Two-stage partial melting during the Variscan extensional tectonics (Montagne Noire,France)

International Journal of Earth Sciences - One of the striking features that characterise the late stages of the Variscan orogeny is the development of gneiss and migmatite domes, as well as...     

Return mapping of phases and the analysis of the gravitational clustering hierarchy

We present spectroscopic and high-speed photometric data of the eclipsing polar V895 Cen. We find that the eclipsed component is consistent with it being the accretion regions on the white dwarf. This is in contrast to Stobie et al. who concluded that the eclipsed component was not the white dwarf. Further, we find no evidence for an accretion disc in our data. From our Doppler tomography results, we find that the white dwarf has   M ≳0.7 M_⊙  . Our indirect imaging of the accretion stream suggests that the stream is brightest close to the white dwarf. When we observed V895 Cen in its highest accretion state, emission was concentrated along field lines leading to the upper pole. There is no evidence for enhanced emission at the magnetic coupling region.     

Axially-symmetric Velocities in the 15 May 2000 Eruptive Prominence

Large Doppler velocities with unique, almost regular elliptical features were observed in the H spectra of the May 15, 2000 eruptive prominence. These features were interpreted in the frame of axially symmetric models of the eruptive prominence. The rotational (7–60 km s^–1), expansion (30–44 km s^–1), axial (3–19 km s^–1), and global (66–160 km s^–1) prominence plasma velocities were derived. The plasma velocity patterns were compared with the observed helical structures of the H prominence. The velocities of selected H blobs in the image plane were determined. The axially symmetric detwisting process of the magnetic flux rope of the eruptive prominence was recognized.     

Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Magnesite forms a series of 1‐ to 15‐m‐thick beds within the ≈2·0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680‐m‐thick formation is composed of a stromatolite–dolomite–‘red bed’ sequence formed in a complex combination of shallow‐marine and non‐marine, evaporitic environments. Dolomite‐collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by δ¹³C values from +7·1‰ to +11·6‰ (V‐PDB) and δ¹⁸O ranging from 17·4‰ to 26·3‰ (V‐SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high δ¹³C values ranging from +9·0‰ to +11·6‰ and δ¹⁸O values of 20·0–25·7‰. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water‐derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high δ¹³C values reflect a combined contribution from both global and local carbon reservoirs. A ¹³C‐rich global carbon reservoir (δ¹³C at around +5‰) is related to the perturbation of the carbon cycle at 2·0 Ga, whereas the local enhancement in ¹³C (up to +12‰) is associated with evaporative and restricted environments with high bioproductivity.     

Modelling of the sensing system of a vibrational magnetometer by digital computer

Summary An analytico-numerical computation of the transfer function of the sensing system of a vibrational magnetometer is presented. In discussing the results, the properties of the method are pointed out and a proposal for the sensing system is made with regard to the required sensitivity of the instrument.     

Thermal precursors in satellite images of the 1999 eruption of Shishaldin Volcano

Shishaldin Volcano, Unimak Island Alaska, began showing signs of thermal unrest in satellite images on 9 February 1999. A thermal anomaly and small steam plume were detected at the summit of the volcano in short-wave thermal infrared AVHRR (advanced very high resolution radiometer) satellite data. This was followed by over 2 months of changes in the observed thermal character of the volcano. Initially, the thermal anomaly was only visible when the satellite passed nearly directly over the volcano, suggesting a hot source deep in the central crater obscured from more oblique satellite passes. The "zenith angle" needed to see the anomaly increased with time, presumably as the thermal source rose within the conduit. Based on this change, an ascent rate of ca. 14 m per day for the thermal source was estimated, until it reached the summit on around 21 March. It is thought that Strombolian activity began around this time. The precursory activity culminated in a sub-Plinian eruption on 19 April, ejecting ash to over 45,000 ft. (13,700 m). The thermal energy output through the precursory period was calculated based on geometric constraints unique to Shishaldin. These calculations show fluctuations that can be tied to changes in the eruptive character inferred from seismic records and later geologic studies. The remote location of this volcano made satellite images a necessary observation tool for this eruption. To date, this is the longest thermal precursory activity preceding a sub-Plinian eruption recorded by satellite images in the region. This type of thermal monitoring of remote volcanoes is central in the efforts of the Alaska Volcano Observatory to provide timely warnings of volcanic eruption, and mitigate their associated hazards to air-traffic and local residents.     

Magnetostratigraphy of Cave Sediments: Application and Limits

The application of the magnetostratigraphy for dating of clastic and chemogenic cave sediments has been limited by the complex conditions underground and the lack of age constraints on these deposits for correlation with geomagnetic polarity timescale (GPTS). Without age constraints any correlation of obtained results cannot be explicit. Additionally, the dynamic character of cave infilling, exhumation and fossilisation expressed by unconformities within preserved sedimentary profiles can hide a substantial part of the geological time. The detailed internal division of deposits (breaks in deposition and related processes) and scarcity of fossils make the correlation of obtained magnetostratigraphic log with geomagnetic polarity timescales sometimes problematic. The analytical results confirmed that the complete step/field procedure offered by demagnetisation methods must be applied. The application of complete analysis only to pilot samples and shortened, selected field/step approach, to other samples did not offer sufficient data set for reliable interpretation.     

Muscovite recrystallization and saddle-shaped Ar/Ar age spectra: example from the Blond granite (Massif Central, France)

Muscovites from the Blond granite (West French Massif Central) were dated by the ⁴⁰Ar/³⁹Ar single-grain method. The ⁴⁰Ar/³⁹Ar ages obtained vary from 305.5 ± 0.3 Ma to 311.3 ± 0.5 Ma, and most of the age spectra are slightly saddle-shaped. The analyzed muscovites show phengitic recrystallization under optical microscope observations, SEM images, and electron microprobe chemical analyses. It is proposed that the saddle-shaped age spectra result from a partial recrystallization, which produced three different isotopic reservoirs in the analyzed white mica single grains: domains of early muscovite, domains of neocrystallized muscovite formed by phengitic and Al-Fe substitutions, and “low-activation energy sites.”     

Multipath mitigation by conventional antennas with ground planes and passive vertical structures

An antenna ground plane (GP) plays a major role in the mitigation of multipath coming from underneath the antenna. A distinct trend in user antenna size and weight reduction has been observed during recent years. With this focus in mind, an overview of conventional flat conductive and impedance GP is provided. Results of modern developments of passive vertically stacked structures are discussed. The possibility of obtaining a reasonable level of multipath mitigation with vertical structures of several centimeters is shown.