Experimental transport of Si,Al and Mg in hydrothermal solutions: an application to vein mineralization during high-pressure,low-temperature metamorphism in the French Alps

A study of hydrothermal vein mineralization in meta-argillites subjected to high-pressure, low-temperature metamorphism reveals that ferromagnesian (e.g., chlorite) and pure aluminosilicate (e.g., pyrophyllite) mineralization can be correlated with regimes of increasing and decreasing temperature, respectively. An experimental study of the transport of silica, aluminum and magnesium in hydrothermal solutions has been undertaken to simulate variations in the physical conditions during metamorphism and the accompanying mass transport in a closed system. Thermodynamic and kinetic analysis of the experimental results indicates that local equilibrium among aqueous and mineral phases controls the distribution and composition of hydrothermal vein mineralization and that vein mineralogy can be used to infer the sense of variation of pressure and temperature during metamorphism.     

Oxygen isotope systematics of gem corundum deposits in Madagascar: relevance for their geological origin

The oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in Madagascar to provide a gemstone geological identification and characterization. Primary corundum deposits in Madagascar are hosted in magmatic (syenite and alkali basalt) and metamorphic rocks (gneiss, cordieritite, mafic and ultramafic rocks, marble, and calc-silicate rocks). In both domains the circulation of fluids, especially along shear zones for metamorphic deposits, provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite, sakenite, and corundumite. Secondary deposits (placers) are the most important economically and are contained in detrital basins and karsts. The oxygen isotopic ratios (¹⁸O/¹⁶O) of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of δ¹⁸O (Vienna Standard Mean Ocean Water) between 1.3 and 15.6‰. Metamorphic rubies are defined by two groups of δ¹⁸O values in the range of 1.7 to 2.9‰ (cordieritite) and 3.8 to 6.1‰ (amphibolite). “Magmatic” rubies from pyroxenitic xenoliths contained in the alkali basalt of Soamiakatra have δ¹⁸O values ranging between 1.3 and 4.7‰. Sapphires are classified into two main groups with δ¹⁸O in the range of 4.7 to 9.0‰ (pyroxenite and feldspathic gneiss) and 10.7 to 15.6‰ (skarn in marble from Andranondambo). The δ¹⁸O values for gem corundum from secondary deposits have a wide spread between −0.3 and 16.5‰. The ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of Madagascar have consistent δ¹⁸O values between 3.5 and 6.9‰. Ruby from the placers of Vatomandry and Andilamena has δ¹⁸O values of 5.9‰, and between 0.5 and 4.0‰, respectively. The placers of the Ilakaka area are characterized by a huge variety of colored sapphires and rubies, with δ¹⁸O values between −0.3 and 16.5‰, and their origin is debated. A comparison with oxygen isotope data obtained on gem corundum from Eastern Africa, India, and Sri Lanka is presented. Giant placer deposits from Sri Lanka, Madagascar, and Tanzania have a large variety of colored sapphires and rubies with a large variation in δ¹⁸O due to mingling of corundum of different origin: mafic and ultramafic rocks for ruby, desilicated pegmatites for blue sapphire, syenite for yellow, green, and blue sapphire, and skarn in marbles for blue sapphire.     

Precipitation as a proxy for climate variables: application for hydrological modelling

It has been proposed that linear regression curves can be used to estimate monthly climate variables from observed precipitation. This approach was explored by applying the MGB hydrological model to the Paraná Basin (Brazil). Linear regressions were obtained for 54 climate gauges, and most of them showed at least six months of significant correlation between monthly climate variables (sunlight hours and relative humidity) and precipitation. The regression equations were applied to 5201 raingauges to estimate monthly climate variables and evapotranspiration, and the results were compared with a scenario using long-term climate averages only. The main differences occurred in wetter periods, where negative correlations between monthly precipitation and evapotranspiration were obtained when using precipitation as a proxy. Long-term changes in the hydrological regime were assessed and showed that the effect of precipitation on relative humidity and sunlight hours seems to have a minor effect on the alterations observed in river discharge in the Paraná Basin.     

Dynamics of an outburst flood originating from a small and high-altitude glacier in the Arid Andes of Chile

The rise of total water levels at the coast is caused primarily by three factors that encompass storm surges, tides and wind waves. The accuracy of total water elevation (TWE) forecast depends not only on the cyclonic track and its intensity, but also on the spatial distribution of winds which include its speed and direction. In the present study, the cyclonic winds are validated using buoy winds for the recent cyclones formed in the Bay of Bengal since 2010 using Jelesnianski wind scheme. It is found that the cyclonic winds computed from the scheme show an underestimate in the magnitude and also a mismatch in its direction. Hence, the wind scheme is suitably modified based on the buoy observations available at different locations using a power law which reduces the exponential decay of winds by about 30%. Moreover, the cyclonic wind direction is also corrected by suitably modifying its inflow angle. The significance of modified exponential factor and inflow angle in the computation cyclonic winds is highlighted using statistical analysis. A hydrodynamic finite element-based Advanced Circulation 2D depth integrated (ADCIRC-2DDI) model is used here to compute TWE as a response to combined effect of cyclonic winds and astronomical tides. As contribution of wave setup plays an important role near the coast, a coupled ADCIRC + SWAN is used to perceive the contribution of wind waves on the TWE. The experiments are performed to validate computed surge residuals with available tide gauge data. On comparison of observed surge residuals with the simulations using modified winds from the uncoupled and coupled models, it is found that the simulated surge residuals are better compared, especially with the inclusion of wave effect through the coupled model.     

Pasisar: Performances of a High and Very High Resolution Hybrid Deep-Towed Seismic Device

The Pasisar seismic acquisition system combines a source at the sea surface and a deep-towed single channel streamer. This unconventional device geometry reduces the width of the first Fresnel zone which increases the lateral resolution. However, the device acquisition geometry generates artifacts on seismic profiles and induces large incidence angles of the seismic signal. A specific processing sequence must be applied to the data to obtain a readable seismic section. Penetration of the seismic signal depends on the energy of the signal reaching the seafloor and on its incidence angle. Because of smaller source energy, 800 Joules Sparker data cannot be acquired in water depth larger than 1500 m for example, whereas there is no depth limit for the use of this system with air gun sources. Differential acoustic absorption of seismic frequencies (below 1000 Hz) in the water column is negligible when compared with wave fronts expansion. Thus, the horizontal resolution of any seismic system strongly depends on the frequency spectrum of the seismic source and on the travel distances. Pasisar and conventional seismic profiles being usually simultaneously recorded, we illustrate the interest of using a hybrid seismic device by comparing horizontal resolutions as well as signal-to-noise ratio obtained with both the Pasisar and conventional systems. In addition, by carefully picking time arrivals of a reflection on simultaneously recorded surface and deep-towed seismic records, it is possible to estimate the average interval seismic velocity. We present the simplified example of a horizontal reflector.     

Bedload transport measurements with impact plate geophones: comparison of sensor calibration in different gravel‐bed streams

Indirect bedload transport measurements have been made with the Swiss plate geophone system in five gravel‐bed mountain streams. These geophone sensors record the motion of bedload particles transported over a steel plate mounted flush with the channel bed. To calibrate the geophone system, direct bedload transport measurements were undertaken simultaneously. At the Erlenbach in Switzerland, a moving‐basket sampler was used. At the Fischbach and Ruetz streams in Austria, a Helley–Smith type bedload sampler provided the calibration measurements. A Bunte‐type bedload trap was used at the Rofenache stream in Austria. At the Nahal Eshtemoa in Israel, Reid‐type slot bedload samplers were used. To characterize the response of the geophone signal to bedload particles impacting on the plate, geophone summary values were calculated from the raw signal and stored at one second intervals. The number of impulses, i.e. the number of peaks above a pre‐defined threshold value of the geophone output signal, correlated well with field measured gravel transport loads and was found to be a robust parameter. The relations of impulses to gravel transport loads were generally near‐linear, but the steepness of the calibration relations differed from site to site. By comparing the calibration measurements from the different field sites and utilizing insights gained during preliminary flume experiments, it has been possible to identify the main factors that are responsible for site specific differences in the calibration coefficient. The analysis of these calibration measurements indicates that the geophone signal also contains some information about the grain size distribution of bedload. Copyright © 2013 John Wiley & Sons, Ltd.     

X-ray determination and equilibrium composition of clinopyroxenes in the system CaO-MgO-Al2O3-SiO2

Ternary clinopyroxenes have been synthesized in the plane Di-CaTs-En. The variation of their crystallographic parameters has allowed the development of three determinative grids, which utilize b-, 2 _22¯1–2 ₃₁₀ and 2 ₃₃₀ –2 ₂₀₂ respectively. These grids show significant differences in comparison with the previously proposed ones. Present results have been used to review some data on clinopyroxenes equilibria in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) (Biggar 1969; Bruno and Facchinelli 1978; Herzberg 1978; O'Hara and Schairer 1963; Boyd 1969). In particular the petrogenic grid correlating P, T and CaTs content of clinopyroxenes in spinel-lherzolite assemblage (Herzberg 1978) has been revised, and consequently equilibrium temperatures rise by one hundred degrees.     

The carbon dioxide solubility in alkali basalts: an experimental study

Experiments were conducted to determine CO₂ solubilities in alkali basalts from Vesuvius, Etna and Stromboli volcanoes. The basaltic melts were equilibrated with nearly pure CO₂ at 1,200°C under oxidizing conditions and at pressures ranging from 269 to 2,060 bars. CO₂ solubility was determined by FTIR measurements. The results show that alkalis have a strong effect on the CO₂ solubility and confirm and refine the relationship between the compositional parameter Π devised by Dixon (Am Mineral 82:368–378, 1997) and the CO₂ solubility. A general thermodynamic model for CO₂ solubility in basaltic melts is defined for pressures up to 2 kbars. Based on the assumption that O²⁻ and CO₃²⁻ mix ideally, we have:

Sedimentation in the floodplains of the Mekong Delta,Vietnam. Part I: suspended sediment dynamics

Suspended sediment is the primary source for a sustainable agro‐ecosystem in the Mekong Delta by providing nutrient input for the subsequent cropping season. In addition, the suspended sediment concentration (SSC) plays an important role in the erosion and deposition processes in the Delta; that is, it influences the morphologic development and may counteract the deltaic subsidence and sea level rise. Despite this importance, little is known about the dynamics of suspended sediment in the floodplains of the Mekong Delta. In particular, quantitative analyses are lacking mainly because of data scarcity with respect to the inundation processes in the floodplains. In 2008, therefore, a comprehensive in situ system to monitor the dynamics of suspended sediment in a study area located in the Plain of Reeds was established, aiming at the characterization and quantification of suspended sediment dynamics in the deeply inundated parts of the Vietnamese part of the Mekong Delta. The monitoring system was equipped with seven water quality–monitoring stations. They have a robust design and autonomous power supply suitable for operation on inundated floodplains, enabling the collection of reliable data over a long period of time with a high temporal resolution. The data analysis shows that the general seasonal dynamics of suspended sediment transport in the Delta is controlled by two main mechanisms: the flood wave of the Mekong River and the tidal backwater influences from the coast. In the channel network, SSC decreases exponentially with distance from the Mekong River. The anthropogenic influence on SSC could also be identified for two periods: at the start of the floodplain inundation and at the end of the flood period, when subsequent paddy rice crops are prepared. Based on the results, we recommend an operation scheme for the sluice gates, which intends to distribute the sediment and thus the nutrients equally over the floodplain. Copyright © 2013 John Wiley & Sons, Ltd.