Episodic and localized illite mineralization is documented in the hydrothermally altered Soultz-sous-Forêts granite (Upper Rhine Graben, France). Separated grain-size fractions of altered granite and argillite vein samples contain mixtures of 2M1 and 1M trans-vacant illite varieties. The platy pseudohexagonal 2M1 illite phases dominate the vein fillings, whereas the 1M illite occurs largely as a fibrous pore-filling variety, which is particularly abundant in the granite matrix. Multiple phases of fluid injections into the granite body have resulted in different illite assemblages, each sample containing a mixture of polytype generations formed during different crystal growth events. On the basis of mineralogical and K–Ar isotopic constraints, the ages of these vein-mineralizing events are determined by plotting the K–Ar values of the various grain-size fractions against polytype abundance and the fitted volume-weighted crystallite thickness distributions. The results suggest a Permian age for the formation of the studied argillite veins, characterized by successive injections of hydrothermal fluids. Secondary episodes of illite crystallization occurred during Jurassic and Cretaceous (or even younger times) in both the veins and the granite matrix. There are indications that the polytype structure and composition of illite were strongly influenced by variations in fluid chemistry and the degree of fluid–rock interaction as the granite was progressively sealed during post-Variscan, episodic hydrothermal activity. 相似文献
For the disposal of high-level waste (HLW) in a deep geological formation as Boom Clay, safety assessment studies have shown that long lived 79Se is one of the more critical fission products. Therefore, the knowledge of its migration properties (diffusion, retention) through the geological barrier (Boom Clay) is of paramount importance. The migration behaviour of selenium strongly depends on its speciation. Under reducing conditions, selenide would be the dominant species and selenium migration would mainly be controlled by the low solubility of Se(−II)-bearing minerals. However Se species are often found in redox disequilibrium and more oxidized species might also coexist. Therefore, the study of selenate migration requires attention, as it might be the most mobile selenium species in the host rock. Electromigration experiments performed with a 75Se-labeled selenate in Boom Clay indicate a high mobility for this species. The apparent diffusion coefficient (Dapp) of selenate in Boom Clay is estimated from electromigration experiments performed under different electric fields. Using two independent approaches, the value of Dapp for selenate is shown to fall in the range from 1.7×10−11 to 6.2×10−11 m2 s−1. Moreover, no reduction of selenate in Boom Clay was observed. 相似文献
A combined analysis of magnetic susceptibility, total organic carbon (TOC), biogenic silica (opal), and TiO2 content of the 12.6 m long composite core PG1351 recovered from Lake El’gygytgyn, Chukotka Peninsula, indicate a clear response
of the lacustrine sedimentary record to climate variations. The impact is not direct, but through variations in oxygenation
of the bottom waters. Mixing of the water body is typical for warmer climates, whereas the development of a stratified water
body associated with anoxic conditions at the lake floor appears during cold climates. Oxic conditions lead to a good magnetite
preservation and thus to high magnetic susceptibilities, but also to a large-scale degradation of organic matter, as reflected
by low TOC (total organic carbon) values. During anoxic conditions, magnetite is severely dissolved yielding very low susceptibility
values, whereas organic matter is best preserved, reflected by high TOC values. Hence, in general, neither susceptibility
reflects the lithogenic fraction, nor does TOC reflect bioproductivity in case of the studied El’gygytgyn sediments. Based
on available infrared stimulated luminescence (IRSL) dating, the obtained susceptibility pattern of core PG1351 shows an obvious
correlation to northern hemisphere insolation variations, with a dominating impact of the Earth’s 18 and 23 kyr precessional
cycles for the upper half of PG1351, that is, during the past 150 ka. Therefore, the whole susceptibility record, together
with biogenic silica (as a proxy for bioproductivity), TOC (as an indicator for redox conditions), and TiO2 (as a proxy for lithogenic input), was systematically tuned to the northern hemisphere insolation yielding an age of about
250 ka for the base of the composite core.
This is the fifth in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and
its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue. 相似文献
In the last few years ecological evaluation of lowland freshwaters was controversely discussed. Due to the fact that most basic information of assessment systems derived from upland streams, the results could not be applied to the lowland situations. Additionally, it is not sufficient for an integral freshwater evaluation only to focus on the stream to show degradation in the tight coupling of running waters and their adjacent landscape. In this study we critically tested a new method, the Standorttypie-Index (STI), to assess lowland streams using caddis-fly communities.
During 1994 and 1996 we investigated the caddisfly community at 15 sampling sites of a river system in the “Westfälische Tieflandbucht” (Northrhine-Westphalia, Germany). As a result we found 72 species belonging to 16 families of the order Trichoptera. The calculation of the “Standorttypie-Index” is based on four defined ecological categories. Although the definitions were inaccurate, each species was classified into one category. In view of stream regulation and streamwidth, we calculated for all study sites the STI of the immature stages, the adults and a combined total STI.
Although there is a correlation between the “Standorttypie-Index” and the state of degradation of the study sites, the separate analysis of larval and adult communities showed that the results easily lead to misinterpretation. The STI of adults devalues the combined total STI of regulated as well as natural study sites. As a consequence, until now it is not possible to evaluate the state of degradation in the coupling of the stream and the adjacent landscape using the STI of the caddis-fly community. 相似文献
Large samples of field horizontal branch (FHB) stars make excellent tracers of the Galactic halo; by studying their kinematics, one can infer important physical properties of our Galaxy. Here we present the results of a medium-resolution spectroscopic survey of 530 FHB stars selected from the Hamburg/ESO survey. The stars have a mean distance of ∼7 kpc and thus probe the inner parts of the Milky Way halo. We measure radial velocities from the spectra in order to test the model of Sommer-Larsen et al., who suggested that the velocity ellipsoid of the halo changes from radially dominated orbits to tangentially dominated orbits as one proceeds from the inner to the outer halo. We find that the present data are unable to discriminate between this model and a more simple isothermal ellipsoid; we suggest that additional observations towards the Galactic Centre might help to differentiate them. 相似文献
Summary Accuracy, reproducibility, stability and response behaviour of mercury and digital deep-sea reversing thermometers have been investigated usingin situ and laboratory observations. Low range (–2 °C to 10 °C) mercury thermometers have an overall accuracy of 5 mK and an equilibrium time of 3 to 4 minutes under normal oceanic conditions. The respective values for digital reversing thermometers are 10 mK and less than one minute.
Sind Quecksilber-Tiefseeumkippthermometer nicht mehr zeitgemäß?
Zusammenfassung Auf der Basis von Labor- und Felduntersuchungen wurden Genauigkeit, Reproduzierbarkeit, Stabilität und Anpaßverhalten von digitalen und mechanischen (Quecksilber) Tiefseeumkippthermometern untersucht. Die hochauflösenden Quecksilberthermometer (–2 °C bis 10 °C) haben eine Genauigkeit von 5 mK und benötigen unter normalen ozeanischen Bedingungen eine Anpaßzeit von 3 bis 4 Minuten. Die entsprechenden Werte für digitale Thermometer sind 10 mK und weniger als eine Minute.
Les thermomètres profonds a renversement au mercure sont-ils depassés?
Résumé Des observations in situ et des tests en laboratoire ont permis d'etudier la precision, la reproductibilité, la stabilité et la reponse des thermomètres profonds a renversement au mercure et de les comparer a celles de leurs equivalents digitaux. Les thermomètres au mercure destinés aux gammes de temperatures basses ont une precision globale de 5 mK et un temps de mise a l'équilibre de 3 jusqu'à 4 minutes dans des conditions oceaniques normales. En comparaison, les caracteristiques des thermométres a renversement digitaux sont respectivement 10 mK et moins d'une minute.
Silica chimneys were discovered in 1985 at 86°W in the rift valley of the Galapagos Spreading Center at 2600 m depth (“Cauliflower Garden”). The inactive chimneys lack any sulfides and consist almost entirely of amorphous silica (up to 96 wt.% SiO2, opal-A); Fe and Mn oxides are minor constituents. Oxygen isotope data show that formation of the silica chimneys took place at temperatures between 32°C (+29.9‰ δ18O) and 42°C (+27.8‰ δ18O).Th/Udating reveals a maximum age of 1440 ± 300y. Amorphous silica solubility relations indicate that the silica chimneys were formed by conductive cooling of pure hydrothermal fluids or by conductive cooling of a fluid/seawater mixture. Assuming equilibrium with quartz at 500 bars, initial fluid temperatures of more than 175°C (i.e., a concentration of > 182 ppm SiO2) were required to achieve sufficient supersaturation for the deposition of amorphous silica at 40°C and 260 bars. If the silica chimneys originate from the same or a similar fluid as higher-temperature ( < 300°C) sulfide-silica precipitates found nearby (i.e., 2.5 km away), then subsurface deposition of sulfides may have occurred. 相似文献