendig waren, entstammen vermutlich tiefliegenden Sepentiniten, auf die es Hinweise im Untergrund der oststeirischen Gneisskomplexe und des südpenninischen Tauernfensters gibt.

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Résumé Cette note présente les résultats d'une étude géochimique des éléments majeurs et en trace d'échantillons de magnésite et de dolomite qui se présentent en masses alternées dans la mine de magnésite de Oberdorf/Laming (Steiermark, Autriche). Une attention particulière a été apportée aux terres rares, considérées comme fournissant des arguments importants dans la discussion de l'origine des gisements. Les teneurs en Al et en terres rares montrent une corrélation positive, ce qui permet de rapporter à un composant argileux la source principale des terres rares. Les résultats obtenus plaident en faveur d'une origine métasomatique des minéralisations en magnésite d'Oberdorf et d'autres endroits des Alpes orientales. Au cours de ce processus, la calcite, d'origine sédimentaire marine, a été le siège d'une dolomitisation avec, subséquemment, formation de corps de magnésite grâce à la circulation de solutions riches en Mg.Au cours de la métasomatose, la distribution des terres rares des carbonates originels n'est pas modifiée: les magnésites montrent des répartitions très semblables à celles des carbonates sédimentaires (lesquels renferment un constituant argileux porteur des terres rares). Des magnésites d'origine purement sédimentaires, formées en milieu évoporitique, doivent présenter un contenu total en terres rares moins élevé et une distribution différente de ces éléments, eu égard à la brièveté de leur séjour dans l'eau de mer. Des analyses de pseudomorphoses de magnésite en talc montrent de même que la distribution des terres rares n'est pas modifiée de manière significative au cours de la talcification. Les solutions riches en Mg responsables de la formation de la magnésite sont probablement dérivées de serpentines situées sous les complexes gneissiques de Styrie et les unités penniques méridionales de la fenêtre des Tauern.

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Molecular bands of supernova 1987A

We have re-investigated the earlier calculations of Sharp and Höfflich (1989) for the first overtone of CO as observed in SN 1987A. The cooling effect of CO has been included in the models. We found a similar but slightly lower CO-enrichment. The sensitivity of the band structure on temperature is also demonstrated.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Particle acceleration in impulsive solar flares

A model for second-step electron acceleration in impulsive solar flares is presented. We have extended the theory of stochastic particle acceleration to include Coulomb energy losses which become important at low coronal heights. This inclusion successfully explains the observed steepening of interplanetary electron spectra below 3 MeV following impulsive solar flares taking place at low coronal heights. It also explains the observed spectral differences of relativistic electrons in long-duration and impulsive flares.     

Interaction of interstellar pick-up ions with the solar wind

The interaction of interstellar pick-up ions with the solar wind is studied by comparing a model for the velocity distribution function of pick-up ions with actual measurements of He⁺ ions in the solar wind. The model includes the effects of pitch-ang'e diffusion due to interplanetary Alfvén waves, adiabatic deceleration in the expanding solar wind and the radial variation of the source function. It is demonstrated that the scattering mean free path is in the range 0.1 AU and that energy diffusion can be neglected as compared with adiabatic deceleration. The effects of adiabatic focusing, of the radial variation of the neutral density and of a variation of the solar wind velocity with distance from the Sun are investigated. With the correct choice of these parameters we can model the measured energy spectra of the pick-up ions reasonably well. It is shown that the measured differential energy density of the pick-up ions does not vary with the solar wind velocity and the direction of the interplanetary magnetic field for a given local neutral gas density and ionization rate. Therefore, the comparison of the model distributions with the measurements leads to a quantitative determination of the local interstellar gas density.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Inversion Reconstruction of Gravity Potential Based on Gravity Gradients

A new method was developed for the inversion reconstruction of gravity potential. This method offers the possibility to determine the potential function and all of its important derivatives using the common inversion of gravity gradients and the first derivatives of potential. Gravity gradients can be originated from Torsion balance measurements, while the first derivatives of potential can be derived from the deflections of the vertical data. Different fields having great importance can be originated from this reconstructed potential function at any points of the investigated area. The advantage of this method is that the solution can be performed by a significantly over-determined inverse problem. Test computations were performed for the inversion reconstruction of gravity potential. There were 248 torsion balance measurements and 13 points where the deflections of the vertical are known in our test area. This inversion algorithm is rather stable. Gravity potential, the first and the second derivatives of the potential were determined for the test area by this suggested method. This method gives a good possibility for a useful geodetic application; deflections of the vertical based on torsion balance measurements can be determined for the whole area for each torsion balance stations.     

On-line with the people in line: Internet development and flexible control of the net in Vietnam

Information and communication technologies in general and the internet in particular are often praised as a means for enhancing democracy and providing new spaces for the development of an egalitarian civil society, in which all members of society can participate equally. However, there are various possibilities to monitor, manipulate and control cyberspace, of which the internet is an essential part. This paper examines the efforts of the Vietnamese government and the Vietnamese Communist Party to control cyberspace as well as the physical spaces through which the virtual world is accessed. There are attempts to control the internet in a similar fashion as the traditional print and broadcast media. Any such control is neither absolute nor without effect. Instead control is exercised in a highly flexible manner, allowing for some officially unwanted or illegal activity to occur. At the same time authorities can apply internet regulations, if it serves their political objectives as for example strengthening the Party’s official monopoly on political power. The paper traces the development of the internet as well as the regulatory environment surrounding it and analyses the inconsistent enforcement of regulations. The analysis is framed in the theoretical works of Michel Foucault and Jürgen Habermas.     

Petrogenesis of Permian alkaline lamprophyres and diabases from the Spanish Central System and their geodynamic context within western Europe

Basic to ultrabasic alkaline lamprophyres and diabases intruded within the Spanish Central System (SCS) during Upper Permian. Their high LREE, LILE and HFSE contents, together with positive Nb–Ta anomalies, link their origin with the infiltration of sublithospheric K-rich fluids. These alkaline dykes may be classified in two distinct groups according to the Sr–Nd isotope ratios: (1) a depleted PREMA-like asthenospheric component, and (2) a BSE-like lithospheric component. A slight enrichment in radiogenic ²⁰⁷Pb and ²⁰⁸Pb allows the contribution of a recycled crustal or lithospheric component in the mantle sources. The intrusion of this alkaline magmatism is likely to have occurred due to adiabatic decompression and mantle upwelling in the context of the widespread rifting developed from Carboniferous to Permian in western Europe. The clear differences in the geochemical affinity of Lower Permian basic magmas from north-western and south-western Europe might be interpreted in terms of a more extensive separation of both regions during that period, until they were assembled during Upper Permian. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The importance of talc and chlorite “hybrid” rocks for volatile recycling through subduction zones; evidence from the high-pressure subduction mélange of New Caledonia

The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models of slab devolatilization. The eclogite-facies mélange belt of northern New Caledonia represents previously subducted oceanic crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large quantities of H₂O and CO₂ and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a mélange of mafic, ultramafic and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at sub-arc depths and at significantly higher temperatures (∼ 800°C) than other lithologies (400–650°C). Fluids released under these conditions could carry high trace-element contents and may trigger partial melting of adjacent pelitic and mafic rocks, and hence may be vital for transferring volatile and trace elements to the source regions of arc magmas. In contrast, these hybrid rocks are unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon into the deep mantle. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Investigation of a shallow alluvial aquifer using geoelectrical methods: a case from Turkey

Near-surface geophysical methods are commonly used to solve a wide class of geological, engineering and environmental problems. In this study, a geoelectrical survey was performed to investigate an alluvial aquifer. The study area is located in the southwest of the Çubukluda? graben, situated in the south of ?zmir, Turkey. The geophysical studies included the electrical resistivity imaging and self-potential (SP) methods. The resistivity data were acquired along eight profiles in the northern part of the study area by a Wenner-Schlumberger electrode configuration and the data processing was achieved by a tomographic inversion technique. The SP data were collected by gradient technique along 16 profiles. Total field values were calculated for each profile by addition of the successive gradient values, then a total field SP map was obtained. The water-saturated zone in the northern part of the study area was clearly revealed by the electrical resistivity imaging and the SP survey yielded useful information on the subsurface fluid movement.     

Palaeohydrogeology: A methodology based on fracture mineral studies

Two sites on the east coast of Sweden (Forsmark and Laxemar/Simpevarp) are currently being investigated as potential geologic hosts for a deep repository isolating high-level nuclear waste. In this paper, a methodology for fracture mineral studies is suggested with focus on the variation in depth of the fresh/saline water interface and location of the redox front in the bedrock. The most commonly precipitated fracture minerals in crystalline rocks are chlorite, calcite, quartz, K-feldspar, Ca–Al-silicates like epidote, prehnite and laumontite, sulphides and Fe-oxides. Of these, calcite is the mineral best suited for palaeohydrological studies since it precipitates during a wide range of conditions including low-temperature conditions during the Pleistocene and Holocene epochs. Sulphides and Fe-oxides/hydroxides provide information on the position of the redox front. In order to carry out palaeohydrological studies, a number of prerequisites are required such as; high quality drill core material, geological knowledge of the sequence of fracture mineralizations; the post-glacial (Holocene) evolution in the area; high quality groundwater chemistry, including stable isotopes; and a conceptual model of the hydrogeochemistry that is to be tested. The choice of methods used here is based on the fact that both the Forsmark and Laxemar/Simpevarp sites are situated in Palaeoproterozoic crystalline rocks with reactivation of fractures over at least 1.5 Ga, and they have been exposed to glaciations/deglaciations and transgressions/regressions of the Baltic Sea during the Quaternary. This has resulted in a palaeohydrology with a range of groundwaters of quite different chemistry and stable isotopic composition. The suggested scheme for solving the variation in depth of the fresh/saline water interface focuses on fracture calcite. It includes a step-by-step procedure with;

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Initial δ¹⁸O and δ¹³C, analyses and complementary petrographic studies of thin sections and crystal morphology followed by