Rb-Sr geochronology in favour of polymetamorphism in the Pan African Damara belt of Namibia (South West Africa)

New Rb-Sr whole rock age data are reported from two metasedimentary sequences of the Damara Supergroup in the central Pan African Damara belt of Namibia (South West Africa).Calc-granofels rocks of the Karibib Formation (Swakop Group) near Usakos are dated at 665±34 Ma (⁸⁷Rb=1.39×10^–11 a^–1) which is interpreted as reflecting a high-grade metamorphic event predating widespread granite intrusion.Pyroxene-bearing feldspathic gneisses of the Khan Formation (Nosib Group) from the Khan-Swakop River area east of Swakopmund show incomplete homogenization at 474±16 Ma. In view of similar ages obtained on the nearby Rössing alaskite granite and on biotites from a variety of rock assemblages this age is interpreted as reflecting a second Damaran metamorphic event rather than a specific stage in a long cooling history as previously thought.It is probable that the new ages characterize two distinct Pan-African tectono-metamorphic events previously named Katangan and Damaran episodes respectively (Clifford, 1967). The younger of these has affected large areas of south western Africa both within orogenic zones and on the neighbouring Kalahari Craton and may reflect crustal processes of sub-continental proportion during the closing stages of the Pan-African tectogenesis.

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Zusammenfassung Neue Rb-Sr-Gesamtgesteins-Isochronenalter von zwei sedimentären Abfolgen der spätpräkambrischen Damara-Supergruppe in der Zentralzone des Pan-Afrikanischen Damara-Orogens von Namibia (Südwest-Afrika) deuten auf eine mehrphasige metamorphe Geschichte hin.Kalk-Granofels-Gesteine der Karibib-Formation (Swakop-Gruppe) südöstlich von Usakos ergaben ein Alter von 665±34 M. J. (⁸⁷Rb=1,39×10^–11 a^–1) und wir interpretieren dieses Alter als das Resultat isotoper Homogenisierung während einer intensiven Regionalmetamorphose, die vor dem Eindringen weitverbreiteter Granite stattfand.Pyroxenführende und feldspathaltige Gneise der Khan-Formation (Nosib-Gruppe) aus dem Gebiet der Khan- und Swakop-Flüsse östlich von Swakopmund zeigen unvollständige Homogenisierung ihrer Sr-Isotope um 474±16 M. J. In Anbetracht ähnlicher Alter für den benachbarten Rössing-Alaskitgranit und für Biotite verschiedener Gesteinstypen aus dem gesamten Zentralbereich des Orogens interpretieren wir obiges Alter eher als den Höhepunkt einer zweiten Regionalmetamorphose als ein bestimmtes Stadium in der post-orogenen Abkühlungsgeschichte.Es ist wahrscheinlich, daß unsere Altersbestimmungen zwei deutlich voneinander getrennte pan-afrikanische tektono-metamorphe Phasen charakterisieren, vor allem wenn man sie in Zusammenhang mit publizierten Daten betrachtet, und wir nehmen an, daß diese den vonClifford (1967) postulierten Katanga- und Damara-Episoden entsprechen. Die jüngere dieser Episoden hat nicht nur die pan-afrikanischen orogenen Bereiche im südwestlichen Afrika, sondern auch weite Teile des Kalahari-Kratons erfaßt und mag auf tektogenetische Prozesse von nahezu kontinentalen Ausmaßen in und unter der Kruste im frühen Paläozoikum zurückzuführen sein.

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Résumé Deux séquences métasédimentaires du supergroupe du Damara ont été datées par la méthode Rb/Sr.Des gneiss calciques («granofels») de la formation de Karibib (groupe de Swakop), récoltés près d'Usakos ont été datés à 665±34 Ma (avec⁸⁷Rb=1.39× 10^–11 a^–1). Cet âge est interprété comme le résultat d'un événement métamorphique de degré é levé antérieur à la mise en place des grandes masses de granite.Des gneiss feldspathiques à pryoxène de la Formation de Khan (Groupe de Nosib), échantillonnés dans la région de la Khan River — Swakop à l'Est de Swakopmund, montrent une homogénéisation isotopique incomplète à 474±16 Ma. Des âges voisins ayant été obtenus pour l'Alaskite de Rössing qui affleure dans la même zone et pour les biotites de différentes roches, cet âge est interprété comme le résultat d'un second métamorphisme damarien plutôt que comme un stade de la longue histoire de refroidissement comme cela avait été imaginé auparavant.Ces nouveaux âges caractériseraient les deux épisodes tectonométamorphiques distincts de l'orogénie Pan-Africaine, qui avaient été appelés respectivement épisode katangien et épisode demarien parClifford (1967). Le plus récent de ces événements a affecté une grande partie de l'Afrique du S.W. aussi bien dans les zones orogéniques que dans le craton voisin du Kalahari; il peut refléter une évolution crustale d'échelle subcontinentale se produisant pendant les derniers stades du tectogène Pan-Africain.

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Rb/Sr - , - ; , .- , , 665±34 (⁸⁷Rb=1,39 10^–1a^–1); , . , , , , 474 ± 16 . - ; , , . , - - , , . , 1967 , . - - , ; , , .

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Contribution No. 27 of the South African Geodynamics Project, presented at the 9th Colloquium on African Geology, Göttingen, April 1977.     

On wave motions in a tropical spectral experiment

An attempt is made to use the barotropic vorticity equation in spectral form in order to study barotropic instability when the basic current has east-west quasi-stationary asymmetries on the scale of long waves. This is done by expressing the spectral equations in three different ways. In the first experiment a 9-component system is integrated and the long waves are allowed to propagate freely. In the second experiment the long waves are constrained to propagate slowly and in the third experiment, they are removed altogether.The motivation behind the present investigation is due to observations made in the motion fields of the tropics. These are characterized by quasi-stationary long waves and very energetically active and propagating short waves.The presence of quasi-stationary long waves seems to enhance the energy exchanges between the short waves and the mean zonal current and also allow for larger energy values for the short waves. Long term integration (90 days) shows a 6-day mode in thev time spectra at latitude 10°N and a 15-day mode in theu time spectra at individual grid points in the equatorial latitudes. Any possible connection between this peak and the observed peak of 15 days in the completely different physics of Kelvin waves is left as a conjecture.     

The coprecipitation of strontium,magnesium, sodium,potassium and chloride ions with gypsum. An experimental study

The coprecipitation of Sr²⁺, Mg²⁺, Na⁺, K⁺ and Cl^? into gypsum was studied as a function of temperature, brine concentration and growth rate. The concentrations of the studied cations in the gypsum increase with growth rate (kinetic effect), with a tendency to reach a limiting value at high growth rates. The partition coefficients of Sr tend to increase with brine concentration and decrease with temperature. The partition coefficients of the other cations also decrease with temperature but depend only very slightly on brine concentration. The concentrations of coprecipitated chloride are negligibly small.The coprecipitation behavior is explained in terms of the relation between the rate of desorption of the coprecipitating ions from the surface of the growing crystal, and the rate of growth. The studied cations may substitute for Ca²⁺ in its normal lattice sites and/or reside in interstitial positions among the structural water molecules. The relative amount of foreign cations occupying interstitial positions increases with increasing growth rate.The elucidation of the behavior of coprecipitated ions in gypsum given here forms a basis for the utilization of these ions as geochemical indicators for the environment of deposition of gypsum. These indicators may help in reconstructing important parameters such as temperature, brine concentration and growth rate.     

New data on magmatic H2O contents of pantellerites,with implications for petrogenesis and eruptive dynamics at Pantelleria

Infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at Pantelleria, Italy, show that the magmas contained moderate pre-eruptive H₂O contents, ranging from 1.4 to 2.1 wt.%. Melt H₂O concentrations increase linearly with incompatible elements, demonstrating that H₂O contents were not buffered significantly during fractionation by any crystalline or vapor phase. The relatively low H₂O contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt. Preeruptive H₂O concentrations do not correlate with the volume or explosivity of pantellerite eruptions; decompression history is critical in determining the style of pantellerite (and other) eruptions.     

Distribution of222Rn over the north Pacific: Implications for continental influences

The atmospheric distribution of²²²Rn over the north Pacific is simulated with a three-dimensional chemical tracer model using meteorological input from the NASA-GISS general circulation model (4°×5° resolution). Radon-222 (half-life 3.8 days) is a tracer of continental air. Model results are in good agreement with measurements from ships and aircraft. Strong Asian influence is found throughout the tropospheric column over the north Pacific in spring, reflecting a combination of frequent convection over the continent, strong westerly winds at altitude, and subsidence over the ocean. In summer, the upper troposphere over the north Pacific is heavily affected by deep convection over China; however, Asian influences at the surface are then at their yearly minimum. In winter, strong Asian influence is found near the surface but not at high altitudes. Transport of American air over the Pacific is important only at low latitudes. American sources account for 11% of total²²²Rn in the model at Midway, 30% at Mauna Loa and 59% at Oahu. Results for Hawaii indicate two seasonal peaks of American influence, one in summer and one in winter. The tropical western Pacific is particularly remote from continental influences year round.     

Multisite campaign on the open cluster M67 – II. Evidence for solar-like oscillations in red giant stars

Measuring solar-like oscillations in an ensemble of stars in a cluster, holds promise for testing stellar structure and evolution more stringently than just fitting parameters to single field stars. The most-ambitious attempt to pursue these prospects was by Gilliland et al. who targeted 11 turn-off stars in the open cluster M67 (NGC 2682), but the oscillation amplitudes were too small (<20 μmag) to obtain unambiguous detections. Like Gilliland et al. we also aim at detecting solar-like oscillations in M67, but we target red giant stars with expected amplitudes in the range 50–  500 μmag  and periods of 1 to 8 h. We analyse our recently published photometry measurements, obtained during a six-week multisite campaign using nine telescopes around the world. The observations are compared with simulations and with estimated properties of the stellar oscillations. Noise levels in the Fourier spectra as low as  27 μmag  are obtained for single sites, while the combined data reach  19 μmag  , making this the best photometric time series of an ensemble of red giant stars. These data enable us to make the first test of the scaling relations (used to estimate frequency and amplitude) with an homogeneous ensemble of stars. The detected excess power is consistent with the expected signal from stellar oscillations, both in terms of its frequency range and amplitude. However, our results are limited by apparent high levels of non-white noise, which cannot be clearly separated from the stellar signal.     

Overcoming Challenges Associated with the Analysis of Nacre by Atom Probe Tomography

In this study atom probe tomography was used to study nacre, an important biocomposite material that is challenging to prepare and analyse by atom probe and, when successful, yields data that is challenging to interpret. It was found that these challenges mainly arise from the insulating and heterogeneous nano‐scale properties of nacre. We outline our current best practice for preparing and running atom probe tips, such as using a low acceleration voltage (< 3 kV) and current (≤ 50 pA) to avoid damage to the microstructure, and using transmission electron microscopy to confirm that the region of interest is located close to the apex of the atom probe tip. Optimisation of the preparation parameters led to several successful atom probe experiments, with one of the data sets containing part of an organic membrane and others showing organic inclusions within the reconstruction.     

Combined thermodynamic and rare earth element modelling of garnet growth during subduction: Examples from ultrahigh-pressure eclogite of the Western Gneiss Region, Norway

Major and trace element zonation patterns were determined in ultrahigh-pressure eclogite garnets from the Western Gneiss Region (Norway). All investigated garnets show multiple growth zones and preserve complex growth zonation patterns with respect to both major and rare earth elements (REE). Due to chemical differences of the host rocks two types of major element compositional zonation patterns occur: (1) abrupt, step-like compositional changes corresponding with the growth zones and (2) compositionally homogeneous interiors, independent of growth zones, followed by abrupt chemical changes towards the rims. Despite differences in major element zonation, the REE patterns are almost identical in all garnets and can be divided into four distinct zones with characteristic patterns.In order to interpret the major and trace element distribution and zoning patterns in terms of the subduction history of the rocks, we combined thermodynamic forward models for appropriate bulk rock compositions to yield molar proportions and major element compositions of stable phases along the inferred pressure-temperature path with a mass balance distribution of REEs among the calculated stable phases during high pressure metamorphism. Our thermodynamic forward models reproduce the complex major element zonation patterns and growth zones in the natural garnets, with garnet growth predicted during four different reaction stages: (1) chlorite breakdown, (2) epidote breakdown, (3) amphibole breakdown and (4) reduction in molar clinopyroxene at ultrahigh-pressure conditions.Mass-balance of the rare earth element distribution among the modelled stable phases yielded characteristic zonation patterns in garnet that closely resemble those in the natural samples. Garnet growth and trace element incorporation occurred in near thermodynamic equilibrium with matrix phases during subduction. The rare earth element patterns in garnet exhibit distinct enrichment zones that fingerprint the minerals involved in the garnet-forming reactions as well as local peaks that can be explained by fractionation effects and changes in the mineral assemblage.