Showing papers by "IPG Photonics published in 1998"

Aluminium X-ray absorption Near Edge Structure in model compounds and Earth’s surface minerals

Abstract: Aluminium K-edge X-ray absorption near edge spectra (XANES) of a suite of silicate and oxides minerals consist of electronic excitations occurring in the edge region, and multiple scattering resonances at higher energies. The main XANES feature for four-fold Al is at around 2 eV lower energy than the main XANES feature for six-fold Al. This provides a useful probe for coordination numbers in clay minerals, gels, glasses or material with unknown Al-coordination number. Six-fold aluminium yields a large variety of XANES features which can be correlated with octahedral point symmetry, number of aluminium sites and distribution of Al-O distances. These three parameters may act together, and the quantitative interpretation of XANES spectra is difficult. For a low point symmetry (1), variations are mainly related to the number of Al sites and distribution of Al-O distances: pyrophyllite, one Al site, is clearly distinguished from kaolinite and gibbsite presenting two Al sites. For a given number of Al-site (1), variations are controlled by changes in point symmetry, the number of XANES features being increased as point symmetry decreases. For a given point symmetry (1) and a given number of Al site (1), variations are related to second nearest neighbours (gibbsite versus kaolinite). The amplitude of the XANES feature at about 1566 eV is a useful probe for the assessment of AlIV/Altotal ratios in 2/1 phyllosilicates. Al-K XANES has been performed on synthetic Al-bearing goethites which cannot be studied by 27Al NMR. At low Al content, Al-K XANES is very different from that of α-AlOOH but at the highest level, XANES spectrum tends to that of diaspore. Al-K XAS is thus a promising tool for the structural study of poorly ordered materials such as clay minerals and natural alumino-silicate gels together with Al-subsituted Fe-oxyhydroxides.

Radiation induced paramagnetic centres in nuclear glasses by EPR spectroscopy

Abstract: An electron paramagenetic resonance (EPR) investigation of a series of glasses irradiated at room temperature with β or X radiation sources has been made in order to predict the long term behavior of glasses used in the nuclear waste disposal. EPR spectra of the irradiated glasses have been measured and treated using computer simulations. Simulation of the g factor and hyperfine interaction distributions have led to the identification of different paramagnetic centers in these irradiated glasses. The following paramagnetic defects were identified: boron oxygen hole center (BOHC), HC1 center, two different hole centers, peroxy radical (Oxy defect), E′ centers, and electron trapping processes for iron and zirconium ions: Zr4++ e−→ Zr3+ and Fe3+ + e−→ Fe2+. In particular, no paramagnetic defects associated with aluminum ions are detected in these irradiated glasses.

Equatorial electrojet at African longitudes: first results from magnetic measurements

Abstract: . In the framework of the French participation in the International Equatorial Electrojet Year (IEEY), ten magnetotelluric stations were installed between November 1992 and November 1994 along a 1200-km-long meridian profile, between Lamto (latitude 6.2°N, Cote d'Ivoire) to the south and Tombouctou (latitude 16.7°N, Mali) to the north. These stations measured digitally the three components of the magnetic field and the two components of the telluric electric field, and operated over a period of 20 months. The magnetic data is used to study the features of the equatorial electrojet (EEJ) in West African longitude. The measurement of the telluric electric field variations will be presented elsewhere. Hourly mean values are used to study the morphological structure of the regular diurnal variation SR of the three components (H, D, and Z) of the earth magnetic field and to characterize the EEJ during magnetically quiet days. The occurrences of the counter-electrojet (CEJ) are set forth, emphasizing its seasonal variability. Assumed to be a current ribbon, the EEJ main parameters (the position of its center, its width, and the amplitude of its current density at the center) are determined. A preliminary analysis of the time variations of these parameters is presented over period of 20 months (from January 1993 to August 1994). Results are compared with those obtained during previous experiments of the same kind.

Pre‐ and syn‐eruptive geochemistry of volcanic gases from Soufriere Hills of Montserrat, West Indies

Abstract: Soufriere Hills fumaroles contained magma-derived volatiles before and during the eruption initiated in 1995 but also preserved a typical and quite steady hydrothermal composition. Chemical changes due to increased boiling and a greater input of oxidizing magmatic gas occurred only at Galway's Soufriere, the most active fumarolic field. Hydrothermal buffering of the fumaroles has been favoured by their remote location (1–2 km) from the eruptive vents and by a preferential degassing of the uprising magma through intrusive conduits under the crater. High temperature (720°C) gas collected from the extruding lava dome in Feb. 1996 was chemically and isotopically representative of the magmatic gas stream. Its composition allows assessment of average eruptive fluxes of H2O, CO2 and HCl which require the degassing of only 2.5–3 times more magma than erupted.

Constraints on the structure of the Azores spreading center from gravity data

Abstract: The Azores Archipelago is believed to be the site of the third arm of a Triple Junction between the Eurasia/Africa/North America plates. However, to the present no study has been able to identify its segmentation pattern, the spreading mechanism and its relationship with the well-known topographic features of the Mid-Atlantic Ridge (MAR). Here we present a new gravity compilation obtained with the existing National Geophysical Data Center (NGDC) data, merged with new gravity profiles collected during the ESCAPE cruise in 1995. This compilation is used to calculate a Free Air Anomaly (FAA) map, which is used to test two different models, the Mantle Bouguer Anomaly model and the elastic plate model, for the study of the thermal regime of the Terceira Axis. The analysis of the results from both models demonstrates that the elastic plate model successfully models the gravity data from the Azores Plateau and that there is no gravity evidence for the existence of a spreading axis. The elastic plate thickness Te, with a value of 7–8 km, suggests a very young lithosphere (about 10 Ma) at the time of the load of the Azores Plateau.

Aluminum K-XANES spectra in minerals as a source of information on their local atomic structure

Abstract: For Al containing compounds with different symmetry types of Al environment (from fourfold to 12-fold coordinated Al) it is revealed that Al K-XANES spectra are well reproduced by the XAFS code, based on the method of Hartree-Fock MT-potential generation, considering the photoelectron single-, double- and triple-scattering processes on approximately linear two- and three-atom chains, originated at the absorbing Al atom. The empirical `selection rules' for choosing these chains are obtained and the origins of features in spectra are interpreted using the SELCOMP code. The proposed EXAFS-like approach for Al K-XANES description is applied also to explain the differences in Al K-XANES spectra, caused by the replacement of Fe atoms by Al atoms in goethite (-FeOOH).

A study of transient variations in the Earth's electromagnetic field at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast)

Abstract: . In the framework of the French-Ivorian participation to the IEEY, a network of 10 electromagnetic stations were installed at African longitudes. The aim of this experiment was twofold: firstly, to study the magnetic signature of the equatorial electrojet on the one hand, and secondly, to characterize the induced electric field variations on the other hand. The first results of the magnetic field investigations were presented by Doumouya and coworkers. Those of the electric field experiment will be discussed in this study. The electromagnetic experiment will be described. The analysis of the electromagnetic transient variations was conducted in accordance with the classical distinction between quiet and disturbed magnetic situations. A morphological analysis of the recordings is given, taking into consideration successively quiet and disturbed magnetic situations, with the results interpreted in terms of the characterization of external and internal sources. Particular attention was paid to the effects of the source characteristics on the induced field of internal origin, and to the bias they may consequently cause to the results of electromagnetic probing of the Earth; the source effect in electromagnetic induction studies. During quiet magnetic situations, our results demonstrated the existence of two different sources. One of these, the SRE source, was responsible for most of the magnetic diurnal variation and corresponded to the well-known magnetic signature of the equatorial electrojet. The other source (the SR*E source) was responsible for most of the electric diurnal variation, and was also likely to be an ionospheric source. Electric and magnetic diurnal variations are therefore related to different ionospheric sources, and interpreting the electric diurnal variation as induced by the magnetic field diurnal variation is not relevant. Furthermore, the magnetotelluric probing of the upper mantle at dip equator latitudes with the electromagnetic diurnal variation is consequently impossible to perform. In the case of irregular variations, the source effect related to the equatorial electrojet is also discussed. A Gaussian model of equatorial electrojet was considered, and apparent resistivities were computed for two models of stratified Earth corresponding to the average resistive structure of the two tectonic provinces crossed by the profile: a sedimentary basin and a cratonic shield. The apparent resistivity curves were found to depend significantly on both the model used and the distance to the center of the electrojet. These numerical results confirm the existence of a daytime source effect related to the equatorial electrojet. Furthermore, we show that the results account for the observed differences between daytime and night-time apparent resistivity curves. In particular, it was shown that electromagnetic probing of the Earth using the classical Cagniard-Tikhonov magnetotelluric method is impossible with daytime recordings made at dip latitude stations. Key words. Electromagnetics (Transient and time do- main) Geomagnetism and paleomagnetism (geomagne- tic induction) Ionosphere (equatorial ionosphere)

Signature morphologique de l'activité de la faille des Cévennes (Languedoc, France)

Abstract: The Tertiary Cevennes faul! presents typical geomorphic characters of active faults. Its trace is sharp on SPOT images and air photographs. It is segmented (15–30 km) and has locally multiple strands. Along the main fault trace, abandoned valleys and terrace risers of the Herault and Gardon rivers are offset sinistrally by several hundreds of metres. This suggests that the fault is an active Quaternary fault whose left slip-rate may range between 0.1 and 2 mm·yr1.

Dégazage magmatique diffus à la Soufrière de Guadeloupe, Antilles

Abstract: A soil gas profiling made along the southern basis of Soufriere summit lava dome, in Guadeloupe, reveals the existence of diffuse emanations of magma-derived CO 2 in coincidence with a major volcanic (Ty) fault, where CO 2 concentrations at 70 cm depth in the ground reach 35–96 % and are associated with a thermal convective cell. Outside, a few ‘cold’ gaseous anomalies of volcanic origin (lack of methane) provide reliable conditions for continuous radon monitoring of soil degassing.

Structure of Pd–Te precipitates in a simulated high-level nuclear waste glass

Abstract: Structural and bonding characteristics of simplified (Pd, Te) precipitates have been determined in a simulated nuclear French glass using extended x-ray absorption fine structure (EXAFS) and x-ray diffraction. In this sample, these precipitates have a homogeneous composition, with about 10 wt.% Te. They retain a face-centered cubic structure as in pure Pd with a cell parameter which obeys Vegard’s law. Pd K-edge EXAFS shows the presence of Te in the Pd coordination shell, with (Pd–Te) distances of 2.80 A. These distances, higher by 0.05 A than the (Pd–Pd) distances, may result in a lower packing efficiency of the CFC lattice. The comparison with the average distances derived from x-ray diffraction shows the nonmetallic character of the Pd–Te bond in these precipitates. These bonding modifications may cause the limited solubility of Te in metallic Pd.

Preuves paléomagnétiques de la non-interstratification des évaporites du Trias dans l'Albien du Maghreb oriental

Abstract: In northwestern Tunisia, a paleomagnetic study was carried out on Albian levels, objects of controversy. They are considered to be either overturned strata on the limb of diapirs, or in their upright position as the floor of a salt glacier. After correction for the apparent dip, the polarity of the primary magnetization is always reversed, and the magnetization directions are relatively scattered. The Earth's magnetic field was always of normal polarity during the Albian, so that these levels are overturned. The scattering of the magnetization direction points out moreover that the tilting occurred around axes of various orientations. All these observations show that the evaporitic bodies were emplaced in diapirs, and that the notion of a salt glacier interbedded within the Albian sediments has to be given up.

Données paléomagnétiques du bassin permien de Rodez et rotations dans la bordure sud-ouest du Massif central

Abstract: The palaeomagnetic study of the Rodez Permian basin yields two poles for the formations attributed to the Thuringian: 48.0 °N, 166.1 °E, K = 453, A95 = 2.7 ° and 52.4 °N, 165.9 °E, K = 68, A95 = 3.9 °. Comparison of these new data with those previously acquired in the neighbouring Permian basins shows that the post-Saxonian rotation already known in the Saint-Affrique basin very likely affected also the Rodez and Brive basins.

Diagenese précoce en milieu sulfuré réducteur une étude isotopique dans le Jurassique basai du Bassin parisien

Abstract: Lower Jurasssic argilo-limestones from a continental platform environment have been drilled at Montcornet in the Paris Basin (Andra, 1989). The isotopic analysis of calcite yield δ 13 C values between −3 and +1 %o, and δ 18 O between +21 and +27 ‰ Depletion in 13 C is caused by secondary crystallisation of calcite in a sulphate-reducing environment, where isotopically light dissolved inorganic carbon, resulting from organic matter mineralization, is added into the pore fluids. The depletion in δ 18 O results from the isotopic equilibrium between pore water and calcite, which is controlled by temperature, fluid/rock ratio and the δ 18 O of interstitial fluid. This study clearly shows that stable isotopes are a powerful tool for the study of local diagenetic mechanisms, knowledge of which is a key to the understanding of the sedimentary record from geochemical and geophysical points of view.

The Spectral Source Model: A Tool for Deterministic and Probabilistic Seismic Hazard Assessment

Abstract: Seismic‐hazard assessment at small scales requires the computation of realistic broad‐band accelerograms near the sources. Methods that use only natural records are often limited because of the limited data set. Among recently developed methods for source modelling, the spectral‐source model, based on the k−2 dislocation distribution seems particularly promising as it enables computation of synthetics for any source‐receiver distance and directivity configuration. The spectral model takes into account important source characteristics, such as stress drop and directivity effects predicting acceptable high‐frequency levels, whatever the station location (Bernard et al., 1996b). Firstly, some tests to explain the directivity properties of the model are presented, followed by the main results of a deterministic study carried out for the 1992 Erzincan earthquake, Turkey. Finally, results of a probabilistic study in the Moyenne‐Durance region (France) are used to demonstrate the ability of this method ...

Paléomagnétisme de l'unité inférieure autunienne de la formation rouge du bassin d'Abadla (Algérie)

Abstract: A new palaeomagnetic pole situated at 29.1°S and 57.8°E has been obtained in the lower unit, of Autunian age ( Doubinger and Fabre, 1983 ) of the Abadla redbeds formation. The proximity of this pole with that of Morel et al. (1981) , determined in the azoic and undated upper unit of the same formation, suggests an Autunian age for this upper unit. Thus, the subsidence of the Abadla basin and the deposition of the whole series occurred within a period no longer than 20–25 My.

Les cratères d'impacts: principaux effets de choc dans les roches et minéraux

Abstract: The basic principles of the physics of shock waves are summarised, showing how toxic pressures, shock and pot-shock temperatures, and shock durations can be estimated in the case of large meteorite impacts on Earth. In a second part, the pertinence of laboratory high-pressure dynamic experiments for simulating large meteorite impact events and for calibrating their physical conditions is discussed. It is concluded that most shock features are common to natural and laboratory shocks, although the lifetime of experimental shocked states is shorter by several orders of magnitude. Then, a review is, made of the major shock effects observed in minerals and rocks. Quartz has been, by far, the most extensively studied shocked mineral. Particularly, planar deformation features (PDFs), interpreted as resulting from relaxations at the shock front, are unambiguous shock indicators, for shock pressures approximately between 15 and 35 GPa. At higher pressure the formation of high-pressure polymorphs of SiO 2 in shocked quartz is also discussed. Shock effects in some other selected minerals, although less extensively studied, are also reviewed, with special emphasis on the discovery of diamonds at impact sites and of all the high-pressure polymorphs of olivines and pyroxenes, including silicate perovskite, in shocked meteorites. Finally, the controversial links between large impacts and major environmental effects are discussed in a fourth part.