Atmosphärische aerosolstreuung im sichtbaren und nahen infrarot

Zusammenfassung Die atmosphärische Aerosolstreuung wird im Bereich 0.52.1 m und 4°40° experimentell untersucht. Sie scheint im Infraroten weitgehend durch die Verhältnisse im Sichtbaren charakterisierbar zu sein, wenn auch die Streudispersion ein unerwartet gegensätzliches Verhalten zeigt. BeiA-ähnlichen, visuellen Streutypen (oder Typen nahe 1) verschwindet diese ab etwa 1.1 m fast vollständig. DieB-ähnlichen, visuellen Streutypen (oder Typen nahe 2) tritt in einem Zwischenbereich (etwa 0.8 bis 1.3 m) starke Streudispersion auf. Für den Gesamtstreutyp wird eine Klassifizierung in Anlehnung an die im Sichtbaren gebräuchliche [11–14]²) vorgeschlagen.

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Summary The scattering by the atmospheric aerosol of radiation in the region 0.52.1 m and 4°40° is investigated experimentally. The features in the infrared can be characterized by that in the visible, but the scattering dispersion is found to be different to that expected. At scattering types nearA (or 1) in the visible there is almost no scattering dispersion for 1.1 m. At types nearB (or 2) in the visible a high scattering dispersion in the region between 0.8 m and 1.3 m was observed. For the whole region of wavelength including the infrared a classification similar to that used in the visible [11–14]²) is suggested.

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Mitteilungen des Astronomischen Instituts der Universität Tübingen Nr. 77     

Geochemistry of lavas from Mohns Ridge,Norwegian-Greenland Sea: implications for melting conditions and magma sources near Jan Mayen

 Mohns Ridge lavas between 71 and 72°30′N (∼360 km) have heterogeneous compositions varying between alkali basalts and incompatible-element-depleted tholeiites. On a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island Jan Mayen and Mohns Ridge tholeiites. The variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts (∼5%) close to Jan Mayen but also shows its signature much further north on Mohns Ridge. Three lava types with different incompatible element compositions [e.g. chondrite-normalized (La/Sm)_N<1 to >2] occur in the area at 72°N and were generated from this heterogeneous mantle. The relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source. The elements Ba, Rb and K of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite. That such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures, at odds with a hotspot origin for Jan Mayen. Instead we suggest that the melting may be induced by the lowered solidus temperature of a “wet” mantle. Mohns MORB (mid ocean ridge basalt) and Jan Mayen area alkali basalts have high contents of Ba and Rb compared to other incompatible elements (e.g. Ba/La >10). These ratios reflect the signature of the mantle source. Ratios of Ce/Pb and Rb/Cs are normal MORB mantle ratios of 25 and 80, respectively, thus the enrichments of Ba and Rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid, as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts. Geophysical and petrological models suggest that Jan Mayen is not the product of hotspot activity above a mantle plume, and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment, a fracture zone and a spreading axis in this part of the North Atlantic. Received: 3 May 1995 / Accepted: 6 November 1995     

Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study

 Zircons from a metasedimentary and a meta-igneous quartz-feldspar granulite from the Val Sesia and Val Mastallone area of the Ivrea Zone (Southern Alps) differ in their response to granulite facies metamorphism with respect to crystal morphology and U/Pb ages. Detrital zircons in the metasediment developed an isometric overgrowth by the addition of Zr derived mainly from co-existing minerals, most probably biotite, decomposing during anatectic melting. The overgrowth started in the pelitic layer of the metasediment in the Late Carboniferous at approximately 296 Ma, significantly earlier than in the adjacent psammitic layer where it started only at 261 ± 4 Ma (95% confidence level). These ages are ascribed to the differential initiation of anatexis in the two layers. The delay of melting in the psammitic layer was probably due to the different position and less steep slope of its solidus in P-T-space, as compared to the solidus in the pelitic layer. Accordingly, the melting in the psammitic layer at 261 Ma was initiated by a thermal pulse and/or by a decompression event. Decompression melting is supported by a characteristic shell morphology of the zircon overgrowth in the psammitic layer, which might have grown under shear movements during high-temperature extensional faulting. The typically magmatic zircon population of the meta-igneous granulite crystallized at 355 ± 6 Ma (95% confidence level). The morphology of the zircons and the chemistry of the rock suggest that the magma was calcalkaline. A minor subpopulation of crystals is morphologically similar to the zircons in the pelitic layer of the metasediment. This points to the admixture of a minor sediment component and, thus, to a largely volcaniclastic origin of the protolith. In contrast to the detrital zircons in the metasediment, the magmatic zircons show rare and little overgrowth and, instead, have been strongly resorbed by anatectic melt. In addition, they became partially recrystallized and the rejuvenated ages from the most thoroughly recrystallized domains indicate that the rock was subject to prograde metamorphism after 279 Ma. This may correspond to the regional temperature increase prior to the climax of metamorphism or to a local thermal pulse due to nearby mafic intrusions. An Upper Triassic event at 226 ± 5 Ma is reflected by distinct peripheral zones in the overgrowths of some zircons in the metapelite. These are interpreted as a second metamorphic pulse, possibly induced by the infiltration of fluids. Received: 2 June 1995 / Accepted: 15 September 1995     

Nd,Pb, and Sr isotope composition of Late Mesozoic to Quaternary intra-plate magmatism in NE-Africa (Sudan,Egypt): high-μ signatures from the mantle lithosphere

The isotopic composition of mafic small-volume intra-plate magmatism constrains the compositions of the sub-continental mantle sources. The Nd, Pb, and Sr isotope signatures of widespread late Mesozoic to Quaternary intra-plate magmatism in NE Africa (Sudan, South Egypt) are surprisingly uniform and indicate the presence of a high-μ (μ = ²³⁸U/²⁰⁴Pb) source in the mantle. The rocks are characterized by small ranges in the initial isotopic composition of Nd, Pb, and Sr and most samples fall within ε Nd ca. 3–6, ²⁰⁶Pb/²⁰⁴Pb ca. 19.5–20.5, ²⁰⁷Pb/²⁰⁴Pb ca. 15.63–15.73, ²⁰⁸Pb/²⁰⁴Pb ca. 39–40 and ⁸⁷Sr/⁸⁶Sr ca. 0.7028–0.7034. We interpret this reservoir as lithospheric mantle that formed beneath the Pan-African orogens and magmatic arcs from asthenospheric mantle, which was enriched in trace elements (U, Th, and light REE). Combining our new data set with published data of intra-plate magmatic rocks from the Arabian plate indicates two compositionally different domains of lithospheric mantle in NE-Africa–Arabia. The two domains are spatially related to the subdivision of the Pan-African orogen into a western section dominated by reworked cratonic basement (NE-Africa; high-μ lithospheric mantle) and an eastern section dominated by juvenile Pan-African basement (easternmost NE-Africa and Arabia; moderate μ lithospheric mantle). The compositions of the Pan-African lithospheric mantle and the MORB-type mantle of the Red Sea and Gulf of Aden spreading centers could explain the Nd–Pb-Sr isotopic compositions of the most pristine Afar flood basalts in Yemen and Ethiopia by mixtures of the isotopic composition of regional lithospheric and asthenospheric sources. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Electromagnetic Brans-Dicke-Bianchi type-V solutions

We derive new solutions for the locally rotational-symmetric Bianchi type-V space-time in the Brans-Dicke theory of gravitation. The solutions represent anisotropic cosmological models filled with stiff matter and an electromagnetic null field, including the vacuum case. It is shown that the scalar field is dynamically an essential factor, since it influences the structure of the singularities and the expansion of the models.