Etude et Mise au Point d'un Microdébitmetre Très Sensible et à Haute Résolution Applications à des Mesures et à des Tests au Sol

Sommaire Nous avons mis au point un microdébitmètre électronique permettant des mesures de pression et pression différentielle, très sensible et à haute résolution.Le module étudié a une fréquence de coupure de 300 Hertz. Il permet de travailler dans une gamme de différence de pression de ±100 Pascal avec une résolution de 0,1 Pascal.L'appareil mis au point a permis de réaliser des series de tests de mesures au sol de la variation de pression dynamique de l'air et du coefficient de structure de ce paramètre.

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Geochemistry of the late Cenomanian-early Turonian chalks of the Paris Basin: Manganese and carbon isotopes in carbonates as paleooceanographic indicators

In the North Atlantic DSDP/IPOD cores, carbon isotope data on the bulk carbonates show significant fluctuations. In sediments now exposed on land coeval fluctuations in the carbon isotope concentrations are also recorded in pelagic and epeiric facies. For instance, in the Upper Cretaceous chalks of the Paris Basin, there is a major break at the Cenomanian-Turonian boundary. At this time, the manganese content of the chalks was also at a maximum and consequently a positive relation can be demonstrated between δ¹³C and manganese concentrations. The same positive correlation is also recorded in many pelagic limestones.In the North Atlantic cores, carbon isotope events are related to the black shale facies and to global oceanic anoxic events and one can suppose that in sediments deposited on the continental margins they are also related to mildly anoxic conditions. Considering the manganese geochemistry in carbonate rocks, a high manganese content in such a reducing environment can be found in the sediments only if the Mn concentration of the interstitial solutions are abnormally high. As a high Mn content in marine pore waters is believed to originate from hydrothermal process, Mn and δ¹³C positive excursions are ultimately related to mid-oceanic ridge activity and to a closely connected phenomenon, the great transgressive pulses during which mid-depth waters may have been anoxic. Consequently, major Mn and carbon isotope events would seem to be useful tools in paleooceanographic reconstructions.     

A 2.5 ka History of Dacitic Magmatism at Nevado de Toluca, Mexico: Petrological, 40Ar/39Ar Dating, and Experimental Constraints on Petrogenesis

After 11·5 ka of quiescence (24·5–13 ka),the Nevado de Toluca volcano started a 2500 year period of activity.This period was characterized by a dome destruction event at13 ka, a small Plinian event at 12·1 ka, and a largePlinian eruption at 10·5 ka. About 10 km³ of magma waserupted that was homogeneous in composition (63·3–65·7SiO₂ wt % whole-rock) and in mineralogy. Pumice consists ofplagioclase (An_30–59) > orthopyroxene (En_56–59)> hornblende >> Fe–Ti oxides + rare apatite (inopx) + biotite, set in a rhyolitic matrix (72–76 SiO₂wt %). ⁴⁰Ar/³⁹Ar analysis of single biotite crystals yieldedages (0·81–4·7 Ma), that do not correspondto eruption ages. The biotite represents partially assimilatedxenocrysts, which could have resided in the magma for only ashort period of time. Mineral chemical data, coupled with hydrothermalexperiments, indicate that prior to eruption the dacitic magmastagnated at a depth of 4·5–6 km below the summitat water pressures of 160–210 MPa and a temperature of824 ± 12°C on the basis of Fe–Ti oxide thermometry,and under water-saturated conditions. To stabilize a homogeneousmagma body of >10 km³ at 824°C in the upper crust, wepropose that reheating of the dacitic reservoir by hotter magmabatches was able to maintain the equilibrium between the temperatureof the magma and the assimilation of wall-rock over a periodof 2500 years. Based on similarities among the juvenile products,we suggest that the three eruptions were fed from the same magmabody. KEY WORDS: explosive volcanism; xenocrystic contamination; Nevado de Toluca, Mexico     

The methodology of quantification of volcanic explosions from broad-band seismic signals and its application to the 2004–2005 explosions at Volcán de Colima, Mexico

A methodology is proposed for the quantification of volcanic explosions based on three parameters derived from broad-band seismic signals: the counter force of the eruption F , the power of the explosion P and the duration of the upward movement of the gas slug in the conduit to the free surface of magma, D . This methodology was applied to the 2004–2005 sequence of explosions at Volcán de Colima, Mexico. The broad-band records of more than 100 explosive events were obtained at a distance of 4 km from the crater. We determined the counter force of the eruption by modelling the low-frequency impulse of the seismic records of 66 volcanic explosions and estimated the power of 116 explosions from the spectra of the high-frequency impulse. The power of Colima explosions spans five orders of magnitude; the counter force spans four orders of magnitude. We show that the power of a volcanic explosion is proportional to the counter force of the eruption. These parameters may be used for the elaboration of a scale of volcanic explosions.     

Radiocaesium dispersion and fixation in the lagoon systems of “Ria de Aveiro”, Portugal

Radiocaesium (¹³⁷Cs) dispersion and Cs⁺ fixation were studied in the sediments collected from the lagoon systems of “Ria de Aveiro”. The Cs⁺ sorption was tested for the fine mica grains and for the < 2 μm clay fractions extracted from silty clays. The Cs⁺ exchange is found strongly onto mica-rich fractions than smectite-rich fractions. The distribution coefficient increases if the silty material is constituted by rich-mica clay fractions or if the non-clay minerals are removed from the silty-clay material. The samples studied behave as multisite ion exchange, where Cs⁺ engages in ion-exchange reactions with hydrated cations on planar sites on expansible layer silicates. Higher concentrations of the ¹³⁷Cs were found associated with mica-rich silty clays. The ¹³⁷Cs ranges from 3.2 to 3.9 Bq kg^− 1 in the < 38 μm fractions and from 2.9 to 3.3 Bq kg^− 1 in the < 64 μm fractions.     

New data on the palaeobiogeography of Loftusia genus (Foraminiferida). An in situ presence of the genus in eastern Greece (Boeotia)

An Upper Maastrichtian horizon rich in Loftusia is for the first time described in situ in Greece in the province of Boeotia. It is found in a continuous undisturbed carbonate sequence of the eastern Greece platform (Subpelagonian zone) of Maastrichtian–Paleocene age followed by flysch sedimentation. Loftusia is found in a facies reflecting an outer shelf environment, associated with debris of rudists, Orbitoides spp., Siderolites calcitrapoides, Omphalocyclus macroporus, Hellenocyclina beotica, Sulcoperculina sp., and echinoderms. Similar fauna, but without Loftusia, is found in the surrounding levels of Late Maastrichtian age as well. This recovery of the genus in the western part of its distribution area, where it is not as abundant as in the eastern part, is considered significant for the palaeobiogeography of the genus in Tethys Ocean during Late Cretaceous. To cite this article: A. Zambetakis-Lekkas, A. Kemeridou, C. R. Geoscience 338 (2006).     

Les auditeurs polonais des cours de minéralogie de René Just Haüy au Muséum national d'Histoire naturelle,Paris

The paper analyses a list of thirty-four Poles, listeners to the mineralogy lectures given by R.-J. Haüy at MNHN in Paris. These students played an important role in the history of Polish Earth Sciences (S. Staszic and F. Drzewiński), but also in other scientific fields (J. Markowski, I. Ab?amowicz, F. Drzewiński, M.A. Paw?owicz, J.K. Skrodzki, E.K. Nowicki), and even in political and cultural life of Poland (A. Downarowicz, J. Weyssenhoff, S. Plater, J. Bieliński, F. Potocki). This paper presents later relations between R.-J. Haüy and his Polish students. A possibility of the Poles' attendance to mineralogy lectures given at other Parisian scientific institutions, like the ‘École des mines’ and the ‘College de France’, is also discussed. To cite this article: R. Tarkowski, P. Daszkiewicz, C. R. Geoscience 338 (2006).     

New Applications of the Model of de Wijs in Regional Geochemistry

This paper is concerned with the lognormal, and its logbinomial approximation, in connection with a three-parameter version of the model of de Wijs. The three parameters are: overall average element concentration value (ξ), dispersion index (d), and apparent number of subdivisions of the environment (N). Multifractal theory produces new methods for estimating the parameters of this model. In practical applications, the frequency distribution of element concentration values for small rock samples is related to self-similar spatial variability patterns of the element in large regions or segments of the Earth's crust. The approach is illustrated by application to spatial variability of gold and arsenic in glacial till samples from southern Saskatchewan. It is shown that for these two elements the model of de Wijs is satisfied on a regional scale but degree of dispersion decreases rapidly toward the local, sample-size scale. Thus the apparent number of subdivisions (N) is considerably less than would be expected if degree of dispersion were to extend from regional to local scale. A random-cut variant of the model of de Wijs produces an empirical frequency distribution of relative element concentration values that can be related to random dispersion index     

Volcanic hazards zonation of the Nevado de Toluca Volcano,Central Mexico

Nevado de Toluca Volcano (NTV), located in central Mexico, is a large stratovolcano, with an explosive history. The area is one of the most important developing centers (>2 millions) in Mexico and in the last 30 yrs large population growth and expansion have increased the potential risk in case of a reactivation of the volcano. As part of a study to assess volcanic risk, this paper presents the results of the volcanic hazard analysis for the NTV. A total of 150 stratigraphic sections were made in the field and three new ages were obtained. Eruptions from NTV produced a complex sequence of pyroclastic deposits that have affected the area at least 18 times during the last 100,000 yrs. Eight vulcanian, four plinian and one-ultraplinian eruptions as well as the destruction of at least three domes occurred in the last 42,000 yr BP as well as two sector collapses in the last 100,000 yrs. Isopach and isopleth maps for the main ulraplinian eruption were also made. The original cone height (5,080 m.a.s.l) was reconstructed through geomorphologic methods. The maximum distance calculated with the energy line for the block and ash flows was 41 km, 35 km for pumice flows and 45 km for debris avalanches. The dominant wind direction at altitudes of 20–30 km is to the east-northeast from November to March, west-northwest in April and west from May to October. Five hazards maps (block and ash flows, pumice flows, ash fall, debris avalanches, and lahars) were made for the NTV. The pyroclastic flows and lahars represent very high to medium hazard for Toluca, Villa Guerrero, Coatepec, Tianguistengo, Metepec, Tenango, Lerma and Zinacantepec. A new debris avalanche would probably affect the south and northeast because of active faulting (E–W and NW–SE) and existing topographic differences in height.