Symplectitic augite: evidence for discontinuous precipitation as an exsolution mechanism in Ca-rich clinopyroxene

Abstract Part of the augite in the Artfjället gabbro consists of symplectitic intergrowths between augite and blebs or lamellae of orthopyroxene. Mineral compositions are consistent with formation of these symplectites by exsolution of orthopyroxene from magmatic augite at a temperature of ca . 900–1000°C. The microstructures indicate that the exsolution mechanism is discontinuous precipitation, whereby the boundary of an augite grain sweeps through a neighbouring augite, leaving the symplectite in its wake. The formation of this symplectitic augite is catalysed by the presence of an intergranular water-rich fluid phase, which promotes grain boundary mobility.     

A case study to verify methods for estimating transmissivity distributions along boreholes

Two methods, both using data from fixed-interval-length hydraulic tests and geological mapping on different scales, have been utilised to estimate transmissivity distributions of individual fractures. Individual fracture properties are of importance in estimating transport of water and solutes; they influence the spreading of grout when sealing tunnels. One of the methods is non-parametric, while the other is based on combinatorics and the multiplication principle. The study verified the usefulness of both methods for estimating the probability of conductive and non-conductive fractures, and the transmissivity distributions. Data for estimating transmissivity distributions originate from flow logging of a borehole in Sweden. Fracture frequency data are from core logging and images using a borehole image processing system (BIPS). Estimated transmissivity distributions, based on flow log data from 3-m sections (test scale length, L: 3 m, and step length, dL: 3 m), were compared with transmissivities obtained directly from detailed, overlapping flow logging which identify individual conductive fractures (test scale, L: 0.5 m, step length, dL: 0.1 m). The latter data are considered to be an acceptable approximation of the individual fracture transmissivities; the agreement, compared with the calculated transmissivity distributions based on 3-m section data, is good for both methods.

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Resumen Se han utilizado dos métodos, ambos procesando datos de pruebas hidráulicas de longitud e intervalo fijo y mapas geológicos en distintas escalas, para estimar las distribuciones de transmisividad de fracturas individuales. Las propiedades de fracturas individuales son de importancia para estimar el transporte de agua y solutos; las propiedades influyen en la propagación de lechada al sellar túneles. Uno de los métodos es no paramétrico mientras que el otro se basa en el principio de multiplicación y combinatorio. El estudio verifica la utilidad de ambos métodos para estimar la probabilidad de fracturas conductivas y no conductivas y las distribuciones de transmisividad. Los datos para estimar las distribuciones de transmisividad se originan del registro de flujo de un pozo en Suecia. Los datos de frecuencia de fracturas provienen de registros de núcleo e imágenes usando un Sistema de Procesamiento de Imágenes de Pozo (BIPS). Se compararon las distribuciones estimadas de transmisividad basadas en datos del registro de flujo en secciones de 3 m (longitud de escala de la prueba L: 3 m, y escalón de longitud dL: 3 m), con transmisividades obtenidas directamente de registros detallados de flujo superpuesto el cual identifica fracturas conductivas individuales (escala de la prueba, L: 0.5 m, escalón de longitud, dL: 0.1 m). Los últimos datos se consideran una aproximación aceptable de las transmisividades de fracturas individuales; la consistencia, comparada con las distribuciones de transmisividad calculada basadas en datos de sección de 3 m, es buena para ambos métodos.

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Résumé Deux méthodes ont été testées afin d’estimer la distribution des transmissivités de fractures unitaires; toutes deux utilisent des données issues de tests hydrauliques effectués à intervalles réguliers et de cartographies géologiques à plusieurs échelles. La connaissance des propriétés des fractures unitaires est importante pour estimer le transport d’eau et de solutés; elles influencent en outre l’étalement du coulis lors du scellement des tunnels. La première méthode est non-paramétrique, et la seconde repose sur l’analyse combinatoire et le principe de multiplication. Cette étude a vérifié l’utilité des deux méthodes pour estimer la probabilité d’occurrence de fractures conductrices et non-conductrices, et la distribution des transmissivités. Les distributions des transmissivités ont été estimées à partir de diagraphies effectuées sur un forage situé en Suède. Les densités de fracturation proviennent de carottages et d’imagerie BIPS (Borehole Image Processing System). Les distributions estimées des transmissivités, basées sur des diagraphies de flux effectuées sur des sections de 3 m (échelle du test L:3 m; pas dL:3 m), ont été comparées aux transmissivités obtenues sur des diagraphies de flux détaillées et se chevauchant, identifiant les fractures conductrices unitaires (échelle du test L: 0.5 m ; pas dL: 0.1 m). Il est admis que ces dernières données représentent une approximation satisfaisante des transmissivités des fractures unitaires; les deux méthodes sont en accord avec les distributions des transmissivités calculées à partir des données acquises sur des sections de 3 m.

     

Downscaling a twentieth century global climate simulation to the North Sea

The regional ocean model system (ROMS) is used to downscale a 26-year period of the twentieth century 20C3M experiment from the global coupled Bergen climate model (BCM) for the North Sea. Compared to an observational-based climatology, BCM have good results on the mean temperature, except for too low winter temperature. This is connected to a too weak inflow of Atlantic water. The downscaling gives added value to the BCM results by providing regional details, doubling the Atlantic inflow, and improving the mean winter temperature. For mean salinity, BCM has values very close to the climatology, whereas the downscaling becomes too fresh. The downscaling, however, improves the sea surface salinity, the vertical structure, and the Norwegian Coastal Current. It is concluded that the downscaling procedure as presented here is a suitable tool for assessing the future Atlantic inflow and sea temperature in the North Sea based on a global climate projection.     

TWO REFERENCE MATERIALS, TRACHYTES MDO-G AND ISH-G, FOR ARGON DATING (K-AR AND 40AR/39AR) OF PLEISTOCENE AND HOLOCENE ROCKS

With the recent extension of K-Ar dating methods within the Upper Pleistocene time, the use of new standards is recommended to allow calibration and inter-laboratory comparisons. For this purpose, two pure microlitic groundmass reference materials were prepared from trachy-basaltic lavas: Trachyte (MDO-G) from Mont Dore, Puy de Sancy, Massif Central is around 250 000 years old; the second Trachyte (ISH-G) from the Isle of Ischia, Gulf of Naples (Italy), is historical: Arso flow erupted on Christmas 1301 - January 1302 AD.     

Composition and genesis of glacial hummocks, western Wisconsin, USA

Glacial hummocks associated with the Superior Lobe in western Wisconsin are stagnant-ice features composed of melt-out till, meltwater-stream sediment, and flow till. The greater proportion of melt-out till in these hummocks than in hummocks described elsewhere suggests that a model of extensive, supraglacial reworking of supraglacially released debris does not apply to the western Wisconsin hummocks. Interpretation of melt-out till in hummock exposures is based on its strong fabric oriented parallel to regional ice-flow direction. Other features of this melt-out till include poorly developed stratification (color banding and discontinuous thin sandy lenses), and minor faulting, both of which support a melt-out origin. We suggest that as stagnant, debris-rich ice began to melt, supraglacially released debris was deposited as flow till and meltwater-stream sediment (with some debris-flow sediment and lake sediment), but as the thickness of supraglacial debris increased, debris melting out at depth was stabilized, allowing features characteristic of melt-out till to be retained. Because the supraglacial debris was sandy and the stagnant ice was likely at the pressure-melting point, the supraglacial debris was well drained and did not readily fail and flow. Debris volume in the glacier generally was greater at the glacier margin, but lateral and longitudinal variations within this zone were caused by thrusting, freezing-on, or ice-margin fluctuations, which in turn resulted in variations in hummock relief. Ice-walled-lake plains are commonly associated with the hummocks and developed where debris volume was small.     

Mobility of Al, Co, Cr, Cu, Fe, Mn, Ni and V in sulphide-bearing fine-grained sediments exposed to atmospheric O2: an experimental study

 The major aim was to increase our knowledge on the behaviour of Al, Co, Cr, Cu, Fe, Mn, Ni and V in sulphide-bearing fine-grained sediments exposed to atmospheric oxygen. Samples of this type of sediment collected in a previous investigation at eight sites in western Finland were digested in HClO₄-HNO₃-HCl-HF at 200  °C and in HCl:HNO₃:H₂O at 95  °C (aqua regia), and subjected to extractions with ammonium acetate and hydrogen peroxide. Metals and S in the leachates were determined with ICP-AES. The results of the chemical analyses are compared with previously reported experimental data. The concentrations of Al and Fe in the sulphide-bearing fine-grained sediments are about 7% and 5%, respectively. Of the trace metals studied, Mn is most abundant followed in decreasing order by V>Cr>Ni>Cu>Co. On oxidation of the sediments, high proportions of Co, Mn and Ni, intermediate proportions of Cu but low proportions of Fe, Al, Cr and V are released. The extent of the release of a metal on oxidation is controlled either by (1) the level to which the pH of the sediments drops on oxidation (Al, Cu, Cr, V), (2) the amount of the metal associated with easily reduced phases (metal sulphides) in the sediments (Ni, Co) or (3) the sum of the amount associated with reduced phases and adsorbed on soil compounds (Mn). No control of the release of Fe on oxidation of the sediments was identified. Based on the results of the study it is argued that artificial drainage and the subsequent oxidation of sulphide-bearing sediments will result in extensive leaching of Co, Mn and Ni, moderate leaching of Cu and limited leaching of Cr and V into drainages. The major elements, Fe and Al, have the potential to be mobilised and leached in large amounts, though the proportions mobilised/leached will remain low. It is suggested that the identification of sulphide-bearing sediments with a high potential of metal release should be based on determination of metals in easily mobilised reduced compounds (dissolved e.g. in H₂O₂) and of the level to which the pH of the sediments drops on oxidation. Received: 16 October 1997 · Accepted: 9 March 1998     

Sampling and Characterizing Rare Earth Elements in Groundwater in Deep-Lying Fractures in Granitoids Under In Situ High-Pressure and Low-Redox Conditions

Several countries are preparing to dispose of radioactive nuclear waste deep underground in crystalline rock. This type of bedrock is commonly extensively fractured and consequently carries groundwater that serves as a medium for transporting metals and radionuclides. A group of metals of particular interest in this context is the rare earth elements (REEs), because they are analogues of actinides contained within radioactive waste and are tracers of hydrological pathways and geochemical processes. Concentrations of REEs are commonly low in these groundwaters, leading to values below detection limits of standard monitoring methods, particularly for the heavy REEs. We present a new technical set-up for monitoring REEs (and other trace metals) in groundwater in fractured crystalline rock. The technique consists of passing the fracture groundwater, commonly under high pressure and containing reduced chemical species, through a device that maintains the physicochemical character of the groundwater. Within the device, diffusive gradient in thin-film (DGT) discs are installed in triplicate. With this set-up, we studied REEs in groundwater in fractures at depths of approximately ?144, ?280, and ?450 m in granitoids in the Äspö Hard Rock Laboratory in southern Sweden. The entire REE suite was detected (concentrations down to 0.1 ng L^?1) and was differently fractionated among the groundwaters. The shallowest groundwater, composed of dilute modern Baltic Sea water, was enriched in the heavy REEs, whereas the deeper groundwaters, dominated by old saline water, were depleted in the heavy REEs. Deployment periods varying from 1 to 4 weeks delivered similar REE concentrations, indicating stability and reproducibility of the experimental set-up. The study finds that 1 week of deployment may be enough. However, if the overall setting and construction allow for longer deployment times, 2–3 weeks will be optimal in terms of reaching reliable REE concentrations well above the detection limit while maintaining the performance of the DGT samplers.     

Characterization of aquifers in the Vientiane Basin,Laos, using Magnetic Resonance Sounding and Vertical Electrical Sounding

The aim of this study is to define and characterize water bearing geological formation and to test the possibility of using geophysical techniques to determine the hydrogeological parameters in three areas in the Vientiane basin, Laos. The investigated areas are part of the Khorat Plateau where halite is naturally occurring at depths as shallow as 50 m in the Thangon Formation. Magnetic Resonance Sounding (MRS) has been used in combination with Vertical Electrical Sounding (VES) in different geological environments. In total, 46 sites have been investigated and the MRS and VES recognized the stratigraphic unit N₂Q_1–3, consisting of alluvial unconsolidated sediments, as the main water bearing unit. The aquifer thickness varies usually between 10 and 40 m and the depth to the main aquifer range from 5 to 15 m. The free water content is here up to 30%, and the decay times vary between 100 and 400 ms, suggesting a mean pore size equivalent to fine sand to gravel. The resistivity is highly variable, but usually around 10–1500 Ω-m, except for some sites in areas 1 and 2, where the aquifer is of low resistivity, probably related to salt water. Hydraulic and storage-related parameters such as transmissivity, hydraulic column, have been estimated from the MRS. The MRS together with VES has been shown to be a useful and important tool for identifying and distinguishing freshwater from possible salt-affected water as well as the salt-related clay layer of the Thangon Formation. This clay layer is characterized by very low free water content and a resistivity lower than 5 Ω-m and can be found in all 3 areas at depths from 15 to 50 m.     

Remote sensing of snow and ice / La télédétection de neige et de glace

Abstract

Monitoring of snow and ice on the Earth's surface will require increasing use of satellite remote sensing techniques. These techniques are evolving rapidly. Active and passive sensors operating in the visible, near infrared, thermal infrared, and microwave wavelengths are described in regard to general applications and in regard to specific USA or USSR satellites. Meteorological satellites (frequent images of relatively crude resolution) and Earth resources satellites such as Landsat (less frequent images of higher resolution) have been used to monitor the areal extent of seasonal snow, but problems exist with cloud cover or dense forest canopies. Snow mass (water equivalent) can be measured from a low-flying aircraft using natural radioactivity, but cannot yet be measured from satellite altitudes. A combination of active and passive microwave sensors may permit this kind of measurement, but not until more is known about radiation scattering in snow. Satellite observations are very useful in glacier inventories, correcting maps of glacier extent, estimating certain mass balance parameters, and monitoring calving or surging glaciers. Ground ice is virtually impossible to monitor from satellites; ice on rivers and lakes can be monitored only with very high-resolution sensors. Microwave sensors, due to their all-weather capability (the ability to see through clouds) provide exciting data on sea ice distribution. Analysis of digital tapes of satellite data requires the archiving and scanning of huge amounts of data. Simple methods for extracting quantitative data from satellite images are described.