Genesis of the three concentrically-zoned granitoid Plutons of Cauterets-Panticosa (French and Spanish Western Pyrenees)

The petrogenesis of the Hercynian plutons of Cauterets-Panticosa was deduced from detailed cartographic, petrographic, chemical (> 300 analyses) and mineralogical evidence. The three adjacent calc-alkaline plutons were derived from a siliceous continental magma modified to varying degrees by hybridization with a mafic mantle-derived magma. The mafic inclusions are evidence for the mafic parent magma. Each of the three plutons is made up of several more or less concentrically disposed granitoid units. This concentric zonation is either recurrent (West Cauterets Pluton), or normal (Panticosa Pluton), or both normal and recurrent (East Cauterets Pluton). The combination of several processes (hybridization, differentiation, multiple intrusion, subterranean collapse) was responsible for the genesis of the three plutons.

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Zusammenfassung Diese Publikation gibt eine Übersicht der kartographischen, petrographischen und chemisch-mineralogischen (> 300 Analysen) Arbeiten, die in den granitoïden Massiven von Cauterets-Panticosa unternommen wurden. Es handelt sich um drei kalkoalkaline Massive, die nebeneinanderliegen, und die während der variskischen Orogenese in einem metasedimentären, gefalteten, epizonalen Zusammenhang entstanden sind. Die Herkunft der Granitoïde von Cauterets-Panticosa wird auf Grund der Ergebnisse der vergleichenden chemisch-mineralogischen Arbeiten zwischen diesen Granitoïden mit ihren basischen Einschlüssen diskutiert. Diese Granitoïde stammen aus vier Grund-Magmen. Jedes dieser vier Magmen soll durch Hybridation mit einem erdmantel-basischen Magma eine mehr oder weniger stark veränderte Fraktion eines sialischen, sauren Magmas darstellen. Die Einschlüsse weisen auf das erdmantel-basische Magma hin. Der Ursprung all dieser Granitoïde läge gleichzeitig in der kontinentalen Kruste und im Mantel. Jeder dieser drei Massive von Cauterets-Panticosa besitzt eine zonale, konzentrische Struktur. Im westlichen Massiv von Cauterets ist diese Struktur rekurrent. Im Massiv von Panticosa ist sie normal und regelmäßig und komplex im östlichen Massiv von Cauterets. Die Beobachtungen im Gelände und die zahlreichen chemisch-mineralogischen Angaben geben Aufschluß über den Ursprung jeder dieser drei Strukturen ebenso wie über die Granitoïd-Einheiten, die sie zusammensetzen. Die Genese der drei Granitoïd-Einheiten des westlichen Massivs wird durch magmatische Hybridationsprozesse erklärt. Die rekurrente Struktur dieses Massivs ist hingegen an vielfache Injektionen und an ein intrakrustales Zusammenschmelzen älterer Granitoïde gebunden. Die magmatische Differentiation einer einzigen Intrusion — hauptsächlich in größerer Tiefe — vor und während der Platznahme, wäre für die Genese der vier granitoïden Haupteinheiten, und für die normale und gleichzeitige Struktur des Massivs von Panticosa verantwortlich. Die fünf granitoïden Haupteinheiten und die komplexe Struktur des östlichen Massivs würde aus einer Kombination der Prozesse, die im westlichen Massiv und im Massiv von Panticosa beobachtet wurden, hervorgehen.

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Résumé Cette note dresse le bilan pétrogénétique des travaux cartographiques, pétrographiques et chimico-minéralogiques (> 300 analyses) détaillés récemment effectués sur les granitoïdes et leurs enclaves des massifs de Cauterets-Panticosa. Il s'agit de trois massifs calcoalcalins, juxtaposés sur le terrain et d'âge hercynien. L'étude comparée des granitoïdes et de leurs enclaves «microgrenues basiques» montre que ces massifs dériveraient d'un magma continental acide plus ou moins modifié par hybridation avec un magma mantellique basique. Par ailleurs, chacun des trois massifs se compose de plusieurs unités granitoïdes disposées de façon concentrique. Cette structure concentrique est soit récurrente (massif occidental de Cauterets), soit normale et régulière (massif de Panticosa), soit, enfin, tout à la fois normale et récurrente (massif oriental de Cauterets). La combinaison de plusieurs processus (hybridation sélective ou globale, différenciation, intrusions multiples, effondrements souterrains) serait responsable de la genèse des ces trois massifs.

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On the lognormality of the small-scale structure of turbulence

Higher-order moments of turbulent velocity gradients and their behavior with Reynolds number were measured in the nearly isotropic turbulent field generated by a square-mesh grid and in a turbulent boundary layer along a flat plate with zero pressure gradient. Hot-wire anemometry and instrumentation combining analog and digital methods were used to measure moments up to the fourteenth order. Measurements of such high-order moments required that particular attention be given to their validity. Involved herein was the evaluation of such effects as nonlinearity, averaging intervals, and the adequacy of the statistics for the tails of the probability density distributions. The results obtained are compared with those of other investigators for a variety of flow configurations in the laboratory as well as in the atmosphere. The concept of the intermittency of the small-scale structure and the theoretical approach involving lognormality of the probability density distribution of the dissipation rate are evaluated.     

5α(H)-cholestan-3α-ol in sediments: Characterization and geochemical significance

The top five centimeters of sediment collected at the deepest point of Lake Léman (Switzerland), 309 meters below water level, contain concentrations of 5α(H)-cholestan-3α-ol (epicholestanol) up to 10% of the total sterol content. Isobutane chemical ionization mass spectrometry and coinjection with an authentic standard on a SP-1000 glass capillary column were used in order to successfully characterize this epimer of cholestanol.The distribution of this stanol throughout the sediment core studied suggests in situ bacterial production. The change in concentration of epicholestanol with depth is different from those found for other C₂₇ stanols, such as cholestanol or coprostanol. It is probably the result of a change in the bacterial fauna in the most recently deposited sediment, related to the increasing eutrophication of the lake rather than the consequence of its lower stability due to the axial conformation of the OH substituent.     

Behaviour of granites and limestones subjected to slow and homogeneous temperature changes

We have studied the evolution of the texture in two granites and two limestones subjected to slow and uniform temperature change. Each granite has a different grain size and each limestone a different texture: Carrare crystalline limestone and Crepey oolitic limestone. Temperature was varied from 200°C to 700°C. Scanning Electron Microscope observations of different rock samples show that during thermal cycling intercrystalline boundaries in granites widen out progressively and porosity increases. New microcracks appear in crystals between 500°C and 600°C. In Carrare crystalline limestone, intercrystalline cracks appear at temperatures as low as 200°C. Due to its heterogeneous cracks appear at temperatures as low as 200°C. Due to its heterogeneous increasing temperature. Longitudinal wave velocity and intrinsic permeability show good correlation with the Scanning Electron Microscope observations.     

The phoshate mineral associations of the Tsaobismund pegmatite,Namibia

A detailed mineralogical investigation using the classical methods of identification by X-ray diffraction and by optical properties in thin sections, has revealed thirty one phosphate minerals occurring in the Tsaobismund pegmatite. This investigation is complemented by wet chemical and, mainly, electron microprobe analyses performed on the phosphates known to be typomorphic or considered to be relevant to the hydrothermal alteration. Additionally, microprobe analyses are also given for garnet, gahnite, and ferrocolumbite associated with the phosphates. On the basis of their chemical composition, particularly in terms of their Fe, Mn, and Mg contents, three types of triphylites are distinguished. Triphylite 1 only occurs as a primary phase, triphylite 2 shows exsolution lamellae of sarcopside, and triphylite 3 is partly replaced by a fluorophosphate of the triplite-zwieselite series. These minerals constitute three generations of the parent phases, which were progressively transformed by metasomatic processes, hydrothermal alteration, and by weathering, to give finally three types of complex associations. The Li(Fe,Mn)PO₄ minerals appear to be more sensitive to such transformations than those of the (Fe,Mn)₂PO₄F series. Four main stages of hydrothermal alteration processes have been recognized in the Tsaobismund pegmatite: (i) the Mason-Quensel sequence results from a progressive oxidation of Fe and Mn, and a concomitant Li-leaching of triphylite yielding ferrisicklerite and heterosite, successively; (ii) the metasomatic exchange of Na for Li produces alluaudite; in the present case, the formation of hagendorfite from triphylite 2 is considered to be earlier than the generation of alluaudite-Na occurring in the three associations; (iii) the hydration phase mainly transforms the parent Li(Fe,Mn)PO₄ phase into grey hureaulite, associated with barbosalite and tavorite; (iv) the formation of fluorapatite, not particularly widespread, replaces alluaudite-Na, as well as zwieselite s.l. The following crystallization sequence of the initially formed phosphate minerals is proposed: triphylite 1 triphylite 2 + sarcopside (associated with garnet) triphylite 3 + zwieselite s.l. The most prominent feature of this succession is the increase in the Mg and Zn contents in the composition of the phosphates, as well as the decrease in their Li contents. The variations of the Fe/Mn ratios in this sequence are discussed. The succession triphylite-zwieselite within weakly differentiated and Li-poor pegmatites is of general significance.     

18O16O ratios in Luna 16 fines

Partial fluorination experiments developed on a 26 mg sample of Luna 16 fines show a big ¹⁸O enrichment in the first oxygen evolved similar to those observed by Epstein and Taylor on Apollo samples and probably related to the high solar wind content of this sample.     

Metasomatism in the subcontinental mantle beneath the Eastern Carpathians (Romania): new evidence from trace element geochemistry

A wide range of trace elements have been analysed in mantle xenoliths (whole rocks, clinopyroxene and amphibole separates) from alkaline lavas in the Eastern Carpathians (Romania), in order to understand the process of metasomatism in the subcontinental mantle of the Carpatho-Pannonian region. The xenoliths include spinel lherzolites, harzburgites and websterites, clinopyroxenites, amphibole veins and amphibole clinopyroxenites. Textures vary from porphyroclastic to granoblastic, or equigranular. Grain size increases with increasing equilibrium temperature of mineralogical assemblages and results from grain boundary migration. In peridotites, interstitial clinopyroxenes (cpx) and amphiboles resulted from impregnation and metasomatism of harzburgites or cpx-poor lherzolites by small quantities of a melt I with a melilitite composition. Clinopyroxenites, amphibole veins and amphibole clinopyroxenites are also formed by metasomatism as a result of percolation through fracture systems of large quantities of a melt II with a melanephelinite composition. These metasomatic events are marked by whole-rock enrichments, relative to the primitive mantle (PM), in Rb, Th and U associated in some granoblastic lherzolites and in clinopyroxene and amphibole veins with enrichments in LREE, Ta and Nb. Correlations between major element whole-rock contents in peridotites demonstrate that the formation of interstitial amphibole and clinopyroxene induced only a slight but variable increase of the Ca/Al ratio without apparent modifications of the initial mantle composition. Metasomatism is also traced by enrichments in the most incompatible elements and the LREE. The Ta, Nb, MREE and HREE contents remained unchanged and confirm the depleted state of the initial but heterogeneous mantle. Major and trace element signature of clinopyroxene suggests that amphibole clinopyroxenites and some granoblastic lherzolites have been metasomatized successively by melts I and II. Both melts I and II were Ca-rich and Si-poor, somewhat alkaline (Na > K). Melt I differed from melt II in having higher Mg and Cr contents offset by lower Ti, Al, Fe and K contents. Both were highly enriched in all incompatible trace elements relative to primitive mantle, showing positive anomalies in Rb, Ba, Th, Sr and Zr. They contrasted by their Ta, Nb and LREE contents, lower in melt I than in melt II. Melts I and II originate during a two-stage melting event from the same source at high pressure and under increasing temperature. The source assemblage could be that of a metasomatized carbonated mantle but was more likely that of an eclogite of crustal affinity. Genetic relationships between calc-alkaline and alkaline lavas from Eastern Carpathians and these melts are thought to be only indirect, the former originating from partial melting of mantle sources respectively metasomatized by the melts I and II. Received: 17 March 1997 / Accepted: 14 July 1997     

Ordering-based nonlinear solver for fully-implicit simulation of three-phase flow

The Fully Implicit Method (FIM) is often the method of choice for the temporal discretization of the partial differential equations governing multiphase flow in porous media. The FIM involves solving large coupled systems of nonlinear algebraic equations. Newton-based methods, which are employed to solve the nonlinear systems, can suffer from convergence problems—this is especially true for large time steps in the presence of highly nonlinear flow physics. To overcome such convergence problems, the time step is usually reduced, and the Newton steps are restarted from the solution of the previous (converged) time step. Recently, potential ordering and the reduced-Newton method were used to solve immiscible three-phase flow in the presence of buoyancy and capillary effects (e.g., Kwok and Tchelepi, J. Comput. Phys. 227(1), 706–727 9). Here, we improve the robustness of the potential-based ordering method in the presence of gravity. Furthermore, we also extend this nonlinear approach to interphase mass transfer. Our algorithm deals effectively with mass transfer between the liquid and gas phases, including phase disappearance (e.g., gas going back in solution) and reappearance (e.g., gas coming out of solution and forming a separate phase), as a function of pressure and composition. Detailed comparisons of the robustness and efficiency of the potential-based solver with state-of-the-art nonlinear/linear solvers are presented for immiscible two-phase (Dead-Oil), Black-Oil, and compositional problems using heterogeneous models. The results show that for large time steps, our nonlinear ordering-based solver reduces the number of nonlinear iterations significantly, which leads to gains in the overall computational cost.