Les minéralisations stratiformes à F-Ba de Hammam Zriba,Jebel Guébli (Tunisie nord orientale): l'apport des études d'inclusions fluides à la modélisation génétique

Résumé Le gîte de H. Zriba-J. Guebli (réserves: 5 millions de tonnes à 15–35% CaF₂) est constitué pour l'essentiel d'amas stratoïdes encaissés dans des formations phosphatifères argileuses et carbonatées, d'âge Campanien inférieur. Quatre formations minéralisées (Mo, Fr, F₂ et F₃) ont été distinguées. La plus ancienne (formation Mo) ne contient pas de fluorine; elle est à dominante quartzeuse ou barytique avec quelques sulfures. Le minerai à fluorine rubanée (formation F₁) est le plus important sous l'angle économique. Trois types d'inclusions fluides ont été observés dans la fluorine: 1) d'abondantes inclusions aqueuses, 2) de rares inclusions gazeuses et 3) de fréquentes inclusions à hydrocarbures liquides jaunes, fluorescentes en éclairage U.V. Les études microthermométriques montrent une augmentation des températures d'homogénéisation (100 à 130°C dans la fluorine F₁, 150–185°C dans la fluorine F₃) avec augmentation parallèle de la salinité des solutions piégées. On propose un modèle de mise en place de la fluorine par voie épigénétique à partir de solutions semblables aux eaux de formation des champs pétrolifères, suivi par des remaniements liés à des circulations hydrothermales récentes.

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In the H. Zriba-J. Guebli mine (5 millions tons of ore with 15%–35% CaF₂) the ore deposits are mostly stratiform bodies hosted by lower-Campanian clay-carbonate sediments with significant phosphate contents. Four successive ore formations are apparently fluorite-free and either quartz-rich or barite-rich with minor amounts of sulfides. The fluorine rubannée (formation F₁) is the main economic ore. Three major fluid inclusions types have been observed in fluorite: (1) abundant aqueous inclusions, (2) rare gas inclusions, and (3) usually yellow oil-bearing inclusions with intense UV fluorescence. In microthermometric investigations on primary aqueous inclusions a general increase in the homogenization temperature from about 100°C to 130°C in fluorite F₁ up to 150°–185°C in fluorite F₃ with a simultaneous increase in salinity was observed. Finally, fluorite crystallized epigenetically in formation-water-type brines; the last geodic filling by fluorite F₃ is tentatively related to Miocene hydrothermal circulation.

     

Petroleum isoprenoid hydrocarbons derived from catagenetic degradation of archaebacterial lipids

The three GC coeluting C₄₀ bis-phytanes with head-to-tail, tail-to-tail and head-to-head linkages considered to be archaebacterial markers, have been recognized in petroleum by enhanced mass-fragmentography. Supporting evidence has been obtained by the simultaneous occurrence of two C₂₁–C₂₄ series of isoprenoid and quasi-isoprenoid hydrocarbons that are supposedly formed during the catagenetic degradation of the isomeric C₄₀ isoprenoids. The C₂₁–C₂₄ quasi-isoprenoid hydrocarbons have been identified conclusively for the first time by comparison with authentic standards.     

Thermal radiation from cloudless skies in Granada

Summary Data on thermal radiation from clear skies obtained at Granada from May to November 1983 were utilized to carry out a detailed comparison between measured and calculated values of longwave radiation with several analytical and empirical equations. The goal of this work is to check the aplicability of these equations to Granada conditions. The results obtained for all the equations are quite satisfactory. Nevertheless, Idso's formula significantly overestimates our experimental results, probably as a consequence of the differences in atmospheric dust conditions between our test area and the location for which that equation was originally derived. A reduction in the independent coefficient, in accordance with previous results of other authors, provides a better fit to our data set and clearly shows the climatological dependence of the constants in the estimation equations.

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Zusammenfassung Die von Mai bis November 1983 in Granada gesammelten Daten über Wärmestrahlung bei klarem Himmel wurden dazu verwendet, um einen detaillierten Vergleich zwischen gemessenen und errechneten Werten der langwelligen Strahlung aus verschiedenen analytischen und empirischen Gleichungen durchzuführen. Das Ziel dieser Arbeit ist es, die Anwendbarkeit dieser Gleichungen auf die Verhältnisse in Granada zu überprüfen. Die gewonnenen Ergebnisse sind für alle diese Gleichungen ganz zufriedenstellend. Idsos Formel überschätzt jedoch deutlich unsere experimentellen Ergebnisse, möglicherweise als Folge der unterschiedlichen atmosphärischen Staubbedingungen in unserem Testgebiet und dem Standort, für den die Gleichung ursprünglich abgeleitet wurde. Eine Reduktion des unabhängigen Koeffizienten, in Übereinstimmung mit früheren Ergebnissen anderer Autoren, bietet eine bessere Anpassung an unseren Datensatz und zeigt deutlich die klimatologische Abhängigkeit der Konstanten in Schätzungsgleichungen.

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With 3 Figures     

The two-step shape and timing of the last deglaciation in Antarctica

The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record.     

Grain-size-sensitive flow and shear-stress enhancement at the brittle–ductile transition of the continental crust

Localized shear zones along low-angle normal faults have been identified in regions of extension at the brittle-ductile transition of the continental crust. The possibility of the strain localizing at a depth of 10 km is interpreted here as a consequence of an increase in the equivalent shear stress applied to the flow of the lower crust. This enhancement of the flow stress is seen as a prerequisite for the triggering of brittle deformation mechanisms leading to strain localization. The lower crust rheology used to examine this stress increase is strain-rate, temperature and grain-size dependent, due to the coupling of dislocation and diffusion creep. The model structure proposed consists of a top layer, the upper crust, gliding rigidly above a bottom layer, the lower crust, which deforms in simple shear. During a short time interval (1400 years), the equivalent shear stress is found to increase by a factor of up to 3 (67 MPa for anorthite and 17 MPa for quartz). For anorthite, this stress could explain the activation of a Mohr-Coulomb failure with a friction coefficient of 0.2, which is reasonable at the depth of 10 km. Dislocation creep is activated during a rapid change in the prescribed velocity, whereas diffusion creep dominates if the velocity is held constant, highlighting the importance of grain-size sensitivity for lower crustal rheology.     

Chemical composition of Luna 20 soil and rock fragments

Abundances of 22 elements, including 9 rare earth elements (REE), have been determined by ‘monostandard’ instrumental neutron activation analysis of samples from the Luna 20 soil and in 6 rock fragments, including a crystalline rock of highland origin, a breccia of similar composition, a glass and a feldspar grain. The soil appears to have been contaminated with W and Mo. The REE content of the soil is very low, being close to 2.3 times below the level in the Luna 16 soil. Sampling errors, for most elements, are negligible in the case of analyses performed on one or several tens of mg of soil, but they become significant on crystalline rock fragments in the 1–2 mg range.     

Flexible Automatic Scheduling for Autonomous Telescopes: The MAJORDOME

We have developped a new method for the scheduling ofastronomical automatic telescopes, in the framework of theautonomous TAROT instrument. The MAJORDOME software canhandle a variety of observations, constrained, periodic,etc., and produces a timeline for the night, which may bemodified at any time to take into account the specificconditions of the night. The MAJORDOME can also handletarget of opportunity observations without delay.     

Voids in the tolman-bondi universe

In the paper the void is modeled as a spherical underdense region surrounded by shells changing into the Friedmannian exterior. This model is in fact a single Tolman-Bondi metric, where at the edge of the void the density need not be continuous. In principle, there may exist 72 variations. These models contain also the cases, when in the void itself there is a Minkowskian vacuum; the shell crossing is not excluded, too. Some technical results are obtained for the Tolman-Bondi metric. Using them, the questions of stability and other theoretical problems are investigated. Some observational facts concerning the voids are also used. As the key result a truncation of the possible models is obtained; only 14 models are physically reasonable. This means that the universe is either hyperbolically expanding (this possibility strengthens the proposition of Bonnor and Chamorro, (1990); (1991)), or there is a shell crossing. Thus the discovery of voids is an observational support either for the open universe or for the shell crossing scenario of galaxy formation (Mészáros, 1991), where no anisotropy of microwave background radiation is needed in a baryon dominated universe.     

On solution of the inverse problem for confined aquifer flow via maximum likelihood

Joint estimation of transmissivity (T) and storativity (S) in a confined aquifer is done via maximum likelihood (ML). The differential equation of groundwater flow is discretized by the finite-element method, leading to equation _t+x _t=u _t. Elements of matrices and , as well as estimated covariance matrix of noise termu _t, are functions of T and S. By minimizing the negative loglikelihood function corresponding to discretized groundwater flow equation with respect to T and S, ML estimators are obtained. The ML approach is found to yield accurate estimates of T and S (within 9 and 10% of their actual values, respectively) and showed quadratic convergence in Newton's search technique. Prediction of aquifer response, using ML estimators, results in estimated piezometric heads accurate to ±0.5 m from their actual, exact values. Statistical properties of ML estimators are derived and some basic results for statistical inference are given.