Depositional mechanics of turbulent nuées ardentes (surges) from their grain sizes

The ability of turbulent nuées ardentes (surges) to transport coarse pyroclasts has been questioned on the basis that settling velocities of coarse fragments in the deposits are much too high for them to have been supported by turbulence in a dilute gas suspension. A computer model is used to evaluate the settling velocity of pyroclasts in suspensions of varying concentration and temperature. Since suspension of grains in low-concentration surges occurs if the shear velocity exceeds the settling velocity, the shear velocities related to the 16th and 84th percentiles, and the mean of the grain-size distribution are compared in surge deposits of the Vulsini, with the shear velocity necessary to move the coarsest grain on the bed surface (the Shields criterion). The results show that the settling velocities do not vary significantly in gaseous suspensions having volume concentrations lower than 15%, and that an increase in concentration to 25% is not sufficient to decrease the settling velocity of the coarser fraction, if it represents flow shear velocity. It is shown that the settling velocity of the mean grain size (M _z ) best depicts the shear velocity of a dilute turbulent suspension. Applying the results to the May 1902 paroxysmal nuées ardentes of Mount Pelée shows that the estimated mean velocities are well within the observed velocities, and sufficient to support all the clasts in dilute, turbulent suspensions.     

Crystal field stabilization energies of almandine-pyrope and almandine-spessartine garnets determined by FTIR near infrared measurements

The band positions of three partially overlapping Fe²⁺ spin-allowed transitions located between 4000 and 9000 cm^–1 in almandine-pyrope and almandinespessartine garnets solid solutions were measured using near-infrared (NIR) spectroscopy. The crystal field stabilization energies (CFSE) along both binaries were calculated assuming a splitting of 1100 cm^–1 for the lower orbitals. The CFSE show a slight increase along the almandine-spessartine binary from 3730 to 3810 cm^–1 and a larger increase from 3730 to 3970 cm^–1 for the almandine-pyrope binary. Dodecahedral Fe²⁺-site distortion increases with an increase in spessartine component and decreases with increasing pyrope component, in agreement with average dodecahedral site distortions determined from diffraction experiments. The excess CFSE's along both joins are negative. For the almandinespessartine binary they are small, but are about 3.5 times larger in magnitude along the join almandine-pyrope, where an interaction parameter of W= -2.9 KJ/mole has been derived from a symmetric mixing model. The excess CFSE are relatively small compared to the magnitudes of the excess enthalpies of mixing that have been assigned to garnet solid solutions. Moreover, they give no indication which could support the positive and asymmetric excess enthalpies of mixing that have been proposed for almandine-pyrope solid solutions.     

Petrology of the mafic rocks of the Xigaze ophiolite,Tibet

The Xigaze ophiolite (Yarlung-Zangbo suture zone, Southern Tibet, China) shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros, by dolerites intrusive throughout the whole ophiolite sequence, and by the injection of dolerites in already serpentinized peridotites. The abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge. All the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea-water circulation, for the spreading centre in which they have been formed, in good agreement with a slow-spreading ridge origin.     

Parameterization of meridional energy flux in a one-dimensional climate model

Summary The zonally averaged meridional energy transport is parameterized in terms of the zonally averaged temperature gradient and its radiative equilibrium value. Two climate regimes are identified: radiative equilibrium and isothermal climates. The transport and temperature gradient are intermediate between corresponding quantities of these two climates. The parameterization assumes a linear increase of transport as temperature gradient departs from its radiative equilibrium value. The parameterization is formulated using a one-dimensional climate model. Ice-albedo feedback provides the mechanism for climate changes. The parameterization works well for climates associated with seasonal changes.

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Parameterisierung des meridionalen Energiestromes in einem eindimensionalen Klimamodell

Zusammenfassung Der zonal gemittelte, meridionale Energietransport wird in Abhängigkeit vom zonal gemittelten Temperaturgradienten und seinem Strahlungsgleichgewicht parameterisiert. Zwei Klimaregime können identifiziert werden: Strahlungsgleichgewicht und isothermes Klima. Der Transport und der Temperaturgradient befinden sich zwischen den korrespondierenden Werten dieser zwei Klimate. Die Parameterisierung nimmt eine lineare Zunahme des Transportes mit der Abweichung des Temperaturgradienten vom seinem Strahlungsgleichgewichtswert an. Die Parameterisierung wird unter Verwendung eines eindimensionalen Klimamodelles formuliert. Die Eis-Albedo-Rückkopplung liefert den Mechanismus für Klimadnderungen. Die Parameterisierung funktioniert gut fur Klimate, welche jahreszeitliche Anderungen beinhalten.

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With 4 Figures     

Aspects of the cycle of inorganic nitrogen compounds in the tropical rain forest of the Ivory Coast

A partial balance of mineral N is given for the basins of two coastal rivers in a forest zone in the Ivory Coast. The dry and wet depositions on the basin surfaces is given for particulate matter (NO₃ ^–, NH₄ ⁺). The quantity of mineral N washed away in the rivers is evaluated. The losses from leaching of the soils by rainwater are about 0.33 to 1.0% of the atmospheric depositions for NH₄ ⁺–N and 2.2 to 5.8% for NO₃ ^––N. The yearly atmospheric input of N compounds to the ecosystem, about 1.4 g N m^–2 y^–1, is at least 14% of mineral N formed in the soils and is therefore quite significant.     

Equilibrium state of diopside-bearing harzburgites from ophiolites: Geobarometric and geodynamic implications

The bulk compositions of coexisting enstatite and diopside in basal lherzolites and clinopyroxene-bearing harzburgites from ophiolitic complexes are typical of solidus/subsolidus equilibria, but for a few texturally distinct magmatic diopsides. They would presumably reflect the state of equilibrium at the time they last coexisted with liquid as the rocks reentered subsolidus conditions. The total lack of correlation between Al and Ca concentrations shows that the compositional scatter observed for any given massif, results from analytical errors related to extensive exsolution and serpentinization, rather than from differences in equilibrium conditions. However, significant differences are found between the residual ophiolitic lherzolites from Hare Bay, Newfoundland, and from Xigaze, Tibet, two massifs selected for their distinct structural and textural features. As for thermobarometry techniques relevant to these rocks, the best barometer found is an empirical relation for the expression of pressure as a virtually temperature-independent function of the ratioK _f=(X _Di ^opx )/(1 –X _Di ^cpx ), in agreement with semi-quantitative models based on natural solid solutions. Temperatures are then simply derived from a surface-fitting expression relating pressure, temperature and diopside-solvus compositions, according to a regularX _En ^cpx solution model (CMS) corrected for the effect of Al in the spinel facies. Application of these techniques yield pressures of 0.4 and 1.4 GPa, i.e. depths from sea-bottom of about 13 and 43 km, for temperatures of 1,170 and 1,300° C for the ophiolitic lherzolites of Tibet and New-foundland, respectively, in good agreement with dry-solidus data by radioactive tracing and with geothermal-model estimates for ridges.     

Photo-interprétation à différentes échelles et relevés de terrain: exemples de corrélations structurales dans le Bassin Méditerranéen et sur le socle Ouest africain

Résumé L'interprétation structurale des linéaments, décelés sur clichés photographiques pris par satellite, est discutée sur les bases suivantes: d'une part en pratiquant au minimum un changement d'échelle (plus grande); d'autre part en utilisant les relevés de terrain. Des exemples pris dans le Bassin Méditerranéen sédimentaire et sur le socle cristallophyllien de l'Afrique de l'Ouest permettent de définir une méthodologie commune. Ainsi en région sédimentaire, partant de prises de vue par exemple au 1/2 500 000, les échelles au 1/15 000 et au 1/50 000 se révèlent être des compléments précieux en photo-interprétation. De la même façon en zone de socle, les échelles au 1/20 000 et au 1/50 000 sont complémentaires du 1/200 000. De plus grâce aux donnés de terrain, il est possible de qualifier l'information photogéologique et d'en exprimer sa nature. Enfin les problèmes de corrélations entre rhegmatisme de socle et fracturation de couverture sont abordés de façon préliminaire: la similitude directionnelle observée devrait orienter les futures recherches en ce domaine.

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The structural interpretation of disclosed lineaments on satellite's pictures ist discused on the following basis: first, with a minimum scale change (making it bigger); on the other hand using field plotting. The examples took from the Mediterranean sedimentary basin and from the western Africa crystalographic shield allow us to define a common methodology. Thus, in sedimentary area, through 1/2 500 000 scale aerial photographys, the 1/15 000 and 1/50 000 scales would be real valuable complements in photo-interpretation. Same way as, in shield zone, the 1/20 000 and 1/50 000 scales would be complements of the 1/200 000 one. More than that, because of field plotting utilisation is possible to quality photogeological information and even to explain its nature. In short correlation problems are exposed in a preliminary approach; the observed directional similitude could orientate the future research.

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Zusammenfassung Die strukturelle Deutung von Lineamenten, die aus Satellitenphotos abzulesen sind, wird nach folgenden Voraussetzungen erörtert: zum einen wird zumindest eine Vergrößerung des Maßstabes vorgenommen, zum anderen werden Geländebefunde herangezogen. An Hand von Beispielen aus dem sedimentären Mittelmeerbecken sowie dem Kristallo-phyllitischen Sockel Westafrikas ist es möglich, eine gemeinsame Methodik zu definieren. So erweisen sich im sedimentären Gebiet die Maßstäbe 115 000 und 150 000 als wertvolle Ergänzungen zur Photointerpretation zum Beispiel einer Aufnahme von 12 500 000. Entsprechend ergänzen im Bereich des Sockels die Maßstäbe 120 000 und 150 000 die Aufnahme von 1200 000. Darüber hinaus ist es mit Hilfe von Geländedaten möglich, die photogeologische Information zu qualifizieren und zu beinhalten. Schließlich werden die Probleme der Korrelation zwischen Bruchbildung im Sockel und Brücken im Deckgebirge angesprochen: Die beobachtete Annäherung in der Ausrichtung soll zukünftige Untersuchungen auf diesem Gebiet orientieren.

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, , : , no- , , — . . , 1: 15.000 1:50.000 12.500.000. 1: 200.000 1: 20.000 1: 50.000. . . .

     

Application of the technique to ellipitical markers deformed by pressure-solution

Compression and extension axes are deduced from quartz deformation lamellae in a quartzite and a graywacke folded into an asymetrical syncline. Deformation lamellae fabrics in the two sandstones are distinctly different. In the graywacke, regardless of bedding orientation or position on the fold, compression axes are normal or nearly normal to the axial planar rough cleavage. Extension axes generally lie in the cleavage plane, parallel to dip. In most quartzite samples, compression axes are parallel or subparallel to bedding, at high angles to the fold axis and extension axes are normal to bedding. Two samples from the very base of the formation indicate compression parallel to the fold axis with extension parallel to bedding, at high angles to the fold axis. One of these two shows both patterns. The lamellae fabric geometry in these two samples suggests the presence of a neutral surface in the quartzite. The lamellae-derived compression and extension axes are in good agreement with the buckling behavior of a viscous layer (quartzite) embedded in a less viscous medium (graywacke and shale below and shale and carbonate above).