Geochemistry of basic dikes in the Lanzo massif (Western Alps): Petrogenetic and geodynamic implications

The Alpine peridotite massif of Lanzo (Italy) contains three generations of basic dikes (gabbros and basalts). The older gabbros are plagioclase-rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes. They both were derived from the N-type mid-ocean ridge basalt (MORB) magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir. The basaltic dikes are the N-type MORB and closely resemble the Alpine-Apennine ophiolitic basalts. They were derived from a different upper mantle source than the parental magmas of the gabbros. The source of the basalts was less depleted in light REE. The presence of basic magmas with N-type MORB affinities in the Lanzo massif is consistent with the close genetic relationship between the Alpine peridotite body and the ophiolites of the Liguro-Piemontese basin.     

Geochemistry of the Adamello massif (northern Italy)

The Tertiary Adamello massif, outcropping over an area of more than 550 km² in the southern Alps (northern Italy) is composed mainly of granitoid rocks (granodiorite, tonalite, quartz diorite) with minor amounts of diorite and gabbro. The major and trace element composition of these rocks is comparable to calc-alkaline volcanic rocks of continental margins. The granitoid rocks display spatial and temporal variations in their composition, particularly in Na, P, Sr, La, Nb and Y contents and ⁸⁷Sr/⁸⁶Sr ratios. The variations were probably produced by concurrent contamination/wall-rock assimilation and fractional crystallization of high-alumina basaltic magma.     

Uranium and thorium in paleozoic basalts of Nova Scotia (Canada)

In the Paleozoic basalts of Nova Scotia (Canada) metamorphosed to a greenschist facies grade, U and Th are closely associated with immobile elements (e.g. Zr and Nb). The coherence of these elements with K, typical of igneous rocks is, however, absent, U and Th are apparently not affected by greenschist facies metamorphism as are alkali and alkali earth elements and their variation thus reflects primary magmatic processes. It seems that during the early stages of metamorphism, U and Th were retained together with several other incompatible elements (Zr, Nb, La and Ce) in stable secondary phases or alternatively these elements may be held in primary accessory minerals such as zircon and apatite.     

Difference between the even and odd 11-year cycles of geomagnetic activity

Summary With the aid of 36 monthly and a double 12-month running average, graphs of the smoothed run of geomagnetic activity were constructed from the monthly values of the geomagnetic activity index aa for the period 1868–1978 and they were then used to determine the run of the geomagnetic activity, expressed during a uniform period and devoid of the secular variation of geomagnetic activity, for the individual 11-year cycles Nos 12–20 and the average runs for even and odd cycles, as well as for the whole period of the 9 cycles. The analysis and comparison of the graphs substantiates and renders more accurate the tendency, observed earlier, towards a regular alternation of geomagnetic activity cycles with double and single maxima. Wheareas a saddle-like shape with a maximum in the neighbourhood of the first and third quarter of the cycle is characteristic of the shape of the even cycle, the second maximum being, as a rule, the main one, the odd cycle is characterized by a peak-like shape with the main maximum in the vicinity of the cycle's centre.     

Comparison between Pollutant Concentration in the Samples of Fog and Rime Water Collected at Mt. Milešovka

The article aims at showing the differences in concentration of pollutants that are contained in the samples of fog and/or low cloud water in comparison with the water from rime. The results follow from the fog and rime measurements made at Mt. Mileovka (eské Stedohoí Mountains). They are compared with the results of other studies that also report the differences in fog and rime chemistry.     

Carbonate metasomatism in the lithospheric mantle: peridotitic xenoliths from a melilititic district of the Sahara basin

The petrological and geochemical study of harzburgitic and dunitic xenoliths from the melilititic district of In Teria (Algerian Sahara) shows that the lighospheric mantle of this region has been affected by a multi-stage metasomatism. The first metasomatic event is related to the injection of alkali silicated (basaltic or kimberlitic) melt and was responsible for the crystallization of phlogopite at depths ranging between 80 and 100 km and the crystallization of amphibole at about 60 km. During this first event, carbonate probably precipitated in the garnet stability field. In a second stage, the spinal peridotites suffered strong mineral changes resulting in an extensive formation of high-Cr endiopside and leading to conversion of harzburgite and dunite into lherzolite and wehrlite. These changes are associated with an enrichment in the most incompatible trace elements including light REE (rare-earth elements), Ta, Th and variable values of ratios such as Th/La and Ta/La. This second event is atributed to the injection (under conditions of decarbonatation and release of CO₂) of a carbonatitic melt resulting from incipient melting of the garnet peridotites, which were previously carbonated. This interpretation is corroborated by the calculation of a diffusion-percolation model which reproduces well the observed distribution of incompatible trace elements in the spinel peridotites. Given the proposed sequence of events, it appears that most of the specificities of the In Teria xenoliths can be explained by the successive geochemical modifications induced within the lithospheric mantle during reheating.     

Frequency analysis of geomagnetic beating-type pulsations in thepc3 range

Summary The spectral analysis of samples of beating-type pc3 pulsations, recorded at the Budkov Observatory during the summer months of 1968 and 1969 was carried out in the frequency interval10 mHz100 mHz. The average limits of the pulsation frequency range are roughly20 mHz60 mHz. The centre of the occurrence frequency graphs for both components is at the frequency f_g 41 mHz. The analysis of three separated daily intervals showed a very slight tendency to a systematic decrease in the frequency of the centre from the morning to the afternoon. The study of the sense of rotation of the pulsation disturbance vector in the (X, Y)-plane showed an overall predominancy of L over R-types during daytime, particularly during the morning.     

Determination of the original shape of pulsations from their spectra

Zusammenfassung Es wird eine einfache auf der Symmetrie der Grundformeln der Fourierschen Transformation gegründete Methode, wie von den Spektralfunktionen zur ursprünglichen Pulsationsform zu gelangen, beschrieben. Mit dieser Methode wird die durchschnittliche Form der Pi2-Pulsationen konstruiert. Diese idealisierte Pulsation hat dieselben Eigenschaften wie ein statistisches Ensemble von Pulsationen. Der wirkliche Verlauf der Intensit?t des magnetischen Feldes bei Pi2-Pulsationen wird durch die Methode aus den Induktionsregistrierungen abgeleitet. Die berechnete Kurve zeigt starke Korrelation mit der direkten Registrierung der Intensit?t des magnetischen Feldes.

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Address: Boční II, Praha 4-Spořilov.     

On the problem of frequency calibration of induction variometers with large air-core loops

Zusammenfassung Es wird eine Methode für experimentelle Untersuchung der Frequenzkennlinien von Induktionsvariometern mit Aufnahmeschleifen nichttraditioneller Ausmasse beschrieben. Die Grundlage des beschriebenen Verfahrens bildet eine genaue Bestimmung jenes Teils des magnetischen Kraftflusses, der durch die Aufnahmeschleife durchgeht.