Sedimentation und Tektonik im mittleren Abschnitt des Rio Toro in der Ostkordillere NW-Argentiniens

Zusammenfassung Die bis 5000 m mächtigen jungkänozoischen Sedimente im Hangenden der Grupo Salta wurden im Mittelabschnitt der Quebrada del Toro auskartiert und gegliedert, wobei ein vorwiegend aus Vulkanitkomponenten bestehendes Konglomerat als Leithorizont diente. Auf diese Sedimente sind von West und Ost ältere Gesteine aufgeschoben, so daß sie heute nur noch in dem Rio Toro-Keilgraben erhalten sind. Anhand der wechselnden Geröllführung und lokaler Diskordanzen läßt sich nachweisen, daß durch die Hebung der westlichen und östlichen Randschollen der kontinentale Sedimentationsraum zunehmend eingeengt wurde. Die terrestrischen Sedimente im Rio ToroKeilgraben wurden erst zu einem relativ späten Zeitpunkt der Schollenbewegungen gefaltet und verworfen.Die jüngsten Sedimente — Terrassen und Schuttfächerablagerungen — lassen sich dem heutigen Flußbett des Rio Toro zuordnen.

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In the central part of the Quebrada del Toro up to 5000 m continental sediments of Upper Cenozoic age on top of the Salta Group have been mapped and divided into several units. A conglomerate, which is mainly composed of volcanic pebbles could be used as a marker. From west and east older rocks have been thrusted on these sediments, which therefore are preserved only in the Rio Toro-Keilgraben. By means of changing gravel associations and local unconformaties it can be shown, that the uplift of the western and eastern border made the basin of continental sedimentation narrower. The terrestrial sediments in the Rio Toro-Keilgraben have been folded and faulted only at a relatively late stage of the block movements.The youngest sediments, alluvial terraces and fan deposits, can be related to the recent bed of the Rio Toro.

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Resumen En la parte central de la Quebrada del Toro hasta 5000 m de sedimentos continentales del Cenozico superior han sido levantados y subdividos en algunos unidades, usando coma estrato guía un conglomerado compuesto en la mayor parte de rodados volcanicos. Del oeste y del este rocas más viejas han sido corridas sobre estos sedimentos, los quales han sido conservados solamente en el Río Toro-Keilgraben. Considerando el cambio en la asociación de rodados y las discordancias locales se puede provar que por el levantamiento de los bordes occidentales y orientales la cuenca sedimentaria continental fue estrechada. En un momento relativamente tarde en el desarollo tectonico los sedimentos continentales en el Rio Toro-Keilgraben fueron plegados y fallados.Los depositos más recientes, terrazzas y conos de deyección pueden ser relacionados con el cauce del Río Toro.

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Résumé Les sédiments d'âge cénozoïque supérieur, de près de 5000 m d'épaisseur qui se trouvent au-dessus du Groupe de Salta, ont fait l'objet de levés dans la partie centrale de la Quebrada del Toro. Un conglomérat se composant en majeure partie de galets volcaniques a servi de repère. Ces sédiments sont surmontés de roches plus anciennes, sous l'effet de chevauchements venant de l'Ouest et de l'Est, si bien que de nos jours on ne les trouve que dans le graben en coin du Rio Toro. On peut prouver, grâce au contenu variable des galets et à des discordances locales, que le soulèvement des bordures occidentale et orientale a causé un resserrement du bassin continental de sédimentation. Les sédiments terrestres dans le graben en coin du Rio Toro ont été plissés et disloqués seulement à une époque relativement tardive au cours des mouvements des blocs.Les sédiments les plus récents — terrasses d'accumulation et cônes d'alluvions — sont à mettre en liaison avec le lit actuel du Rio Toro.

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Geänderte Fassung eines auf der 64. Jahrestagung der Geologischen Vereinigung 1974 in Bochum gehaltenen Vortrages.     

Stochastic riming of plate-like and columnar ice crystals

Riming growth of ice particles is simulated by numerically solving the stochastic collection equations, simultaneously considering coagulation of water droplets. By introducing a special criterion which defines the habit of a riming particle, the influence of this habit on the growth of several kinds of ice particles assumed to be formed during riming was investigated.It was found that (i) hexagonal rimed ice plates are growing more efficiently than lump graupel or rimed columns, (ii) the use of different collection kernels for the lump graupel evolution leads to widely differing results and that (iii) the time dependent decrease of liquid water substance and the size of the resulting ice particles were more strongly influenced by the initial ice crystal concentration than by the initial ice crystal size and the habit of the ice particles. By decreasing the number density of ice crystals gradually a critical ice crystal concentration was found at which the present liquid water was not completely consumed by the riming process even after 1800 s model time, causing large drops of radii >100 m to be formed in appreciable concentrations.     

The genesis of the methane in Lake Kivu (Central Africa)

Large amounts of methane and carbon dioxide, among other gases, are dissolved in the deep water of Lake Kivu. There is no dispute about the primarily magmatic origin of the carbon dioxide, but models of the genesis of the methane have been contradictory up to now. They have been based on too few and partly too inaccurate data.On the basis of new measurements obtained from gas and sediment samples, some of the old concepts have been further developed to a new model. According to this model, the methane is generated mainly by bacteria from the organic carbon of the sediment. It probably also contains minor amounts of thermocatalytic methane.About 70% of the organic carbon of the upper sediment is derived from mainly magmatic carbon dioxide (old carbon), which enters the biozone of the lake from the deep water by eddy diffusion and is assimilated there. The remaining 30% comes from atmospheric carbon dioxide (young carbon) assimilated in the biozone. But because methane also migrates into the lake from deeper sediment, the¹⁴C-content in the methane dissolved in the lake water is not 30% modern but only ca. 10% modern.More isotopic measurements on plankton, methane, carbon dioxide and sediment samples are necessary to support this model.

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Zusammenfassung Im Tiefenwasser des Kivusees sind u. a. große Mengen an Methan und Kohlendioxid gelöst. Während über den hauptsächlich magmatischen Ursprung des Kohlendioxids weitgehend Einigkeit besteht, sind die bisherigen Modellvorstellungen zur Genese des Methans widersprüchlich. Sie beruhen auf zu wenigen und zum Teil zu ungenauen Meß-daten.Mit Hilfe neuer Meßergebnisse an Gas- und Sedimentproben des Kivusees wurden einige der alten Vorstellungen zu einem neuen Modell weiterentwickelt. Danach ist das Methan hauptsächlich bakteriell aus dem organischen Kohlenstoff des Sediments entstanden. Wahrscheinlich enthält es auch geringe Beimengungen thermokatalytischen Methans.Der organische Kohlenstoff des oberen Sediments stammt zu rd. 70% aus dem vorwiegend magmatischen Kohlendioxid (alter Kohlenstoff), das aus dem Tiefenwasser durch turbulenten Austausch in die Biozone des Sees gelangt und dort assimiliert wird. Die restlichen 30% stammen aus dem in der Biozone assimilierten atmosphärischen Kohlendioxid (junger Kohlenstoff). Weil jedoch auch Methan aus tieferen Sedimentschichten in den See wandert, beträgt der¹⁴C-Gehalt des im Seewasser gelösten Methans nicht 30% modern, sondern nur ca. 10% modern.Weitere Isotopenuntersuchungen an Plankton-, Methan-, Kohlendioxid- und Sedimentproben sind notwendig, um das Modell abzusichern.

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Résumé De grandes quantités de méthane et d'oxyde carbonique sont dissoutes dans les eaux profondes du Lac Kicu. Alors qu'on est en général d'accord sur l'origine surtout magmatique de l'oxyde carbonique, les modèles devant représenter la genése du méthane sont contradictoires. Ils reposent sur des données trop peu nombreuses et en partie trop inexactes.A l'aide de nouveaux résultats de mesures faites sur des échantillons de gaz et du sédiment, on a développé un nouveau modèle, à partir des anciennes représentations. D'après celui-ci, le méthane provient pour sa plus grande part du carbone organique du sédiment, transformé par des bactéries. Il contient probablement des traces d'un méthane de thermocatalyse.Le carbone organique du sédiment supérieur provient pour 70% de l'oxyde carbonique surtout magmatique (carbone »ancien«), des eaux profondes parvenu, par échanges turbulents, dans la biozone du lac, où l'oxyde carbonique est assimilé. Les 30% restant proviennent de l'oxyde carbonique atmosphérique (carbone »jeune«) assimilé dans la biozone. Le méthane des couches profondes du sédiment migrant dans le lac, la teneur en¹⁴C de méthane dissous dans les eaux du lac n'est pas de 30% modernes, mais de 10% modernes.D'autres recherches sur les isotopes d'échantillons du plancton, du méthane, de l'oxyde carbonique et du sédiment du Lac Kivu seront nécessaires pour confirmer ce modéle.

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The nature and origin of ureilites

Ureilites are carbon-bearing olivine-pigeonite achondrites which constitute a unique achondrite type. We performed a comprehensive mineralogical and petrological study on eight ureilites (Kenna, Novo Urei, Goalpara, Haverö, Dingo Pup Donga, Dyalpur, North Haig, and ALHA77257) the results of which were used to construct a hypothesis for the origin of ureilites. This hypothesis suggests that ureilites are primarily olivine-pigeonite cumulates which crystallized from a silicate liquid that also contained suspended solid carbon phases, mainly graphite. This carbon caused reduction of the melt and influenced ureilite mineral compositions. Carbonaceous material was trapped by settling cumulus mafic silicates along with other intercumulus material to form the carbon-rich ‘veins’ common to ureilites. Petrofabric analyses show that mafic silicates are oriented in lineated and foliated patterns characteristic of cumulate rocks, specifically adcumulates. Strain state of silicates suggests that ureilites were deformed subsequent to lithification by mild tectonic stress as well as by moderate to severe shock. The latter event caused the formation of micron-sized diamonds and lonsdaleite from graphite and carbon-induced reduction of silicate grain margins during temporal shock-heating.     

Microscope-photometric methods for non-destructive Fe2+ – Fe3+ determination in chloritoids (Fe2+, Mn2+, Mg)2 (Al,Fe3+)4 Si2O10(OH)4

Six natural chloritoid crystals with Fe²⁺ and Fe³⁺ contents ranging from 4.15 to 12.81 and from 0.411 to 0.849g-atoms/l, respectively, as determined by means of microprobe and Mössbauer techniques, served as reference material to develop non-destructive microscope-spectrophotometric methods for quantitative Fe²⁺ – Fe³⁺ determinations in chloritoids from unpolarized spectra of (001) platelets. Fe²⁺ concentrations in g-atom/l can be obtained from [ [Fe³⁺]=C₁xD₁/t where D₁ = log₁₀(I_0/I at 28,000 cm^-1 and t=crystal thickness in cm; C₁ is a conttant that may be influenced somewhat by experimental conditions and is found to be 0.002289 with the experimental set-up used in this study. Fe²⁺ concentrations in g-atom/l can be obtained from [Fe²⁺]=C₁xD₁/D₁-C₃ with D₂=log₁₀(I₀/I) at 16,300 cm^?1 and constants C₄ = 45.36 and C₅ = 3.540. Due to the uncertainties in absorbance measurements, D₁ and D₂ and the thickness measurements, the accuracies are ±0.05 and ±0.15 g-atom/l for [Fe³⁺] and [Fe²⁺], respectively. The determinations may be carried out on chloritoid grains in normal thin sections with an areal resolution of ～10 μm.     

Faziesverzahnung von diamantführenden Konglomeraten und Bändererzen (BIF) in der südlichen Serra do Espinhaço (Minas Gerais,Brasilien)

Zusammenfassung Die untersuchten Gesteine wurden im Altproterozoikum als Schelfsedimente vorwiegend in Flachwasserfazies abgelagert. In ihrer Verbreitung sind Unterschiede zwischen Bildungen turbulenter Bereiche (Sande, Konglomerate und vereinzelte Breccien) und Stillwasserbildungen (Hämatitquarzit, Itabirit und Phyllit) festzustellen. Im Übergangsbereich beider Faziesbereiche kam es zu Verzahnungen von klastischen mit chemischen Sedimenten. Die chemisch gefällten Bändererze sind in zwei Typen zu trennen, zum einen in klastisch beeinflußte aus Bereichen unruhiger Sedimentationsbedingungen, zum anderen in klastisch nicht beeinflußte aus ruhigen Bereichen. Klastische Beeinflussung und intraformationale Aufarbeitung der BIF fand während allen Stadien der Verfestigung statt. Intraformationale Aufarbeitungsprodukte der Bändererze treten gemeinsam mit Konglomeraten großer Gerölldurchmesser entlang einer Nord-Süd streichenden Zone auf, wo sie sich lateral mit ungestörten chemischen Eisenerzen verzahnen. Konglomerate und BIF wurden zeitgleich sedimentiert. Charakteristisch für diese Zone sind außerdem synsedimentär aufgedrungene Vulkanite, deren Vorkommen einen Bereich tektonischer Unruhe nachzeichnet.

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The investigated rocks were deposited as shelf-sediments during the lower Proterozoic, predominantly in a shallow marine environment. It can be distinguished between sediments formed under turbulent conditions (sands, conglomerates and occasional breccias) and sediments of undisturbed environments (hematite-quartzite, itabirite and phyllite). In the transition of both facies-zones detritic and chemical sediments are interfingering. The chemically precipitated banded iron ores can be separated into two types — clastically influenced BIF formed in areas of agitated sedimentary conditions, and BIF with no clastic admixtures formed in quiet regions. Banded iron ores were clastically influenced and intraformationally reworked during all states of lithification. Products of intraformationally reworked BIF appear together with conglomerates of large diameters along a north-south-striking zone where they interfinger laterally with undisturbed chemically precipitated iron ores. Conglomerates and BIF were deposited here at the same time. Significant for this zone are also synsedimentary volcanites accentuating the presence of a tectonically active zone.

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Resumo Os metasedimentos aqui analisados foram predominantemente depositados em ambientes plataformais do Proterozoico inferior. Na sua distribuição constata-sc depósitos de águas calmas (hematita-quartzitos, itabiritos e filitos) e turbulentas) areias, conglomerados e esporadicamente brechas). Na zona de transição entre as duas facies sedimentares ocorreu interdigatação de sedimentos clásticos e químicos. Os minérios bandeados de ferro (BIF), quimicamente precipitados, podem ser classificados em dois grupos: o primeiro com interferência clástica, típico de ambientes agitados de sedimentação; o segundo sem componentes clásticos e característico de ambientes calmas. Interferência clástica como retrabalhamento intraformational ocorreu durante todos os estágios de processo sedimentar e de consolidação de BIF. Produtos do retrabalhamento intraformacional dos minérios bandeados ocorrem simultaneamente com conglomerados (seixos de diámetros grandes) ao longo de uma faixa Norte-Sul e interdigitados lateralmente com sedimentos químicos não perturbados. Conglomerados e BIF foram sedimentados concomitantemente. Característico para esta faixa são vulcanitos de extrusão sinsedimentar cuja presença tipifica um ambiente de instabilidade tectônica.

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Elemental abundances in chondrules from unequilibrated chondrites: Evidence for chondrule origin by melting of pre-existing materials

Bulk abundances of Na, Mg, Al, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, La, Sm, Eu, Yb, Lu, Ir, and Au were determined by neutron activation analysis of chondrules separated from unequilibrated H-, L-, and LL-chondrites (Tieschitz, Hallingeberg, Chainpur, Semarkona) and correlated with chondrule petrographic properties. Despite wellknown compositional differences among the whole-rock chondrites, the geometric mean compositions of their respective chondrule suites are nearly indistinguishable from each other for many elements. Relative to the condensible bulk solar system (approximated by the Cl chondrite Orgueil), chondrules are enriched in lithophile and depleted in siderophile elements in a pattern consistent with chondrule formation by melting of pre-existing materials, preceded or attended by silicate/metal fractionation. Relative to nonporphyritic chondrules, porphyritic chondrules are enriched in refractory and siderophile elements, suggesting that these two chondrule groups may have formed from different precursor materials.     

Temperature-jump induced cation exchange kinetics in (Co<Subscript>0.1</Subscript>Mg<Subscript>0.9</Subscript>)<Subscript>2</Subscript>SiO<Subscript>4</Subscript> olivine: an in situ optical spectroscopic study

In the olivine crystal structure, cations are distributed over two inequivalent octahedral sites, M1 and M2. Kinetics of cation exchange between the two octahedral sites in (Co_0.1Mg_0.9)₂SiO₄ single crystal have been studied in the temperature range from 600 to 800°C by monitoring the time evolution of the absorbance of Co²⁺ ions in M1 or M2 sites using optical spectroscopy after rapid temperature jumps. It was found from such temperature-jump induced relaxation experiments that with increasing temperature the absorbance of Co²⁺ ions in the M1 site decreases while that in the M2 site increases. This indicates a tendency of Co²⁺ cations to populate the M2 site with increasing temperatures and vice versa. The experimental relaxation data can be modeled using a triple exponential equation based on theoretical analysis. Activation energies of 221 ± 4 and 213 ± 10 kJ/mol were derived from relaxation experiments on the M2 site and M1 site, respectively, for the cation exchange processes in (Co_0.1Mg_0.9)₂SiO₄ olivine. Implications for cation diffusion at low temperatures are discussed.