Sur une typologie des sulfures sédimentaires d'après le rapport 32S/34S

Résumé L'auteur développe une théorie explicative du rapport ³²S/³⁴S basée sur les notions d'obtention ou non obtention de l'équilibre isotopique et de milieu ouvert ou fermé qui permet d'expliquer aussi bien les mesures faites sur les gisements de sulfures que lex expériences de laboratoire.

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The author develops a theory of ³²S/³⁴S ratio, based on notions of isotopic quilibrium obtainment or non obtainment, and of open or closed system, permitting to explain both measurements made on sulfur ore desposits and laboratory experiments. From this theory he deduces a typology of sedimentary sulphide deposits.

     

Nitrogen compound emission from biomass burning in tropical African savanna FOS/DECAFE 1991 experiment (Lamto,Ivory Coast)

Gaseous nitrogen compounds (NO _x , NO _y , NH₃, N₂O) were measured at ground level in smoke plumes of prescribed savanna fires in Lamto, in the southern Ivory Coast, during the FOS/DECAFE experiment in January 1991. During the flaming phase, the linear regression between [NO _x ] and [CO₂] (differences in concentration between smoke plumes and atmosheric background) results volumic emission ratio [NO _x ]/[CO₂]=1.37×10^–3 with only slight differences between heading and backing fires. Nearly 90% of the nitrogen oxides are emitted as NO. Average emission ratios of other compounds are: 1.91, 0.047, and 0.145×10^–3 for NO _y , NH₃ and N₂O, respectively. The emission ratios obtained during this field experiment are compred with corresponding values measured during former experiments with the same plant species in combustion chambers. An accurate determination of both the biomass actually burned and of the plant nitrogen content, allows an assessment of emission fluxes of N-compounds from Guinean savanna burns. Preliminary results dealing with the influence of fire on biogenic emissions from soils are also reported.     

Ammonite-based correlations in the Cenomanian-lower Turonian of north-west Europe, central Tunisia and the Western Interior (North America)

The biochronology of Cenomanian-early Turonian ammonite faunas from three key stratotype areas (north-west Europe, central Tunisia and the Western Interior of North America) has been analysed and revised by utilizing the unitary association method. This review is prompted by the huge amount of biostratigraphic data published during recent decades and by a taxonomic homogenisation of the ammonite faunas from these key areas. The Cenomanian and lower Turonian of Tunisia comprise twenty-four Unitary Association zones and the middle Cenomanian-lower Turonian of the Western Interior Basin twenty-three such zones. The unitary association method means a two-fold increase in resolution of these ammonite zonations compared to the standard, empirical schemes. Central Tunisia and the Western Interior are correlated with north-west Europe by constructing a zonation including all taxa common to these areas. These correlations highlight the variable completeness and resolution of the faunal record through space and time, and reveal a significant number of diachronous taxa between the three areas. These correlations enable the designation of a new global marker for the middle/upper Cenomanian boundary, which is characterised by the disappearance of the genera Turrilites, Acanthoceras and Cunningtoniceras and by the appearance of Eucalycoceras, Pseudocalycoceras and Euomphaloceras. The only synchronous datum known is the last occurrence of Turrilites acutus, which may thus be used as a marker for the middle/upper Cenomanian boundary, provided that it does not turn out to be diachronous in the light of any new data.     

The effect of mineralogical and cationic heterogeneities on rheological properties of suspensions with Li-smectites, extracted from Volclay MX-80

The rheological study of colloidal suspensions contributes to the understanding of the aggregation phenomena of colloidal particles within a suspended fluid. The infra-2-μm (equivalent diameter) fraction of a bulk material, Volclay MX-80 bentonite was intensively analysed in order to understand the relative rheological data scattering observed with Li-smectite suspensions, prepared with the usual saturation procedure. A good mineralogical characterisation, especially for secondary minerals, coupled to a verification of cation exchanges on clayey fraction allowed us to propose a new way for investigating the observed differences of the bentonites rheological behaviour. Indeed, for a well Li-saturated smectite obtained from the purified infra-2-μm fraction, rheological data proved to be very accurate and perfectly reproducible. This emphasises the role of other minerals as well as the commonly observed discrepancy between the rheological behaviours of natural and industrial bentonites.     

Experimental investigation of large-ion-lithophile-element-, high-field-strength-element- and rare-earth-element-partitioning between calcic amphibole and basaltic melt: the effects of pressure and oxygen fugacity

The effects of pressure and oxygen fugacity (fO₂) on trace element partitioning between pargasitic amphibole and alkali-basaltic melts have been determined at pressures from 1.5 to 2.5 GPa and oxygen fugacities at 2 log units above and below the nickel–nickel oxide buffer. Amphibole crystallization experiments were performed in a piston cylinder apparatus and partition coefficients between amphibole and quenched melt of large-ion-lithophile elements (LILE: Rb, Sr, Ba), high-field-strength elements (HFSE: Zr, Nb, Ta, Hf, U, Th) and rare-earth elements (REE: La to Lu; +Y) were measured with a LASER ablation inductively coupled plasma – mass spectrometer. Increasing pressure from 1.5 to 2.5 GPa at similar temperatures and approximately constant fO₂ increases D _Rb but decreases D _Zr and D _Hf and D _REE (D _La, D _Ce, D _Pr). An empirical relationship was observed between D _Zr and (Ti/Al)_M2 in the amphibole, which can be described by:

Numerical modelling of hydraulic decompression due to the excavation of tunnel and drifts at the Tournemire underground laboratory

The Tournemire underground laboratory is situated in a clay formation and consists of a disaffected railway tunnel and two perpendicular drifts. The paper presents modelling and 3D simulations of the hydraulic behaviour of the argillaceous formation around the works. Experimental measurements of porosity, permeability and specific storage coefficients allowed us to model the hydraulic properties of the shale. Numerical simulations are performed with the code CASTEM. Numerical results of hydraulic heads are compared to experimental measurements. Calculations show that the excavation of the tunnel and drifts has induced an hydraulic decompression of the indurated argillaceous formations. The zones of decompression are centred around the structures and extend depending on the values of hydraulic parameters such as permeability and specific storage selected in the model. Six years after the excavation of the drifts, the hydraulic steady state in the fractured zone is not yet reached.     

Helium isotopes on the Macdonald seamount (Austral chain): constraints on the origin of the superswell

We present new helium isotope data from the Macdonald seamount (Austral chain). The helium isotopic ratio varies from ⁴He/³He=45 000 (R/Ra=16.0) to 200 170 (R/Ra=3.6). The helium content is between 1.5×10^?8 and 1.1×10^?5 ccSTP/g. These helium results show clearly the presence of primitive mantle material in the source of the Austral chain. Macdonald has the lowest ⁴He/³He ratio among the Polynesian submarine volcanoes, except Hawaii (Loihi). The simplest explanation for the primitive helium signature is the presence under Macdonald of a mantle plume that derives either from the 670 km or 2900 km boundary layers, or, eventually, from the top of a large mantle dome resulting from a stratified two-layer convection. This plume contains less-degassed material with low ⁴He/³He ratio. To cite this article: M. Moreira, C. Allègre, C. R. Geoscience 336 (2004).     

Eaux fortement minéralisées et circulations profondes dans le socle. Exemple des Alpes franco-italiennes

The presence in the French–Italian Alps of several springs characterized by a high sodium–chloride content, depleted in oxygen-18, tritium free and with low carbon-14 activity, is well known. The geological, hydrological, and geochemical studies in progress for the new high-speed railway tunnel crossing the French–Italian Alps, show, in the drilled boreholes at a mean elevation of 700 masl, the presence of brines. Brines appear to be the basis of the geological, structural, and hydrodynamic systems, and fill the largest faults and the main tectonic contacts. The brines squeezed by tectonic activity, are mobilised at depth by freshwaters from the surface. To cite this article: G.-M. Zuppi et al., C. R. Geoscience 336 (2004).     

Recent algal stromatolitic deposits,Andros Island,Bahamas. Preliminary report

After some introductory comments on fresh water bluegreen algal calcareous formations, the general features and the ruling mecanisms of which are of fundamental importance for the interpretation of stromatolites, the distribution and general features of algal deposits across Andros Id, Bahama, and adjacent marine platforms are briefly reported.Morphological differenciations characterizing Recent stromatolitic biostromes in process of formation over the flats of seasonal lakes (N-Eastern Andros) are then briefly analyzed. The interpretation of structures relies on the various data gathered during an observation period of three months encompassing the phase of prolific stromatolitic growth during the complete flooding of the lakes, and ending with the progressive drying up and exposure of the algal flat at the beginning of the dry season.

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Zusammenfassung Nach einem einführenden Kommentar über den Bau und die Funktion von Süßwasseralgen unter besonderer Berücksichtigung ihrer Bedeutung beim Aufbau von Stromatolithen wird die Verteilung und Struktur des Cyanophyceen-Wachstums auf der Insel Andros (Bahamas) und den angrenzenden Meeresplattformen dargestellt. Auf den flats periodischer Seen im nordöstlichen Teil von Andros läßt sich die morphologische Differenzierung stromatolithischer Biostrome zeigen. Es wurden in einem dreimonatigen Beobachtungszeitraum die Wachstumsphasen der Stromatolithe erfaßt, das heißt in einer Periode von der vollen Wässerung des Sees bis zur allmählichen Austrocknung am Anfang der trockenen Jahreszeit.

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Résumé Après un commentaire introductif sur les constructions algaires d'eau douce et l'intérêt qu'elles revêtent pour la compréhension des stromatolithes tant par leur organisation que pour les mécanismes qu'elles révèlent, l'auteur rapporte rapidement la distribution et la structure des tapis à Cyanophycées de part et d'autre d'Andros Island, Bahama, et des plateformes marines adjacentes.Il décrit ensuite brièvement les différenciations morphologiques rencontrées au sein de biostromes stromatolithiques en voie de formation sur les flats de lacs saisonniers de la région nord orientale d'Andros Island. L'interprétation des structures observées se base sur les données recueillies au cours d'une période d'observation de trois mois couvrant la phase de pleine croissance des stromatolithes, au moment d'innondation maximale du lac, jusqu'à la phase de dessication progressive à l'approche de la saison sèche.

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Andros (Bahama) . flats Andros Biostrome. , . e. .

     

Partition coefficient of thallium compared with rubidium

Analyses of 31 minerals (biotite, potash feldspar and plagioclase), occurring in 12 acidic volcanic rocks and 5 granodiorites from a Tuscan igneous series, gave the following mineral-groundmass (l) partition coefficients (D-values) for Tl: D^Kf-l, 0.75; D^bio-l, 1.82; D^pl-l, 0.13.Values for Rb (previously published) are similar for D^Kf-l but smaller for D^bio-l. Model calculations for crystallizing acid magma, using obtained D-values, confirm the observed trends of increased abundances of Tl, Rb in residual liquids and indicate and expected increase in Rb/Tl.In acid plutonic rocks, most of the Tl is in biotite; in equivalent lavas it is in the groundmass.