Single-crystal polarized FTIR spectroscopy and neutron diffraction refinement of cancrinite

We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H₂O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow···O donor–acceptor distances >2.7 Å. The FTIR spectra show two main absorptions. The first at 3,602 cm^?1 is polarized for E ⊥ c and is assigned to the ν₃ mode. The second, at 3,531 cm^?1, is also polarized for E ⊥ c and is assigned to ν₁ mode. A weak component at 4,108 cm^?1 could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300–1,500 cm^?1 range are strongly polarized for E ⊥ c, and are assigned to the vibrations of the CO₃ group.     

铊-锗-硫三元素相平衡的实验研究

本文用抽真空石英管在干体系中对Tl-Ge-S三元系在300℃等温切面相关系和沿Tl_2-GeS_2及Tl_2-GeS变温切面假二元系相平衡进行了实验研究。 Tl-Ge-S三元系在300℃为三个液相区与三个二元相、八个三元相(包括三个固溶体)和十四个两相区、二十四个三相区稳定共存的复杂相关系。本实验首次合成了三个三元相(Tl_2Ge_(1-x)S_2、Tl_5Ge_(13)S_(24)、TlGe_3S_5)和三个固溶体相,即S·S·a(Tl_2~+Ge~(4+)S_3～Tl_2~+Ge_2~(2+)S_3)、S·S·b(Tl~+Ge_(0.5)~(2+)Ge_(0.5)~(4+)S_2～Tl~+Ge_3~(2+)S_4),S·S·c(Tl~+Ge_2~(2+)S_3～Tl_2~+Ge_5S_6)。     

Microgranitoid enclaves from the Swifts Creek Pluton SE-Australia: textural and physical constraints on the nature of magma mingling processes in the plutonic environment

The Early Devonian Swifts Creek Pluton, with an areal extent of 35 km² intruded Ordovician meta-flysch sequences along the southwestern margin of the Omeo Metamorphic Belt, southeastern Australia.Characterized throughout by I-type mineralogy the rocks of the pluton vary from tonalite through granodiorite to granite. The granitic rocks are exposed at topographic highs and along the contacts with the country rocks while the tonalites and granodiorites are found towards the center of the pluton at topographic lows. This pattern is interpreted to reflect a compositionally zoned magma chamber. Microgranitoid enclaves of quartz-diorite to tonalite composition comprise 20–25% of the total plutonic rock volume, but account for more than 60% towards the core of the pluton where they are often extremely flattened and stretched, giving the rock a gneissic appearance. The observed flattening can not be accomodated by solid state deformation alone and is thought to reflect plastic deformation in the presence of liquid. Enclaves with chilled margins are common and display textural and modal variations from margin to center which indicate differentiation of isolated magma globules to produce cumulate enriched chilled margins and leucocratic core regions. Quartz and plagioclase megacrysts found in many enclaves are texturally and compositonally very similar to plagioclase and quartz of the host granitoids. Together with occasional, gradational or interfingering contacts between the two rock types they are evidence for a partial mingling process between an essentially liquid enclave melt and a 40–60% crystalline host rock magma to generate the megacrystic enclave type.The large viscosity contrast between the two magmas rules out simple two end-member bulk mixing. Applying recently developed fluid dynamical and thermomechanical concepts (Campbell &Turner, 1986;Furman &Spera, 1985) it appears that interfacial processes such as heat transfer and shearing of host-enclave interfaces controlled the hybridisation which was triggered by turbulent magma injection along a central conduit, now represented by the gneissic zone, into the magma chamber. Hybridisation occurred 10³ to 10⁴ years after pluton emplacement and was accompanied by convective velocities of the comingling magmas in the order of 10¹ to 10^–1 per year.We therfore conclude that in many cases microgranitoid enclaves are not of resitite origin as has been frequently proposed but represent mafic to intermediate magmas that cool rapidly when brought into contact with the host magma. Whether mixing occurrs or not is dependent on the particular boundary conditions, such as temperature and viscosity contrast, injection velocity and feeder geometry.

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Zusammenfassung Der Unterdevonische Swifts Creek Pluton mit einer flächenhaften Ausdehnung von 35 km² intrudierte ordovizische Metaflyschabfolgen entlang des südöstlichen Randes des Omeo Metamorphic Belt, Süd-Ost Australien.Die Gesteine des Plutons haben eine Zusammensetzung, welche von Tonalit bis zu Granit reicht und sind durch eine I-Typ Mineralogie gekennzeichnet. Die granitischen Gesteine sind sowohl auf den topographischen Höhen als auch entlang einiger Kontakte mit dem Flysch ausgebildet. Tonalite und Granodiorite finden sich zum Zentrum des Plutons hin in topographischen Tiefen. Wir interpretieren dieses Verteilungsmuster der Gesteine als eine ehemalige in der Zusammensetzung zonierte Magmenkammer. Mikrogranitoide Enklaven quarz-dioritischer bis tonalitischer Zusammensetzung bestimmen 20–25% des Gesamtgesteinsvolumens, können jedoch zum Pluton Zentrum hin auf über 60% ansteigen. In diesem Bereich sind die Enklaven extrem ausgedünnt und geben dem Gestein daher ein gneisartiges Erscheinungsbild. Diese Ausdünnung kann nicht alleine auf eine tektonische Deformation zurückgeführt werden, sondern wird als eine plastische Deformation des Magmenkörpers in Gegenwart hinreichender Schmelze gedeutet. Enklaven mit feinkörnigen, schnell kristallisierten Randbereichen weisen Variationen sowohl in Textur als auch Mineralbestand von deren Rändern zum Inneren auf, welche auf eine Differentiation isolierter, kugelförmiger Magmenkörper hindeutet. Hierdurch kam es zur Kumulatanreicherung im Randbereich und zur Ausbildung leukokrater Kerngebiete. Viele Enklaven weisen Quarz und Plagioklas-Megakristalle auf, die sowohl in Textur als auch Zusammensetzung dem Quarz und Plagioklas des granitoiden Wirtsgesteines gleich sind. Im Zusammenhang mit seltener auftretenden, gradierten Übergängen zwischen den beiden Gesteinseinheiten werden diese als Beleg für einen teilweisen Vermengungsprozess zwischen einer im wesentlichen flüssigen Enklaven-Schmelze und einem zu 40–60% kristallisierten Wirtsgesteinsmagma gedeutet.Auf Grund der Viskositätsunterschiede der beiden Magmen halten wir eine vollständige Vermischung der beiden Endglieder für unwahrscheinlich. Die Anwendung erst kürzlich entwickelter flüssigkeitsdynamischer und thermo-mechanischer Konzepte legt die Schlußfolgerung nahe, daß Grenzflächenprozesse zwischen den beiden Magmen wie zum Beispiel der Austausch von Wärme als auch Scherung entlang der gemeinsamen Kontakte die Hybridisierungsprozesse bestimmten. Die Hybridisierung wurde durch turbulente Injektion der Enklavenschmelze entlang eines zentralen Förderganges, welcher nun in Form der gneissartigen Zone vorliegt, ausgelöst und erfolgte 10³ bis 10⁴ Jahre nach Platznahme des Plutons und war durch Magmenkonvektionen in der Größenordnung von 10¹ bis 10^–1 Metern pro Jahr begleitet.Wir kommen zu dem Ergebnis, daß in den meisten Fällen mikrogranitoide Enklaven nicht restitischen Ursprungs sind, wie vielfach gefordert, sondern daß es sich um mafische bis intermediäre Magmen handelt, welche schneller Kristallisation unterliegen, sobald diese in Kontakt mit dem granitoiden Magma kommen. Ob es zur Vermischung der beiden Magmen kommt oder nicht, ist abhängig von den jeweiligen Grenzbedingungen, wie zum Beispiel Temperatur- und Viskositätsunterschiede der beiden Magmen sowie Injektionsgeschwindigkeit und Geometrie des Fördersystems.

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Résumé Le pluton dévonien de Swifts Creek, qui occupe une aire d'environ 35 km², a fait intrusion dans des séquences de métaflysch ordoviciens de la marge sud-ouest de la ceinture métamorphique Omeo, située dans le sud-est de l'Australie.Une minéralogie de type I caractérise les roches du pluton dont la composition varie entre les pôles tonalitique, granodioritique et granitique. Les roches granitiques affleurent sur les hauteurs topographiques et le long des contacts avec les roches encaissantes, tandis que les tonalites et les granodiorites sont situées dans la partie centrale du massif, dans des régions en dépression cette disposition pourrait refléter un réservoir magmatique de composition zonée. Des enclaves microgranitoïdes de composition quartz-dioritique et tonalitique forment 20–25% du volume total de la roche plutonique, mais interviennent pour plus de 60% vers le centre du pluton, où elles sont souvent extrêmement aplaties et étirées, donnant à la roche une apparence gneissique. L' aplatissement ne peut être simplement expliqué par une déformation à l'état solide mais purrait refléter une déformation plastique en présence dün liquide. Des enclaves à bordure figée sont communes, qui montrent des variations de texture et de mode depuis leur marge jusqu' à leur centre. Ces variations indiquent que les globules magmatiques isolés se sont différenciés pour produire une bordure figée enrichie en cumulats et un noyau leucocrate. Les mégacristaux de quartz et de plagioclase observés dans de nombreuses enclaves ont une texture et une composition semblables à celles des cristaux de quartz et de plagioclase du granite hôte. Ces mégacristaux, de même que l'existence occasionnelle de contacts graduels ou interdigités entre les deux types de roches, témoignent d'un processus de mélange partiel entre des enclaves essentiellement liquides et un magma hôte renfermant 40 à 60% de cristaux.L'important contraste de viscosité entre les deux magmas élimine le simple mélange de deux termes de composition extrême. En appliquant les conceptions récentes de dynamique des fluides et de thermomécanique, il devient apparent que les processus interfaciaux, comme le transfert de chaleur et le cisaillement des interfaces enclave-hôte, out déterminé l'hybridation, declenchée par des injections turbulentes de magmas le long d'un conduit central, maintenant représenté par la zone gneissique, dans la chambre magmatique. L'hybridation s'est produite de 1.000 à 10.000 ans après l'emplacement du pluton et a été accompagnée par une vitesse convective de l'ordre de 1 à 10 m/an des magmas mélangés.Nous concluons que dans de nombreux cas, les enclaves microgranitoïdes ne sont pas d'origine restique comme il est souvent suggéré, mais quíls représentent plutôt des magmas de composition mafique à intermédiaire qui se refroidissent rapidement lorsqu'ils sont mis en contact avec le magma hôte. Que le mélange magmatique s' opère ou non dépend des conditions limites particulières, telles que le contraste des températures et des viscosités, la vitesse d'injection et la géométrie des conduits magmatiques.

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Anomalous isotopic compositions of Sr,Ar and O in the Mesozoic diabase dikes of Liberia,West Africa

The Mesozoic diabase dikes of Liberia are tholeiites whose ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr ratios scatter widely on the Rb-Sr isochron diagram. The problem is attributed to differences in the initial ⁸⁷Sr/⁸⁶Sr ratios of these rocks which range from 0.70311 to 0.70792, assuming a uniform age of 186 Ma for the dikes and using (⁸⁷Rb)=1.42 × 10^–11y^–1. The range of values is similar to that observed in the Mesozoic basalt flows and dikes of other Gondwana continents.New whole-rock K-Ar dates confirm previous conclusions that the diabase dikes in the Liberian and Pan-African age provinces of Liberia absorbed extraneous ⁴⁰Ar after intrusion. Only the dikes in the Paynesville Sandstone have K-Ar dates that range from 117 Ma to 201 Ma and may not contain extraneous ⁴⁰Ar. However, dikes from all three age provinces of Liberia have elevated initial ⁸⁷Sr/⁸⁶Sr ratios. These results indicate that contamination with radiogenic ⁸⁷Sr occurred primarily before intrusion of the magma whereas the addition of extraneous ⁴⁰Ar occurred after emplacement and reflects the age and mineral composition of the country rock.The ¹⁸O values of the Liberian diabase range from +5.6/% to +9.10/% and correlate positively with initial ⁸⁷Sr/⁸⁶Sr ratios. The data can be modeled by fractional crystallization and simultaneous assimilation of crustal rocks by the magma. However, samples containing amphibole and biotite replacing pyroxene deviate from the Sr-O isotope trajectories of the model and appear to have been depleted in ¹⁸O and enriched in ⁸⁷Sr by interactions with groundwater at high temperature.Laboratory for Isotope Geology and Geochemistry Contribution No. 76     

Petrogenesis of the Kirkpatrick Basalt,Solo Nunatak,northern Victoria Land,Antarctica, based on isotopic compositions of strontium,oxygen and sulfur

Chemical and isotopic compositions of Jurassic tholeiites of the Kirkpatrick Basalt Group from Solo Nunatak, northern Victoria Land, indicate that these rocks are contaminated with crustal material. The basalts are fine grained and contain phenocrysts of augite, pigeonite, hypersthene and plagioclase. The flows on Solo Nunatak are chemically more similar to average tholeiite than flows from Mt. Falla and Storm Peak in the Central Transantarctic Mountains (TAM) which appear to be more highly differentiated. Initial ⁸⁷Sr/⁸⁶Sr ratios of the flows on Solo Nunatak are high (>0.710) and are similar to those reported for the Kirkpatrick Basalt in the Central TAM. Whole-rock δ¹⁸O values are also high, ranging from +6.0 to +9.3‰ and correlate positively with initial ⁸⁷Sr/⁸⁶Sr ratios, similar to the Kirkpatrick Basalt in the Central TAM. The correlation between initial ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values is explained as the result of simultaneous fractional crystallization and assimilation of a crustal contaminant. Sulfur isotope compositions vary between limits of δ³⁴S= -4.01 to +3.41‰ Variations in (δ³⁴S probably resulted from outgassing of SO₂ under varying oxygen fugacities. Laboratory for Isotope Geology and Geochemistry (Isotopia), Contribution No. 71     

The role of speciation in alkaline igneous fluids during fenite metasomatism

Fenites associated with alkaline igneous rocks show a progression from a high temperature assemblage consisting of sodium-rich alkali feldspar + a sodium-ironrich mafic mineral, to an extreme end member assemblage consisting of pure potassium feldspar + iron oxide. The latter assemblage is only found in association with low temperature carbonatites. Segments of this distribution trend can be found in the contact aureole of single intrusive centers.In the east African Kisingiri nephelinite volcano, ijolite intruded a granodioritic basement, producing a fenitized contact aureole. During metasomatism of granodiorite, according to the mass balance model of Rubie (1982), feldspar only participated in an alkali exchange reaction, while quartz was replaced by sodic pyroxene. Outward from the intrusive contact, with decreasing temperature, feldspar became progressively K-enriched, while pyroxene was enriched in the acmite component. It is predicted that alkaliexchange reactions were controlled by NaCl⁰-KCl⁰ aqueous complexes close to the intrusive contact, while further out in the aureole Na⁺-K⁺ ions dominated at the lower temperatures and enhanced the level of potassium metasomatism of feldspar. With decreasing temperature in the aureole, the K/Na ratio of the fluid decreased and consequently the activity of acmite increased.Around carbonatites, where the level of CO₂ in the escaping fluid can be expected to be high, Na₂CO ₃ ⁰ -K₂CO ₃ ⁰ complexes may dominate. Alkali exchange between feldspar and these aqueous species enhances, even further, the stabilization of pure potassium feldspar. Boiling may also play an important role in potassium metasomatism as carbonatites are frequently associated with pyroclastic rocks. Formation of hematite instead of sodic pyroxene may be attributed to low , high and a CO₂-rich fluid.Important variables which determine the products of alkali metasomatism are shown to be temperature, pressure and CO₂ content of the fluid, as well as the K/Na ratio of the fluid.     

Analysis and interpretation of the paleogeographic distribution of marine sedimentary phosphorites

World-wide distribution of marine phosphorite deposits has been plotted on computergenerated paleogeographic maps of specific geologic time intervals in order to determine if and when optimum arrangements of continents and oceans existed for phosphorogenesis. Geographic input data for the program HYPERMAP consists of digitized continental edges and bathymetry and appropriate coordinates for discrete continental structural blocks. Paleopole and block rotation data were used to construct maps for the following time intervals; Recent, 20, 40, 60, 80, 100, 120, 140, 170–190, 250, 280, 325, 350, 375, 400, 450, 500, and 600 Ma. Phosphorite deposits were plotted directly on the appropriate map according to location and age. The major phosphorites have been divided into four types (A, B, C, D)on the basis of the environment of deposition and facies association. A and B types were deposited in water depths of several hundred meters in association with black shales, chert, and dolomite. In terms of reserves of P ₂ O ₅ they show marked distribution peaks in the Lower and Middle Cambrian and the Permian. C and D types were deposited in water depths of tens of meters in association with quartz siltstones, quartz mudstones, and carbonates. Time distribution peaks in the Lower and Middle Cambrian, the Ordovician, Upper Cretaceous-Eocene, and the Miocene are evident for these deposits. The plotted distributions may be related to a recent model of phosphorite deposition (Sheldon, 1980)which suggests that phosphate levels in ocean water have fluctuated with time. Phosphate content built up during times of high-level warm seas was brought into favorable depositional sites by equatorial and tradewind belt upwelling currents. The paleogeographic analysis shows that maximum phosphorite deposition from equatorial upwelling (type C)occurred north and south of narrow constricted equatorial seaways, at times of high-level warm seas. Maximum deposition of phosphorites from tradewind belt upwelling (types A, B, C, D)occurred when polar oceans were wide and there were extensive north-south trending coasts in low latitudes, at times of transition from high-level warm seas to low-level cool seas.     

A simulation study of the errors of omission and commission for GRACE RL01 gravity fields

This paper examines the influence that certain omission and commission errors can have on the gravity field models estimated from the initial release of data (RL01) from the Gravity Recovery And Recovery Experiment (GRACE) satellite mission. The effects of omission errors were analyzed by limiting the degree and order to which the GPS and K-band range-rate (KBR) measurement partials were extended in the solution process. The commission error studies focused on the impact of an imperfect mean reference gravity field model on the solution. Combinations of both of these error sources were also explored. The nature of these errors makes them difficult to distinguish from the true gravity signal, so the exploration of these error sources was performed using simulations; however, comparisons to real-data solutions are provided. The results show how each of the specific error sources investigated influences the gravity field solution. The simulations also show how all of the errors examined can be sufficiently mitigated through the appropriate choice of processing parameters.     

A novel approach to the analytical modelling of an auger conveyor system for lunar regolith transportation

Acta Geotechnica - Auger-based transportation systems are a promising method to transport lunar regolith for in situ resource utilisation. An analytical model based on terrestrial auger conveyor...