Changement de faunes de bryozoaires dans le Valanginien supérieur des Alpes-de-Haute-Provence. Parallélisme avec la crise observée dans le Jura à la même époque

Dans le Valanginien supérieur (Marnes à Toxaster et Grande Lumachelle) des Alpes-de-Haute-Provence, la succession de deux faunes de bryozoaires est observée. Parmi les causes du changement de faune, le remplacement d'un fond vaseux en eau calme (Marnes à Toxaster) par un fond sableux coquillier en eau assez agitée (Grande Lumachelle) est certainement important. Cependant, ces modifications résultent elles-mêmes d'événements plus généraux.Le changement de faune peut être comparé à celui qui intervient dans le Jura au début de la zone à Trinodosum. La faune des Marnes à Toxaster (zone à Verrucosum) montre certaines des espèces caractéristiques de la “faune 1” du Jura et, de plus, les deux mêmes espèces dominantes. Le milieu de vasière des Marnes à Toxaster, opposé à celui de plate-forme carbonatée du Jura, entraîne seulement un appauvrissement spécifique. Quant à la faune de la grande Lumachelle, elle est absolument semblable à la “faune 2” récoltée dans les Marnes à bryozoaires et le Calcaire à Alectryonia du Jura.Ce parallélisme des deux faunes avec celles du Jura, malgré les différences de faciès sédimentaire, montre que le changement de faune, maintenant reconnu sur près de 400 km, résulte d'une même cause principale. Ainsi, l'hypothèse d'un refroidissement que j'ai proposée pour le Jura semble pouvoir être étendue à la Provence.The change of bryozoan fauna in the upper Valanginian of the Alpes-de-Haute-Provence. Parallelism with the crisis observed in the Jura at the same time.In the upper Valanginian (Marnes à Toxaster and Grande Lumachelle) of the Alpes-de-Haute-Provence, the succession of two bryozoan faunas is observed. Among the reasons for the change of fauna, the replacement of a muddy bottom in calm water (Marnes à Toxaster) by a sandy-shelly bottom in rather agitated water (Grande Lumachelle) is certainly important, but these modifications are the result of more general events. The change of fauna could be compared with the one that took place at the beginning of the Trinodosum Zone in the Jura. The fauna of the Marnes à Toxaster (Verrucosum Zone) shows some characteristic species of the “faune 1” of the Jura and, moreover, the same two dominating species. The muddy basin environment of the Marnes à Toxaster, unlike the carbonate platform environment in the Jura, involves only a specific impoverishment. As for the fauna of the Grande Lumachelle, it is totally identical to the “faune 2” found in the Marnes à bryozoaires and the Calcaire à Alectryonia of the Jura.This parallelism of the two faunas with those of the Jura, in spite of the differences of sedimentary facies, shows the change of the fauna now observed over about 400 km, has the same principal cause. Thus it seems possible to extend the cooling hypothesis I have proposed for the Jura, to Provence.     

Continuous monitoring of distal gas emanations at Vulcano,southern Italy

The increasing activity of Vulcano Island (Italy) since 1985 led to the initiation of continuous geochemical monitoring of the lateral soil gas emissions. On the basis both of their relative geochemical characteristics and of local considerations, three gaseous components were selected for monitoring, namely CO₂, He and ²²²Rn. Monitoring has been performed by means of specific analysers. Gases extracted from a water well located at the foot of the active cone were selected for monitoring, on the basis of their geochemical and isotopic characters that indicate their genetic link with central high temperature fumarolic gases emitted at the crater. Very strong variations of gas composition can be observed within one day (from 1 to about 94% for CO₂). Some variations display a daily character and can be correlated with that of atmospheric pressure. The three monitored gases are highly correlated, suggesting very high kinetics of gas transfer in the system. Because of these considerable variations of chemical composition, bulk concentrations obviously are not suitable for monitoring at Vulcano. However, the evolution with time of ratios such as ²²²Rn/CO₂ and He/CO₂ (the latter being corrected for atmospheric contamination) supplies numerical parameters that the expected to characterize the intensity of the degassing process. A new input of magmatic gases, that would lead to an increase in the ²²²Rn/CO₂ and He/CO₂ ratios, should therefore be detected by such a monitoring station.     

Coarse cliff-top aeolian sedimentation in northern Gaspésie,Québec (Canada)

This paper deals with the effects of wind on rockwall dynamics. On 5 and 6 January, 1988 very strong northwest winds (blizzard) were blowing onto the rockwall of Mount Saint-Pierre (alt.: 424 m), Gaspésie, Québec (Canada). The most violent recorded squall reached 99·4 km h^?1. During this event, the summit plateau received a large amount of aeolian sediments originating from the shale rockwall that forms the mountain's northwest side. In the 15 to 20 m wide by 75 m long belt located along the top of the rockwall, over a 1200 m² area, a continuous layer of debris completely covered the snow. This layer of debris had a mean thickness of 11·4 mm, which represents an accumulated volume in the order of 13 m³. Largely dominated by sand and granules (2–4 mm), the 28 samples collected for grain-size analysis also contained numerous thin shale flakes, many of which were longer than 10 mm. The largest flake measured had a width of 134 mm and a weight of 164·3 g. Six available ¹⁴C dates provide information concerning mean cliff-top aeolian sedimentation rate for the last thousand years (c. 1·8 mmyr^?1).     

Composite magnetic anisotropy fabrics: experiments, numerical models and implications for the quantification of rock fabrics

Magnetic fabrics from rocks with multiple mineral-preferred orientations may have anisotropy ellipsoids whose shape and orientation arise from the addition of two or more component fabrics. Our numerical models and experiments demonstrate that such composite magnetic fabrics do not directly reflect the shapes and/or orientations of the individual mineral fabrics and we provide criteria for the recognition and interpretation of composite fabrics in natural rocks. These criteria include:

1. (1) the orientation of the maximum susceptibility axis is located at the intersection of two planar fabrics, and

2. (2) the shape of the susceptibility ellipsoid changes from oblate to prolate and the degree of anisotropy decreases, as the relative intensity of two planar component fabrics becomes equal and as the angle between the planar fabrics increases.

Composite magnetic fabrics are observed in the shales and slates of the Martinsburg Formation, Lehigh Gap, Pennsylvania. Modeling of the AMS (anisotropy of magnetic susceptibility) and ARMA (anhysteretic remanent magnetization anisotropy) behavior constrains the relative degree of anisotropy of the bedding-parallel and cleavage-parallel fabrics. In particular, ARMA model results allow a good estimate of magnetite fabric strength.

We conclude that, in the presence of composite magnetic fabrics, quantitative measures of finite strain in deformed rocks are limited by the ability to accurately determine the degree of anisotropy and relative susceptibility of each component fabric. Such determinations require knowledge of the mineral(s) that are responsible for the measured magnetic fabric and their behavior during deformation.     

Magnetite activities across the MgAl2O4-Fe3O4 spinel join,with application to thermobarometric estimates of upper mantle oxygen fugacity

The activity of Fe₃O₄ component in MgAl₂O₄-Fe₃O₄ spinels has been measured at 900° and 1000° C and 1 atm total pressure using a zirconia oxygen electrolyte. As previously reported for the dilute Fe₃O₄ concentration region (Mattioli and Wood 1986a), magnetite activity at 1000° C is greater than at 900° C at constant Fe₃O₄ mole fraction, for compositions across the MgAl₂O₄-Fe₃O₄ join between 20 and 80 mol% Fe₃O₄ component. The 1-atm solvus crest lies between 900° and 1000° C and, at 900° C the limbs are at Fe₃O₄ mole fractions of 0.2 and 0.6 approximately.Application of the O'Neill and Navrotsky (1983, 1984) cation distribution model indicates that the unusual activity — composition behavior of Fe₃O₄ is caused by changes in the equilibrium state of disorder of mixed MgAl₂O₄-Fe₃O₄ spinels relative to the disordered Fe₃O₄ standard state. In addition, both stoichiometric volumes (Mattioli et al. 1987) and activities across the MgAl₂O₄-Fe₃O₄ join suggest that short range order is significant for this binary. Excess free energy terms must be added to ideal Fe₃O₄ activities formulated from equilibrium cation distributions in complex MgAl₂O₄-Fe₃O₄ spinels in order to increase Fe₃O₄ activities to values consistent with observation and to generate the apparent region of immiscibility at 900° C.We have applied our activity data to the estimation of upper mantle spinel-lherzolite oxygen fugacities. We calculated that minimum 's are about 2 log units below the synthetic QFM buffer at 15 kbar total pressure for Fe₃O₄ concentration of 2 mol%, in a Cr-free spinel phase. If a preliminary calibration of an additional 25 mol% Fe²⁺-substitution as FeCr₂O₄ or FeAl₂O₄ component is incorporated into Fe₃O₄ activity, then olivine-orthopyroxene-spinel assemblages of depleted-Type 1-spinel-lherzolite xenoliths indicate 's close to QFM at 15 kbar. This is in good agreement with previous thermobarometric estimates and in sharp contrast to 1 atm intrinsic measurements near IW.     

Aperçu sur les amas sulfurés massifs des hercynides Marocaines

Massive sulfide deposits located on Hercynian islets of northwestern Morocco exhibit four main characteristics:

They are strata-bound massive pyrrhotite deposits mined for sulfur and/or base metals occasionally occurring as sulfides of workable grade.

Volcanic rocks with which these massive sulfide orebodies are associated are scarce, although always present as acid flows of submarine emissions of either rhyolitic or more often quartz-keratophyric nature. Later on, basic plutonics intruded the pelitic country rocks.

Stockworks underlying the massive sulfide orebodies are common, but not systematic. When present, they occur in siltite ± phyllite ± carbonate rocks at the wall of massive sulfide lenses. They consist of fissural disseminations transformed by epimetamorphic recrystallization and by one schistosity generally concordant with s₀.

Associated alterites and exhalites belong to three types, i.e., sericitite (or biotite-rich rock), chloritite, and/or chert (jasper).

Generally well located in a back-arc basin environment characterized by a two-phase geological history, i.e., “extension and volcanism, compression and metamorphism”, these volcano-sedimentary deposits exhibit distal features with regard to the volcanism coeval with their sedimentation. They are mostly linked with strongly reducing environmental properties entailing pyrrhotite and/or magnetite syngenetic deposition, whatever the iron activity.     

On the validity of the chapman-enskog description of the solar wind

The validity of the Chapman-Enskog method in the calculation of the heat conductivity of the solar wind is studied. The predictions of the Chapman-Enskog theory are compared with known results of rarefied gas kinetic theory. The results suggest that the use of the Chapman-Enskog theory to describe the transport processes in the solar wind is not strictly justified.