Mineralogy and geochemical behaviour during weathering of greenstone belt under tropical dry conditions in the extreme North Cameroon (Central Africa)

Mineralogy, major, trace and rare earth elements of a weathering profile developed on tertiary greenstone belt in the extreme North Cameroon are reported. The aim of which was to investigate mineralogical evolution and element mobilization and redistribution during weathering under dry tropical climate. The weathering profile consists of four main horizons: (1) a spheroidal weathering zone constituted by a corestone–shell complex, (2) a C horizon, (3) a Bw horizon and an Ah horizon. The results indicate that nontronite, a Fe-rich smectite, is the exclusive clay mineral formed in the exfoliated shells and the C horizon. It is associated with kaolinite in the upper horizons. The coexistence of these two clay minerals induced a decrease of CEC and pH which becomes neutral. The weathering index (WI) values reveal that weathering becomes more and more intensive from the corestone up to Bw horizon, which is the most weathered horizon in the weathering profile. Mass balance calculations, using Th as immobile element, indicate that Ti is quite mobile and that Al and Fe are relatively enriched at the bottom and strongly leached at the top of the profile. Alkalis and alkaline earth elements are strong leached through out the profile, except Ca which displays similar trend as Al and Fe. The same goes for LILE (Cs, Sr), TTE (Cr, Co, Ni) and HSFE (Y, Nb, Hf). In opposite, REE are depleted at the bottom and enriched in the upper horizons, with more enrichment for LREE than for HREE. It appears that weathering of greenstone belt causes a fractionation of HREE and induces a concentration of LREEs. Ce and Eu anomalies display opposite behaviour.     

Coupled discrete element and finite volume solution of two classical soil mechanics problems

One dimensional solutions for the classic critical upward seepage gradient/quick condition and the time rate of consolidation problems are obtained using coupled routines for the finite volume method (FVM) and discrete element method (DEM), and the results compared with the analytical solutions. The two phase flow in a system composed of fluid and solid is simulated with the fluid phase modeled by solving the averaged Navier–Stokes equation using the FVM and the solid phase is modeled using the DEM. A framework is described for the coupling of two open source computer codes: YADE-OpenDEM for the discrete element method and OpenFOAM for the computational fluid dynamics. The particle–fluid interaction is quantified using a semi-empirical relationship proposed by Ergun [12]. The two classical verification problems are used to explore issues encountered when using coupled flow DEM codes, namely, the appropriate time step size for both the fluid and mechanical solution processes, the choice of the viscous damping coefficient, and the number of solid particles per finite fluid volume.     

Criterion of stability and implementation issues of hybrid frequency-time-domain procedure for non-linear dynamic analysis

A criterion of stability pertaining to the hybrid frequency-time-domain procedure is derived. It stems from the initial-value theorem on one side and from the condition of convergence which applies to the harmonic counterpart of the procedure on the other side. The criterion depends on the dynamic stiffnesses of both the original non-linear system and the pseudo-linear system evaluated at the Nyquist frequency. Since at this frequency both stiffnesses are usually controlle by the mass, the criterion of stability is satisfied for most structural systems. The procedure must, however, be applie sequentially to individual time segments. The validity of this criterion is confirmed by a numerical investigation performe for a SDOF system. The solution of non-linear dynamic problems requires consideration of implementation issues which are also discusse in the paper. The analysis of a non-linear soil-structure-interaction system in which the soil's stiffness coefficients an directly defined in the frequency domain also demonstrates the accuracy of the hybrid frequency-time-domain procedure.     

Determination of Fluorine in Eighty International Geochemical Reference Samples by Proton Induced Gamma Ray Emission Spectrometry (PIGE)

Fluorine content has been determined non destructively in eighty international geochemical reference samples (GRS) including rocks, soils, sediments, minerals and ores, by proton induced gamma ray emission spectrometry (PIGE) using the nuclear reaction ¹⁹F(p,αγ) ¹⁶O. Under our operating conditions, the detection limit is around 25 ppm. Our analytical results are compared with the presently preferred values when available or with other published data. The general agreement between this work and reported values is reasonably good. The present paper includes results on several GRS for which existing data are rather scarce or even inexistant.     

Théorie du pendule de Zöllner et du pendule de Lettau

Summary The theory of a Zöllner's horizontal pendulum with perfect wires of suspension is made. We use the Euler's angles of the axes of a system fixed in the moving body relatively to fixed axes and a fourth angle caracterising the amplitude of a motion of translation. We give the conditions for the decoupling of the four types of motion (pure rotation, pitching, rolling, translation) and the equations of forced motions.A rigorous theory of Lettau's double pendulum is initiated. We suppose that the primary pendulum has only one degree of freedom. First the case of a secondary pendulum not disturbing the primary one is examined, and then the case of an indifferent primary pendulum, wich enables us to understand what happens in the normal case. The potential energy of the secondary pendulum is not always a minimum in the central position when the plane of symmetry is vertical and it is possible that parasitesolutions appear, stabler than the central one. This study is extended to the case of a non-vertical plane of symmetry and indications are given for the study of the free motions of the system.In conclusion we could say that this system is simple only at first sight. Its use would be necessary in the case of a real great drift of the underground of the station. But in the contrary case ordinary simple pendulums are sufficient and more easy to understand.

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Resumo La movoj de horizontala Zöllnera pendolo estas difinataj pere de la Euleraj anguloj de triaksa sistemo ligita al la pendolo, rilate al fiksa sistemo, kaj de kvara angulo rilata al translacia movado. Ni donas la kondiojn necesajn por la senkuplado de tiuj kvar movoj (pura rotacio, tangado, ruligado, translacio) kaj la ekvaciojn de la devigataj movoj.La teorio de Lettaùa duobla pendolo estas multe pli kompleksa. Supozente ke la primara pendolo havas nur unu gradon de libereco, ni studas unue la movojn de la sistemo kiam la sekundara pendolo ne perturbas la primaran aù kiam tiu-i estas en indiferenta ekvilibro. Tio permesas kompreni tion kio okazas en la normala kazo. ar la potenciala energio de la sekundara pendolo ne estas iam minimuma en la centra pozicio kiam la simetria ebeno estas vertikala, parazitaj pozicioj povas ekzisti kiuj estas pli stabilaj ol la centra. Ni ekzamenas ankaù la kazon de nevertikala ebeno de simetrio kaj indikas kiel studi la liberajn movojn de la sistemo.

     

Modelling observed and future runoff from a glacierized tropical catchment (Cordillera Blanca, Perú)

Monthly runoff from the 34.3% glacierized tropical catchment of Llanganuco in the tropical Cordillera Blanca, Perú, is successfully simulated and compared with a measured 44 year time series. In the investigation area, the climate is characterized by all-year round homogenous temperature conditions and a strong variability in air humidity and moisture content of the atmosphere. Thus, contrary to the mid latitudes, the seasonal variation in glacier melt strongly depends on moisture-related variables, rather than on air temperature. The here presented ITGG-2.0-R model aims for these requirements. The lack of moisture-related input data other than precipitation demands for an intermediate calibration step. Net shortwave radiation, the emissivity of the atmosphere and a sublimation/melt ratio are related to precipitation amounts. Runoff is well simulated and correlates with the measured record with r² = 0.76. Seasonally obtained r² are only slightly smaller. On a long-term, the cumulative deviation is minor, and the mean annual cycle of runoff is reproduced rather well (r² = 0.99). Based on four different IPCC climate change scenarios, future runoff is simulated. All runoff scenarios are modelled for the respective steady-state glacier extent. This leads to a reduction in the glacier size and a decreased amount of glacier melt. On the other hand, direct runoff increases due to larger glacier free areas. Consequently, mean annual runoff remains almost unchanged, but the seasonality intensifies considerably with more runoff during the wet and less runoff during the dry season.     

Alteration geochemistry of the Nopal I uranium deposit (Sierra Peña Blanca,Mexico), a natural analogue for a radioactive waste repository in volcanic tuffs

Natural analogues provide an approach to characterize and test the long‐term modelling of a repository performance. This article presents geochemical information about the alteration conditions of the Nopal I uranium deposit, Mexico, an analogue for the proposed Yucca Mountain radioactive waste repository. Mineralization and hydrothermal alteration of volcanic tuffs are contemporaneous, according to petrographic observations. Trace element geochemistry (U, Th, REE) provides evidence for local mobilization of uranium under oxidizing conditions and further precipitation under reducing conditions. O‐ and H‐isotope geochemistry of kaolinite, smectite, opal and calcite suggests that argillic alteration proceeded at shallow depth with meteoric water at 25–75 °C, a low‐temperature context, unusual for volcanic‐hosted uranium deposits. This temperature range is compatible with some post‐closure evolution models of the proposed Yucca Mountain repository.