Le minerai de fer d'Hajigak (Afghanistan). Position stratigraphique,cadre géologique et type du gisement

Age and grade of metamorphism of the considerable iron deposit of Hajigak (Central Afghanistan) have been controversial until now. A recent field study on the eastern part of the district produced new data concerning the geological environment and paragenesis of the ore. The iron beds are situated in the Upper Kalu Formation (= Awband Suite) of lower Paleozoïc, may be Silurian to Lower Devonian age. Basic lavas and tuffs, converted into green-schists, always are present near the ore bodies; and straight relationship is evident between basic rocks and iron ore. We consider the succession of events at Hajigak to be as follows: — at the bottom of a Paleozoïc sedimentary basin, an andesitic submarine volcanism brought along tuffs, lavas, and sericitic and chloritic sediments of exhalative origin; in the same time, hematite and magnetite precipitated. — afterwards gabbro and dolerite sills were intruded; as a result, iron oxides were remobilized, filling fractures across gabbro boundaries, and replacing neighbouring dolomites. — at last, but before Upper Devonian, Lower Paleozoïc rocks of Hajigak and Turkman area were affected by two kinds of metamorphism: the first one, of green-schist facies, is contemporaneous with magmatic activity; the second one, sericitic dynamo-metamorphism, appeared when the folding took place, mainly in the eastern region (Khesh and Zerok areas). In conclusion, the Hajigak iron ore appears to be a submarine — exhalative deposit, synchronous with a Lower Paleozoïc basic volcanic activity (but non ophiolitic).     

Geochemistry of the Rare Earth elements in spilites from the oceanic and continental crust

Analytical data including major elements, the Rare Earths, Ni, Cu, Zn, Rb, and Sr are presented for twenty-three spilites from the Mid-Atlantic Ridge, the Hercynian part of the Variscan geosyncline in Germany, and several localities in Switzerland. Low grade metamorphism (up to approximately 400° C) and spilitization of basaltic rocks apparently do not alter the original Rare Earth element (REE) distributions. This fact permits comparison of the spilites and unaltered tholeiitic basalts from the Mid-Atlantic Ridge. The relative REE distributions thus appear suitable for delineating the original basalt types of spilites formed by metamorphism. The spilites from the Hercynian part of the Variscan geosyncline have a REE distribution pattern which is characteristic of continental tholeiites. It is thus probable that during the formation of this geosyncline the principal magma extruded was of tholeiitic composition and that these rocks were later converted to spilites by metamorphism.     

Erzlagerstättenbildung und permischer Vulkanismus im Raume von Trient (Norditalien)

Zusammenfassung Es werden die Befunde einer Reihe — zum Teil noch unveröffentlichter — geologisch-lagerstättenkundlicher Arbeiten zum Problem der Lagerstättenbildung im Raume von Trient zusammenfassend bekanntgegeben. Es gibt drei verschiedenartige und verschieden alte Lagerstättentypen, deren Bildung sich nicht in eine unitaristische alpine Metallogenese zwängen läßt und deren Beziehungen zum permischen Vulkanismus erörtert werden. Es ergeben sich Hinweise auf ein varistisches Alter des Cima-d'Asta-Granites und interessante Parallelen zum triassischen Vulkanismus in Jugoslawien und zu seinen Lagerstätten.     

Environmental isotopes (N, S, C, O, D) to determine natural attenuation processes in nitrate contaminated waters: Example of Osona (NE Spain)

Nitrate-contaminated groundwater from an aquifer in the Osona region (NE Spain) was chemically and isotopically (δ¹⁵N_NO3,δ¹⁸O_NO3,δ³⁴S_SO4,δ¹⁸O_SO4, δD, δ¹⁸O_H2O and δ¹³C_DIC) characterized. Diffuse- contamination reached values of 366 mg/L. Nearly 75% of the 37 sampled sites had higher concentrations than the 50 mg/L in limit for drinking water. To identify the source of pollution δ¹⁵N_NO3 and δ¹⁸O_NO3 were used with results, for most samples, in the range of pig manure . Nitrification processes were evaluated by means of the δ¹⁸O of and water. Isotopic data suggested that natural attenuation of was taking place. This process was confirmed using the δ¹⁸O_NO3 coupled with the ratio, avoiding the influence of continuous inputs. A further insight on denitrification processes was obtained by analyzing the ions involved in denitrification reactions. Although the role of organic matter oxidation could neither be confirmed nor discarded, this approach showed a link between denitrification and pyrite oxidation. Therefore, in areas with no adequate infrastructure (e.g. multipiezometers), such as the one studied, this approach could be useful for implementing better water management.     

Hydrogeological interactions between fault zones and alluvial aquifers in regional flow systems

The hydrological influence of fault zones in tectonic areas is usually difficult to depict from field data. Numerical simulation allows representation of such flow systems and an estimation of flow lines and rates. This paper reports on simulations of the groundwater flow in a range‐and‐basin area affected by a regional fault zone, which may drain or recharge an overlaying alluvial aquifer. Different hydraulic conductivity values for the range rocks, the fault‐zone, and the sedimentary infill of the basin are considered, as well as different fault‐zone widths and boundary conditions. Results show that upward and downward fluxes develop in the upper part of the fault zone, controlled by the action of the alluvial aquifer, influencing the recharge of the sedimentary basin. This paper shows the hydrological efficiency of fault zones as preferential flow; it also analyses the constraints that determine groundwater recharge to the surrounding basins. These results contribute to the understanding of hydrogeological dynamics in tectonic areas. Copyright © 2008 John Wiley & Sons, Ltd.     

The enigma of the oldest 'nova': the central star and nebula of CK Vul

CK Vul is classified as, amongst others, the slowest known nova, a hibernating nova or a very late thermal pulse object. Following its eruption in ad 1670, the star remained visible for 2 yr. A 15-arcsec nebula was discovered in the 1980s, but the star itself has not been detected since the eruption. We here present radio images which reveal a 0.1-arcsec radio source with a flux of 1.5 mJy at 5 GHz. Deep Hα images show a bipolar nebula with a longest extension of 70 arcsec, with the previously known compact nebula at its waist. The emission-line ratios show that the gas is shock-ionized, at velocities  >100 km s⁻¹  . Dust emission yields an envelope mass of  ∼5 × 10⁻² M_⊙  . Echelle spectra indicate outflow velocities up to 360 km s⁻¹. From a comparison of images obtained in 1991 and 2004 we find evidence for expansion of the nebula, consistent with an origin in the 1670 explosion; the measured expansion is centred on the radio source. No optical or infrared counterpart is found at the position of the radio source. The radio emission is interpreted as thermal free–free emission from gas with   T _e∼ 10⁴ K  . The radio source may be due to a remnant circumbinary disc, similar to those seen in some binary post-AGB stars. We discuss possible classifications of this unique outburst, including that of a sub-Chandrasekhar mass supernova, a nova eruption on a cool, low-mass white dwarf or a thermal pulse induced by accretion from a circumbinary disc.     

Three‐dimensional hydrodynamic and salinity transport modelling of Danshuei River estuarine system and adjacent coastal sea,Taiwan

A three‐dimensional, time‐dependent hydrodynamic and salinity model was applied to the Danshuei River estuarine system and adjacent coastal sea in Taiwan. The model forcing functions consist of tidal elevations along the open boundary and freshwater flows from the main stem and tributaries in the Danshuei River system. The bottom roughness height was calibrated and verified with model simulation of barotropic flow, and the turbulent diffusivities were calibrated through comparison of time‐series of salinity distributions. The overall model verification was achieved with comparisons of residual current and salinity distribution. The model simulation results are in qualitative agreement with the available field data. The model was then used to investigate the tidal current, residual current, and salinity patterns under the low freshwater flow condition in the modelling domain. The results reveal that the extensive intrusion of saline water imposes a significant baroclinic forcing and induces a strong residual circulation in the estuary. The downriver net velocity in the upper layer increases seaward despite the enlargement of the river cross‐section in that direction. Strong residual circulation can be found near the Kuan‐Du station. This may be the result of the deep bathymetric features there. Copyright © 2007 John Wiley & Sons, Ltd.     

Recent climatic trends and linkages to river discharge in Central Vietnam

In this study, change in rainfall, temperature and river discharge are analysed over the last three decades in Central Vietnam. Trends and rainfall indices are evaluated using non‐parametric tests at different temporal levels. To overcome the sparse locally available network, the high resolution APHRODITE gridded dataset is used in addition to the existing rain gauges. Finally, existing linkages between discharge changes and trends in rainfall and temperature are explored. Results are indicative of an intensification of rainfall (+15%/decade), with more extreme and longer events. A significant increase in winter rainfall and a decrease in consecutive dry days provides strong evidence for a lengthening wet season in Central Vietnam. In addition, trends based on APHRODITE suggest a strong orographic signal in winter and annual trends. These results underline the local variability in the impacts of climatic change at the global scale. Consequently, it is important that change detection investigations are conducted at the local scale. A very weak signal is detected in the trend of minimum temperature (+0.2°C/decade). River discharge trends show an increase in mean discharge (31 to 35%/decade) over the last decades. Between 54 and 74% of this increase is explained by the increase in precipitation. The maximum discharge also responds significantly to precipitation changes leading to a lengthened wet season and an increase in extreme rainfall events. Such trends can be linked with a likely increase in floods in Central Vietnam, which is important for future adaptation planning and management and flood preparedness in the region. Copyright © 2012 John Wiley & Sons, Ltd.     

The Dakhleh Glass: Product of an impact airburst or cratering event in the Western Desert of Egypt?

Abstract— Impact cratering is a ubiquitous geological process on the terrestrial planets. Meteorite impact craters are the most visible product of impact events, but there is a growing recognition that large aerial bursts or airbursts should occur relatively frequently throughout geological time. In this contribution, we report on an unusual impact glass‐the Dakhleh Glass (DG)–which is distributed over an area of ?400 km ²of the Dakhleh Oasis, Egypt. This region preserves a rich history of habitation stretching back to over 400,000 years before the emergence of Homo sapiens. We report on observations made during recent fieldwork and subsequent analytical analyses that strengthen previous suggestions that the DG formed during an impact event. The wide distribution and large size of DG specimens (up to ?50 cm across), the chemistry (e.g., CaO and Al₂O₃ contents up to ?25 and ?18 wt, respectively), the presence of lechatelierite and burnt sediments, and the inclusion of clasts and spherules in the DG is inconsistent with known terrestrial processes of glass formation. The age and other textural characteristics rule out a human origin. Instead, we draw upon recent numerical modeling of airbursts to suggest that the properties of DG, coupled with the absence of a confirmed crater, can best be explained by melting of surficial sediments as a result of a large airburst event. We suggest that glass produced by such events should, therefore, be more common in the rock record than impact craters, assuming that the glass formed in a suitable preserving environment.     

Himalayan metamorphic CO2 fluxes: Quantitative constraints from hydrothermal springs

Hot springs in the Marsyandi Valley, Nepal, vent CO₂ sourced from metamorphic fluids that mix with shallow groundwaters before degassing near the Earth's surface. The δ¹³C of spring waters ranges up to + 13‰, while that of the coexisting free gas phase is close to ? 4‰. Empirical and thermodynamic modelling of this isotopic fractionation suggests > 97 ± 1% CO₂ degassing. The calculated minimum total CO₂ degassing in the Marsyandi catchment is 5.4 × 10⁹ mol/yr from a Cl-based estimate of the spring water discharge to the Marsyandi River and the fraction of CO₂ degassed. Extrapolated to the whole of the Himalayas, this implies a probable minimum metamorphic CO₂ flux of 0.9 × 10¹² mol/yr, or ～ 13% of solid Earth CO₂ degassing. The calculated flux is a factor of three greater than the estimated CO₂ drawdown by silicate weathering in the Himalayas. Himalayan metamorphic degassing contributes a significant fraction of the global solid Earth CO₂ flux and implies that metamorphism may cause changes in long-term climate that oppose those resulting from the orogenic forcing of chemical weatherability.