Monte guardia sequence: a late-pleistocene eruptive cycle on Lipari (Italy)

This paper documents a complex sequence of interbedded lapilli-fall, base-surge, and pyroclastic-flow deposits, here named the Monte Guardia sequence, that erupted from volcanic centers in the southern part of Lipari (Aeolian Island Arc). Radiocarbon data from ash-flow tuffs above and below this sequence bracket its eruption between 22,600 and 16,800 years ago. Geologic evidence, however, suggests that this single eruptive cycle had a more restricted duration of years to tens-of-years. The basis for our interpretations comes from data measured at 38 detailed sections located throughout the island. The Monte Guardia sequence rests on a series of lower rhyolitic endogenous domes in the southern part of Lipari and it covers the oldest lavas, lahars, and pyroclastic flows in the north. Only in the northeast part of the island is it covered by younger deposits which there consist of lapilli tuffs and lavas of the Monte Pilato rhyolitic cycle. The deposit ranges in thickness from more than 60 m surrounding the vents in the south to less than a few decimeters at 10 km distance in the north. Throughout most of the island the Monte Guardia sequence overlies a thin andesitic lapilli-fall layer which is a key bed for correlation. This lapilli tuff probably erupted from a volcanic center on another island of the Aeolian Arc (possibly Salina). The principal activity of the Monte Guardia sequence started with an explosion that formed a continuous breccia blanket covering most of the island. Some pumiceous blocks within this breccia are composed of alternating bands of acidic and andesitic composition suggesting that the initiation of pyroclastic activity could have been triggered by magma mixing. Typical Monte Guardia sequence consists of explosive products that grade from magmatic (pumice-fall) to phreatomagmatic (base-surge) character. The eruptive cycle is characterized by a number of energy decreasing megarhythms that start with a lapilli-fall bed and end with a base-surge set that progresses through sand-wave, massive, and planar beds. Isopach maps of the fall and surge deposits indicate that both types were directed to the northwest by prevailing winds. Existing topographic relief was an additional factor that affected the emplacement of surge products. At the end of the cycle andesitic pyroclastic flows and rhyolitic endogenous domes were emplaced above the Monte Guardia deposits near the vent.     

Geology and volcanology of the Edd-Bahar Assoli area (Ethiopia)

The paper presents geological and petrological data on one of the alkaline ranges developed along the borders of the Afar depression (Ethiopia). These alkaline ranges occur in a position transversal to the dominant NNW trend of the spreading zones of northern and central Afar which are characterized by magmas of tholeiitic affinity. The Edd-Bahar Assoli volcanic range consists of broad fields of basic lavas and numerous spatter cones outcropping in the area extending between 13°25′ and 13°75′ lat. N and 41°38′ and 42°15′ long. E. The mineralogical assemblage and the chemical data point to an alkaline nature for this range consisting mainly of alkali olivine basalts and basalts tending to hawaiites, the most evolved terms being largely subordinate. Petrologic differences between the Assab, Edd-Bahar Assoli and Erta Ale ranges are shown. The Edd-Bahar Assoli alkaline volcanism would be related to tectonic patterns trending both from NNW-SSE to N-S and from NE-SW to E-W. The supposed similarity with the transverse structure of the equatorial Atlantic ocean would thus not completely be ascertained in this zone. In Afar, the coexistence of an axial volcanism of tholeiitic affinity with an alkaline volcanism at the margin can better be explained by models based upon the upper mantle temperature distribution in a zone under oceanization.     

The abundance of barium in stony meteorites

The concentration of Ba in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites and 1 stony-iron meteorite has been determined by the stable isotope dilution technique using solid source mass spectrometry. Analysis of the C1 chondrite Orgueil indicates a small adjustment of the “cosmic” abundance of Ba to 4.2 on the Si=10⁶ abundance scale. The present work provides a more complete coverage of a number of meteorite classes than has so far been available for the abundance of Ba in stony meteorites.     

Clinopyroxenes and amphiboles in a metadolerite from the northern apennines. Implications on the palaeogeographic role of ophiolites

Summary A metadolerite of the ophiolites of the Northern Apennines (Vignola, Panaro Valley, near Modena, Italy) behaves as ocean-floor tholeiites when Ti, P, Zr, Cr, Y and Nb are considered. However, it deviates from typical oceanic tholeiites in that(1) it contains primary alkali amphiboles, (2) its clinopyroxenes plot in the diagram TiO₂–MnO–Na₂O not in the fields of the ocean-floor basalts, and (3) the calculated uranium content of the liquid in equilibrium with its magmatic minerals is similar to the one reported for alkali basalts.These results put constraints on the generalized significance attributed to the ophiolites of the Northern Apennines, i. e. not all of them were generated at spreading ridges similar to the normal ridge segments of the present-day ocean-floors, as has been generally accepted by now.The metamorphic parageneses indicate that the rock underwent low-pressure transformations, probably in oceanic environment, followed by recrystallizations at higher pressure, reasonably in concomitance with an orogenesis.

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Klinopyroxene und Amphibole in einem Metadolerit aus dem Nördlichen Apennin. Folgerungen für die paläogeographische Stellung der Ophiolithe

Zusammenfassung Ein Metadolerit aus den Ophiolithen des Nördlichen Apennins (Vignola im Panaro-Tal bei Modena, Italien) entspricht in den Gehalten an Ti, P, Zr, Cr, Y und Nb den Tholeiiten der Ozeanböden. Er weicht hingegen von typischen ozeanischen Tholeiiten dadurch ab, daß er 1. primäre Alkaliamphibole enthält, daß 2. seine Klinopyroxene im Diagramm TiO₂–MnO–Na₂O nicht in die Felder der Tholeiite der Ozeanböden fallen und daß 3, der berechnete U-Gehalt für die mit seinen magmatischen Mineralen im Gleichgewicht stehenden liquiden Phase ähnlich jener ist, die für Alkalibasalte angegeben wird.Diese Ergebnisse schränken die generelle Bedeutung, die den Ophiolithen des Nördlichen Apennins bisher allgemein zugeschrieben wurde, ein: Nicht alle entstanden an sich ausdehnenden Rücken ähnlich jenen der heutigen Ozeanböden.Die metamorphen Parageneses zeigen, daß das Gestein bei niedrigem Druck, wahrscheinlich in ozeanischer Umgebung, Umwandlungen erlitt; dann folgten Rekristallisationen unter höherem Druck, die wohl zeitlich mit einer Orogenese zusammenfielen.

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With 4 Figures     

A note on the approximate determination of the dispersion of Love waves in a laterally nonhomogeneous layer lying over a homogeneous half-space

Rayleigh's principle and the concept of the local wave number have been utilised for the approximate determination of the dispersion of Love waves propagating in a laterally heterogeneous layer lying over a homogeneous half-space. The shear wave velocity and the rigidity in the surface layer have been assumed to decrease with the increase of the lateral distance from the origin. The range of validity of the dispersion equation obtained by this method has been examined critically. It was found that: (a) for existence of Love waves the minimum value of shear wave velocity in the layer must be less than that in the matter below, and (b) the phase velocity of Love waves decreases with the increase of the lateral distance from the origin.     

Faecal pollution of Belgian coastal water

In 1978 faecal pollution of Belgian coastal waters was investigated. The degree of pollution is maximal on the coastline and decreases quickly offshore. At 5 km from the coastline, pollution indicators are still detectable. No significant difference was seen between the values of surface and deep-sea samples.     

A Study of Ozone Laminae Using Diabatic Trajectories, Contour Advection and Photochemical Trajectory Model Simulations.

In this paper, we show that the rate of ozone loss in both polar and mid-latitudes, derived from ozonesonde and satellite data, has almost the same vertical distribution (although opposite sense) to that of ozone laminae abundance. Ozone laminae appear in the lower stratosphere soon after the polar vortex is established in autumn, increase in number throughout the winter and reach a maximum abundance in late winter or spring. We indicate a possible coupling between mid-winter, sudden stratospheric warmings (when the vortex is weakened or disrupted) and the abundance of ozone laminae using a 23-year record of ozonesonde data from the World Ozone Data Center in Canada combined with monthly-mean January polar temperatures at 30 hPa.Results are presented from an experiment conducted during the winter of 1994/95, in phase II of the Second European Stratospheric And Mid-latitude Experiment (SESAME), in which 93 ozone-enhanced laminae of polar origin observed by ozonesondes at different time and locations are linked by diabatic trajectories, enabling them to be probed twice or more. It is shown that, in general, ozone concentrations inside laminae fall progressively with time, mixing irreversibly with mid-latitude air on time-scales of a few weeks. A particular set of laminae which advected across Europe during mid February 1995 are examined in detail. These laminae were observed almost simultaneously at seven ozonesonde stations, providing information on their spatial scales. The development of these laminae has been modelled using the Contour Advection algorithm of Norton (1994), adding support to the concept that many laminae are extrusions of vortex air. Finally, a photochemical trajectory model is used to show that, if the air in the laminae is chemically activated, it will impact on mid-latitude ozone concentrations. An estimate is made of the potential number of ozone molecules lost each winter via this mechanism.     

Analysis of Astex-Stratocumulus Observational Data Using a Mass-Flux Approach

A mass-flux approach is applied to observational data obtained in a convective boundary layer topped with stratocumulus clouds. The observational data were obtained from aircraft measurements during the Atlantic Stratocumulus Transition Experiment (ASTEX). A conditional sampling method is used to calculate average updraft and downdraft values. The vertical fluxes calculated with the mass-flux approach are found to be proportional to the real (measured) fluxes, with a proportionality factor being about 0.6. This value is predicted by theory for two variables having a joint Gaussian distribution function; proportionality factor = 2^-1 0.637. The horizontal fractional entrainment and detrainment rates calculated from the data ( 1–2 × 10^-2 m^-1) are an order of magnitude higher than the rates obtained by large eddy simulations for cumulus convection ( 2–3 × 10^-3 m^-1) and two orders of magnitude higher than those used in modelling cumulus convection with a mass-flux scheme in an operational weather forecast model ( 3 × 10^-4 m^-1). A numerical mass-flux model for the thermodynamics was developed and showed that results are in good agreement when compared with measured profiles of the liquid water content.