Le Capelinhos (Faïal,Açores) vingt ans après son éruption: le modèle éruptif «surtseyen» et les anneaux de tufs hyaloclastiques

Phreatomagmatic structures are of two kinds: maars and tuff-rings. Data given by records of Capelinhos activity (Faïal, Açores, 1957–1958), by structures at west point of Faïal island, and by palagonitic breccias of Velay and Cantal areas (France) lead to relate hyaloclastic tuff-rings and shallow subaquatic («surtseyan») eruptions. It is possible to precise causes, characteristics, and mechanism of formation of tuff-rings.     

Identification of a magma chamber in the Ghoubbet-Asal rift (Djibouti) from a magnetotelluric experiment

A very broad band (10^?3 to 10⁴ Hz) magnetotelluric investigation of the axial zone of the Ghoubbet-Asal rift (Djibouti) has revealed a shallow (2–4 km) magma chamber which can be mapped in some detail. The suggested roof of the chamber is shallowest very close to the Ardoukoba volcano which was built during the November 1978 rifting episode.     

H-Alpha survey of the solar corona at Pic-du-Midi

A daily survey of cool material evolution in the inner parts of the solar corona is now performed at the Pic-du-Midi observatory with an H imaging coronagraph. The total field of the coronagraph allows the detection of emission regions all around the solar limb, up to 0.6 solar radius above the limb. This survey is devoted to the study of coronal events and mass motions in a large range of spatial and time scales. The observing modes are associated with a set of numerical treatment processes to produce images which are calibrated in intensity relative to the solar brightness. Position angles and height above the solar limb of the coronal features are determined. Methods of calibration, sky brightness subtraction, and detection of events are discussed.     

Chemistry of some Pb-(Cu,Fe)-(Sb,Bi sulfosalts from France and Portugal. Implications for the crystal chemistry of lead sulfosalts in the Cu-poor part of the Pb2S2-Cu2S-Sb2S3-Bi2-Bi2S3 system

Electron microprobe analysis of Pb-Cu(Fe)-Sb-Bi sulfosalts from Bazoges and Les Chalanches (France), and Pedra Luz (Portugal), give new data about (Bi, Sb) solid-solution and incorporation of the minor elements Cu, Fe or Ag in jaskolskiite, and in izoklakeite-giessenite and kobellite-tintinaite series. Jaskolskiite from Pedra Luz has high Sb contents (from 17.9 to 20.7 wt.%), leading to the extended general formula: Cu _x Pb_2+x (Sb_1–y Bi _y )_2–x S₅, with 0.10 x 0.22 and 0.19 y 0.41. Fe-free, Bi-rich izoklakeite from Bazoges has high Ag contents (up to 2.2 wt. %), leading to the simplified formula Cu₂Pb₂₂Ag₂(Bi, Sb)₂₂S₅₇; in Les Chalanches it contains less Ag content (1.2 wt.%), but has an excess of Cu that gives the formula: Cu_2.00 (Cu_0.49Ag_1.18)_=1.67Pb_22.70(Bi_12.63Sb_8.99)_=21.62S_57.27.In tintinaite from Pedra Luz, the variation of the Fe/Cu ratio can be explained by the substitution: Cu + (Bi, Sb) Fe + Pb; Fe-free kobellite from Les Chalanches has a Cu-excess, corresponding to the formula Cu_2.81Ag_0.54Pb_9.88(Bi_10.37Sb_5.21)_=15.38S_35.09. Eclarite from the type locality, structurally related to kobellite, shows a Cu excess too. In natural samples of the kobellite homologous series, Fe is positively correlated with Pb, and its contents never exceed that of Cu. Ag substitutes for Pb, together with (Bi, Sb). Taking into account the possibility of Cu excess, but excluding formal Cu²⁺ and Fe³⁺, general formulae can be written:     

The Role of Laser Ranging for Calibrating Jason-1: The Corsica Tracking Campaign

The French Transportable Laser Ranging System (FTLRS), a highly transportable Satellite Laser Ranging (SLR) instrument, was set up in Corsica (from January to September 2002) for participating to the JASON-1 altimeter verification phase. In addition to the tracking of oceanographic satellite missions and in order to perform an accurate positioning, the FTLRS also acquired laser ranging data on geodetic satellites, STARLETTE and STELLA essentially.

The paper describes the analysis strategy mainly based on the use of a short-arc orbit technique to compute accurate 1 cm local orbits, and then the geocentric positioning (2-3 mm relative to GPS). Finally, we established the JASON-1 absolute calibration value, based on 9 SLR short-arcs (between cycles 1 and 26), at 108.2 ± 8.7 mm; the 10-day repeatability is of 26.1 mm showing that a great accuracy has been reached.     

Bemerkungen zum Energie-Umsatz im Gewitter

Zusammenfassung Eine Abschätzung der im Gewitter zum Umsatz kommenden Energiebeträge zeigt, daß die in den elektrischen Phänomenen verbrauchte Energie im Gesamt-Energie-Umsatz nicht ins Gewicht fällt. Zieht man jedoch den Vergleich nur zu dem Teilbetrag des mechanisch-thermodynamischen Energieumsatzes, der sich über den normalen Energie-Umsatz des Wetter- bezw. Austauschgeschehens erhebt, und nimmt versuchsweise für diesen den aus dem Niederschlag allein abgeleiteten Betrag an, so ergibt sich, daß die elektrischen Phänomene etwa 1/4 dieses Betrages beanspruchen. — Die Abschätzung des elektrischen Energie-Umsatzes erlaubt es, die mittlere in einem Erdblitz umgesetzte Energie abzuleiten: Sie ergibt sich zu rund 2000 kWh. Eine noch nicht lösbare Diskrepanz ergibt sich für die Feldstärke unterhalb der Gewitterwolken.

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Summary By an estimate of the energy budget of the thunderstorm is shown that the energy of electrical activity is entirely negligible in considerations of the whole budget. But if the energy of electrical activity is only compared with that amount of the whole energy which exceeds the normal energy amount of the «Austausch» it can by shown that the electrical phenomena need about 1/4 of this amount (this amount is estimated only from the precipitation). — By the estimation of the electrical energy budget it is possible to give the mean energy of a lightning: about 2000 kWh. In considerations of the potential-gradient under thunderstorm clouds a contradiction is given which can not be solved until now.

     

Die Tagesschwankungen der Empfangsfeldstärke bei 17,8 KHz

Ohne Zusammenfassung     

Transient States in Diagenesis Following the Deposition of a Gravity Layer: Dynamics of O<Subscript>2</Subscript>, Mn,Fe and N-Species in Experimental Units

Biogeochemical processes induced by the deposition of gravity layer in marine sediment were studied in a 295-day experiment. Combining voltammetric microelectrode measurements and conventional analytical techniques, the concentrations of C, O₂, N-species, Mn and Fe have been determined in porewaters and sediments of experimental units. Dynamics of the major diagenetic species following the sudden sediment deposition of few cm-thick layer was explained by alternative diagenetic pathways whose relative importance in marine sediments is still a matter of debate. Time-series results indicated that the diffusion of O₂ from overlying waters to sediments was efficient after the deposition event: anoxic conditions prevailed during the sedimentation. After a few days, a permanent oxic horizon was formed in the top few millimetres. At the same time, the oxidation of Mn²⁺ and then Fe²⁺, which diffused from anoxic sediments, contributed to the surficial enrichment of fresh Mn(III/IV)- and Fe(III)-oxides. Vertical diffusive fluxes and mass balance calculations indicated that a steady-state model described the dynamic of Mn despite the transitory nature of the system. This model was not adequate to describe Fe dynamics because of the multiple sources and phases of Fe²⁺. No significant transfer of Mn and Fe was observed between the underlying sediment and the new deposit: Mn- and Fe-oxides buried at the original interface acted as an oxidative barrier to reduced species that diffused from below. Nitrification processes led to the formation of a NO₃⁻/NO₂⁻ rich horizon at the new oxic horizon. Over the experiment period, NO₃⁻ concentrations were also measured in the anoxic sediment suggesting anaerobic nitrate production.     

Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal–As–Sb–Au mineralisation during Hercynian strike-slip deformation

In the Saint-Aubin-des-Chateaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Grès armoricain Formation. The hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After the massive sulphidation stage (Fe–As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb–Sb–Au (stage 3) parageneses in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification of structures that developed incrementally via right-lateral simple shear compatible with bulk strain affecting the Central Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids (maximum of 390–350°C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300–275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying Neo-Proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring deposits suggest that large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key control for deformation and fluid flow in the oolitic ironstones, which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic mineralisation of Saint-Aubin-des-Chateaux appears to be very similar to epigenetic sulphidation described in banded iron formation-hosted gold deposits.