Progressive niedriggradige Metamorphose glaukonitführender Horizonte in den helvetischen Alpen der Ostschweiz

Glauconite-bearing formations of Cretaceous and Tertiary age in the Helvetic zone of the Glarus Alps have been investigated by microscopic, X-ray, wet chemical, electron microprobe, Mössbauer spectroscopic, and K-Ar dating methods. 3 different metamorphic zones with increasing grade can be distinguished (Fig. 3). Original, unmetamorphosed sediments containing glauconite-calcite-quartz±chlorite comprise zone I. The glauconite is very rich in potassium (8–9 wt.%) and the chlorite is Fe-rich. In zone II green stilpnomelane forms by the reaction: glauconite±chlorite + quartz = stilpnomelane + k-feldspar + H₂O + O₂. The green stilpnomelane contains as much as ten times the amount of K found in brown stilpnomelane, which is believed to be a weathering feature. In zone III biotite appears by the reaction: chlorite + k-feldspar = biotite + stilpnomelane + quartz + H₂O. Riebeckite is a possible additional phase in all three zones. Generally, zones I–III are arranged nearly parallel to the Alpine border with metamorphic grade increasing to the south. In the Glarnisch Massif, however, the transition from zone I to zone II is clearly controlled by the overburden of the nappe pile (Fig. 6). The beginning of zone II also seems to coincide with the middle of the anchizone, as defined by illite-crystallinity measurements in adjoining marly shales and slates; this corresponds approximately to the transition from the zeolite facies to the prehnitepumpellyite facies.K-Ar-ages on glauconites regularly decrease when approaching the zone I/II-transition. Field evidence and combined K-Ar age determinations on glauconites, stilpnomelanes and riebeckites point to a peak of the metamorphism during Lower to Middle Oligocene, shortly after the main orogenic phase in this part of the Helvetic Alps.

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Die Autoren danken Herrn Prof. E. Niggli für das fördernde Interesse an dieser Arbeit, die Überlassung der Probe EN 8999 sowie die kritische Durchsicht des Manuskriptes. Für wertvolle Diskussionen und Hinweise danken wir den Prof. E. Jäger, R. Herb und P. M. Orville sowie Dr. W. D. Brückner und Dr. N. Clauer. M.F. möchte Herrn E. Weber herzlich für die Einführung auf der Mikrosonde danken. Herrn Prof. Th. Hügi danken wir für die Benützung des geochemischen Labors, Herrn Prof. H. Schwander für einige Na-Bestimmungen auf der Basler Mikrosonde und Herrn Dr. F. Hofmann für die Bestimmung der C-Gehalte. Den Herren Theo Küpfer und J. Fuhrimann verdanken wir verschiedene Laborarbeiten. Diese Arbeit wurde durch die folgenden Stipendien des Schweizerischen Nationalfonds zur Förderung der wissenschaftlichen Forschung unterstützt: Nr. 5358.2 an M.F., Nr. 2.367.70 an J.H. sowie Nr. 2598 an P.R. Die Durchführung der Mikrosondenanalysen wurden zudem möglich dank eines Studienaufenthaltes von M. F. an der Yale Universität, New Haven, Connecticut, wofür dieser Autor der Forschungskommission der Universität Bern zu großem Dank verpflichtet ist.     

New England granites: Trace element evidence regarding their origin and differentiation

The abundances of Sc, rare earths, Zr, Hf, Ta and Th were determined in five New England granitic plutons. Similar data are reported for separated minerals from three of the units. The granites can be divided into the Massachusetts alkaline group (Cape Ann, Peabody, Quincy) and the Rhode Island subalkaline group (Narragansett Pier, Westerly).Analyses for three samples from each pluton indicate that the Westerly granite is heterogeneous in both major and trace elements. Th abundances vary considerably (factor of 2) in both Rhode Island granites. Zr and Hf are heterogeneously distributed in all the granites while the rare earths are more homogeneous (< ± 20% deviation from mean).Eu depletions in all the granites imply that feldspar was involved either as a cumulate during fractional crystallization or as a residual phase during anatexis. Because the Massachusetts granites are associated with syenites and show evidence for low water fugacity and denser rocks at depth, these granites probably developed from basic magmas as a result of extensive feldspar crystallization.Depletion of Tb, Dy, Yb, Lu, Zr, Hf and Ta in the Rhode Island granites suggest participation of zircon or garnet as residues of partial melting or as crystal cumulates. These granites crystallized under nearly water saturated conditions and are characterized by abundant pegmatites. The presence of a water rich phase may also have been important in the depletion of elements which form stable complexes in aqueous solutions.     

Microtektites: a chemical comparison of bottle-green microtektites,normal microtektites and tektites

Trace element abundances in Ivory Coast normal microtektites and Australasian bottle-green microtektites confirm that microtektites are genetically related to tektites in the associated strewn field. Although major and compatible trace element abundances imply that bottle-green microtektites are members of a fractional crystallization sequence, the similar rare earth element distributions in Australasian normal and bottle-green microtektites and tektites cannot be explained by a simple fractionation model. The similar REE abundances in tektites and microtektites of widely different major element composition also preclude simple models calling for sedimentary rock precursors.     

Ultramafic inclusions from San Carlos,Arizona: Petrologic and geochemical data bearing on their petrogenesis

Ultramafic inclusions from San Carlos, Arizona, are classified into two groups. Group I inclusions are dominated by magnesian (Mg/Mg + ΣFe= 0.86 – 0.91), olivine-rich peridotites containing Cr-rich clinopyroxene and spinel. The less abundant Group I pyroxenites (containing Mg- and Cr-rich pyroxenes) occur as discrete inclusions and as portions of composite inclusions where they have a sharp, planar interface with lherzolite. Group II inclusions are dominated by clinopyroxene-rich peridotites containing Al- and Ti-rich augite and commonly abundant, Al-rich spinel. Compared to Group I inclusions, they are more Fe-rich (Mg/Mg + ΣFe= 0.62 – 0.78) and more hetereogeneous in composition and modal proportions. Similar groups occur at many ultramafic inclusion localities.Our petrographic and geochemical results lead to the following conclusions. Olivine-rich Group I inclusions are not genetically related to the host basanite, and they are formed from two components. Component A is a partial melting residue; it comprises the major portion of these inclusions and determines the modal mineralogy and major and compatible trace element composition. Component B results from a small degree (<5%) of garnet peridotite melting (probably, within the low-velocity zone). This highly LIL-element-enriched melt has migrated upwards into the overlying component A where it crystallized primarily as clinopyroxene and amphibole, and thus, introduced LIL elements into the residual component A. Subsequent cooling and subsolidus recrystallization have removed textural evidence of this mixing. This model has also been proposed for olivine-rich Group I inclusions from Victoria, Australia. At Victoria and San Carlos some relatively clinopyroxene-rich Group I lherzolites are not contaminated by component B, and they represent the best estimates of upper mantle composition prior to melting. Group I orthopyroxenites may be fragments of tectonic layers formed in lherzolite, but they could also be early cumulates (now metamorphosed) from the melt in equilibrium with component A. Group I clinopyroxenites have geochemical features of clinopyroxene in equilibrium with a magma. Thus, they could also represent early cumulates (now metamorphosed) from a magma unrelated to the host basanite. Alternatively, their geochemical characteristics could result from more complex models such as residues from partial remelting of pyroxenite dikes and veins or intradike segregation processes such as filter pressing. All Group II inclusions studied appear to be cumulates derived from a SiO₂-undersaturated magma, possibly an early magma in the same volcanic episode which culminated with eruption of the host basanite. The poikilitic texture of amphibole-rich (kaersutite) inclusions is consistent with a cumulate origin. The bulk compositions of Group II inclusions are not equivalent to typical basaltic compositions.     

Optical tomography of the aurora and EISCAT

Tomographic reconstruction of the three-dimensional auroral are emission is used to obtain vertical and horizontal distributions of the optical auroral emission. Under the given experimental conditions with a very limited angular range and a small number of observers, algebraic reconstruction methods generally yield better results than transform techniques. Different algebraic reconstruction methods are tested with an auroral are model and the best results are obtained with an iterative least-square method adapted from emission-computed tomography. The observation geometry used during a campaign in Norway in 1995 is tested with the are model and root-mean-square errors, to be expected under the given geometrical conditions, are calculated. Although optimum geometry was not used, root-mean-square errors of less than 2% for the images and of the order of 30% for the distribution could be obtained. The method is applied to images from real observations. The correspondence of original pictures and projections of the reconstructed volume is discussed, and emission profiles along magnetic field lines through the three-dimensionally reconstructed arc are calibrated into electron density profiles with additional EISCAT measurements. Including a background profile and the temporal changes of the electron density due to recombination, good agreement can be obtained between measured profiles and the time-sequence of calculated profiles. These profiles are used to estimate the conductivity distribution in the vicinity of the EISCAT site. While the radar can only probe the ionosphere along the radar beam, the three-dimensional tomography enables conductivity estimates in a large area around the radar site.Former address: MPE Garching     

Temporal evolution of the kerguelen plume: Geochemical evidence from 38 to 82 ma lavas forming the Ninetyeast ridge

Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have²⁰⁶pb/²⁰⁴Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher²⁰⁶Pb/²⁰⁴Pb, such as that expressed in lavas from St. Paul Island.     

Geochemistry of phonolites and trachytes from the summit region of Mt. Kenya

Two suites of felsic eruptives and intrusives are represented in a set of samples from the summit region of the Plio-Pleistocene volcano, Mt. Kenya. Most of the samples are moderately or strongly undersaturated and have ⁸⁷Sr/⁸⁶Sr initial ratios in the range 0.70360–0.70368 (mean=0.70362). Members of this phonolitic suite are phonolites, nepheline syenites or kenytes and as a group they show a wide variation in TiO₂, FeO, P₂O₅, Sr, Ba, Zr and Nb. The minor and trace element geochemistry reflect variation in the nature of the parental basaltic magmas from which the phonolitic rocks evolved and variation in the crystal fractionation process in individual cases. Crystal fractionation involving plagioclase, alkali feldspar, clinopyroxene, olivine and magnetite is the process by which most of the phonolitic rocks evolved and variation in the relative proportions of these phases in individual cases has led to a broad spectrum of trace and minor element behaviour. The second suite of felsic samples is critically saturated and consists of trachytes showing either slight oversaturation or slight undersaturation with respect to SiO₂. This trachyte suite has lower initial ⁸⁷Sr/⁸⁶Sr ratios (mean=0.70355) and is derived from transitional alkalic basalts by low pressure (crustal) crystal fractionation involving feldspar, clinopyroxene, magnetite and olivine. The range in minor and trace element chemistry observed among the felsic rocks is a consequence of variation in the parental basalts which is related to mantle source variation and to the specific nature of the crystal fractionation process.     

Geology and geochemistry of basaltic lava flows and dikes from the Trans-Koolau tunnel, Oahu, Hawaii

A 200-m section of Koolau basalt was sampled in the 1.6-km Trans-Koolau (T–K) tunnel. The section includes 126 aa and pahoehoe lava flows, five dikes and ten thin ash units. This volcanic section and the physical characteristics of the lava flows indicate derivation from the nearby northwest rift zone of the Koolau shield. The top of the section is inferred to be 500–600 m below the pre-erosional surface of the Koolau shield. Therefore, compared with previously studied Koolau lavas, this section provides a deeper, presumably older, sampling of the shield. Shield lavas from Koolau Volcano define a geochemical end-member for Hawaiian shields. Most of the tunnel lavas have the distinctive major and trace element abundance features (e.g. relatively high SiO₂ content and Zr/Nb abundance ratio) that characterize Koolau lavas. In addition, relative to the recent shield lavas erupted at Kilauea and Mauna Loa volcanoes, most Koolau lavas have lower abundances of Sc, Y and Yb at a given MgO content; this result is consistent with a more important role for residual garnet during the partial melting processes that created Koolau shield lavas. Koolau lavas with the strongest residual garnet signature have relatively high ⁸⁷Sr/⁸⁶Sr, ¹⁸⁷Os/¹⁸⁸Os, ¹⁸O/¹⁶O, and low ¹⁴³Nd/¹⁴⁴Nd. These isotopic characteristics have been previously interpreted to reflect a source component of recycled oceanic crust that was recrystallized to garnet pyroxenite. This component also has high La/Nb and relatively low ²⁰⁶Pb/²⁰⁴Pb, geochemical characteristics which are attributed to ancient pelagic sediment in the recycled crust. Although most Koolau lavas define a geochemical endmember for Hawaiian shield lavas, there is considerable intrashield geochemical variability that is inferred to reflect source characteristics. The oldest T–K tunnel lava flow is an example. It has the lowest ⁸⁷Sr/⁸⁶Sr, Zr/Nb and La/Nb, and the highest ¹⁴³Nd/¹⁴⁴Nd ratio found in Koolau lavas. In most respects it is similar to lavas from Kilauea Volcano. Therefore, the geochemical characteristics of the Koolau shield, which define an end member for Hawaiian shields, reflect an important role for recycled oceanic crust, but the proportion of this crust in the source varied during growth of the Koolau shield. Received: 1 June 1998 / Accepted: 30 August 1998     

Infiltration of fine sediment into a coarse mobile bed: a phenomenological study

Experiments were undertaken to study the nature of granular interaction in running water by examining the influence of fine grain inputs to a coarser sediment bed with a mobile surface. Video recordings of grain sorting by both kinetic sieving and spontaneous percolation are used to diagnose the critical processes controlling the overall bed response. Kinetic sieving takes place in the mobile bed surface, with the finer sediment moving to the bottom of the bedload transport layer at the interface with the underlying quasi‐static coarse bed. We show that the behavior at this interface dictates how a channel responds to a fine sediment input. If, by spontaneous percolation, the fine sediment is able to infiltrate into the underlying quasi‐static bed, the total transport increases and the channel degrades. However, if the fine sediment input rate exceeds the transport capacity or is geometrically unable to infiltrate into the underlying bed, it forms a quasi‐static layer underneath the transport layer that inhibits entrainment from the underlying bed, resulting in aggradation and an increase in bed slope. Copyright © 2016 John Wiley & Sons, Ltd.     

Hydrodynamics of the Bottom-Water Flow from the Arctic to the Atlantic through the Strait of Denmark

Izvestiya, Atmospheric and Oceanic Physics - The overflow of bottom waters through the Strait of Denmark to the Atlantic has been investigated. A unique physical and hydrodynamic effect of...