Infrared vibrational spectra of Beta-Phase Mg2SiO4 and Co2SiO4 to Pressures of 27 GPa

Infrared absorption spectra of the high-pressure polymorphs β-Mg₂SiO₄ and β-Co₂SiO₄ have been measured between 0 and 27 GPa at room temperature. Grüneisen parameters determined for 11 modes of β-Mg₂SiO₄ (frequencies of 300 to 1,050 cm^?1) and 5 modes of β-Co₂SiO₄ (490 to 1,050 cm^?1) range between 0.8 and 1.9. Averaging the mid-infrared spectroscopic data for β-Mg₂SiO₄ yields an average Grüneisen parameter of 1.3 (±0.1), in good agreement with the high-temperature thermodynamic value of 1.35. Similarly, we find a value of 1.05 (±0.2) for the average spectroscopic Grüneisen parameter of β-Co₂SiO₄.     

Observed dependence of the water vapor and clear-sky greenhouse effect on sea surface temperature: comparison with climate warming experiments

This study presents a comparison of the water vapor and clear-sky greenhouse effect dependence on sea surface temperature for climate variations of different types. Firstly, coincident satellite observations and meteorological analyses are used to examine seasonal and interannual variations and to evaluate the performance of a general circulation model. Then, this model is used to compare the results inferred from the analysis of observed climate variability with those derived from global climate warming experiments. One part of the coupling between the surface temperature, the water vapor and the clear-sky greenhouse effect is explained by the dependence of the saturation water vapor pressure on the atmospheric temperature. However, the analysis of observed and simulated fields shows that the coupling is very different according to the type of region under consideration and the type of climate forcing that is applied to the Earth-atmosphere system. This difference, due to the variability of the vertical structure of the atmosphere, is analyzed in detail by considering the temperature lapse rate and the vertical profile of relative humidity. Our results suggest that extrapolating the feedbacks inferred from seasonal and short-term interannual climate variability to longer-term climate changes requires great caution. It is argued that our confidence in climate models' predictions would be increased significantly if the basic physical processes that govern the variability of the vertical structure of the atmosphere, and its relation to the large-scale circulation, were better understood and simulated. For this purpose, combined observational and numerical studies focusing on physical processes are needed.     

Upper mantle lateral heterogeneities and magnetotelluric daily variation data

We use telluric and magnetic data of the diurnal variation recorded in Europe, Australia and North America to study the magnetotelluric tensor in the 6h–24h period range. We use associate directions and we eliminate the effects of deviation of telluric currents. We thus obtain for each observatory reliable phases and apparent resistivity values representative of the neighbouring stratified substratum. It appears that the values obtained in the four European observatories (Saint-Maur, France; Ebro, Spain; Toledo, Spain; Nagycenk, Hungary) give similar results and that these results are different from those obtained either in Tucson (USA) or in Watheroo (Australia).Using Bostick transform we interpret these phase and apparent resistivity values in terms of conductivity of the upper mantle. We discuss then the conductivity heterogeneities in terms of change either in temperature, or partial melting or percentage of fluids of the upper mantle: at depths of about 300 km, the upper mantle appears to be 100 °C hotter under Australia than under Europe; the probable presence of fluids at depths about 100 km in the southwestern North America upper mantle appears to be responsible for the high observed conductivities. All these conductivity values are coherent with tomography results from Woodhouse and Dziewonsky: high (low) conductivities are cohernet with low (high)seismic wave velocities.     

Isotopic study of the Manaslu granite (Himalaya,Nepal): inferences on the age and source of Himalayan leucogranites

A detailed isotopic study of the Manaslu leucogranite was carried out. A U-Pb age of 25 Ma and a whole rock Rb-Sr age isochron of 18 Ma were obtained, suggesting that the magmatic activity lasted at least 7 Ma. Initial Sr isotopic ratios are very high (0.740 to 0.760) and initial Nd isotopic ratios are low ( _Nd ⁱⁿ : –13 to –16), and they show the existence of large isotopic variations even at the metre scale. These are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes (e.g. fusion, mixing by convection). These results also support the crustal origin of this leucogranite. The Tibetan slab paragneisses, whose Sr and Nd isotopic ratios are very similar to those of the granite at an age of 20 Ma, are the most probable parental material. Nd model ages for both the leucogranite and the gneisses are in the range 1.5–2 Ga. A model of formation of the Manaslu granite by coalescence of different batches of magma is in agreement with the present data.     

A new method for sampling and analyzing volcanic sublimates — application to Merapi volcano, Java

A new method for the sampling of sublimates from high-temperature volcanic gases has been used at Merapi volcano, Java, in 1978. The sublimates were collected on the inner walls of silica tubes introduced into fumarolic vents. Volcanic gases were allowed to move freely through the tubes and as they cooled, a fraction of the volatile components condensed on the inner walls of the tubes along the temperature gradient. The sublimates were then analyzed by a combination of light microscopy, scanning electron microscopy, electron microprobe and X-ray diffraction.Six successive zones of different compositions and mineralogical associations have been identified along the covered range of temperatures (900° to around 400°C). From the high to the low temperatures, these zones are composed of: (1) cristobalite, magnetite, hercynite; (2) molybdenite; (3) acmite; (4) halite, sylvite; (5) sphalerite, pyrite; and (6) galena. Equilibrium calculations show that these crystalline phases are stable for pS₂, pC₁, and pO₂, values typical of magma-buffered gases that have not been contaminated by atmospheric oxygen.The deposits observed in the tubes may be useful in aiding the understanding of the mechanisms acting during the cooling of the gaseous phase on its way to the surface and before its emission into the atmosphere.     

Les débits de CO2 et de SO2 volcaniques dans l’atmosphère

Volcanic contribution to the atmosphere can be re-evaluated as 5×10⁷ ton/year of CO₂, which is less than recent industrial contribution. Degassing is more abundant from volcanoes where magma convections are significant. Effective degassing occurs before the magma reaches the surface, that is, when the magma is fluid. Volcanic gas in the atmosphere can affect the population or the vegetation around volcanoes.     

An example of health hazard: People killed by gas during a phreatic eruption: Diëng plateau (Java,Indonesia), February 20th 1979

On February 20th, 1979, 142 inhabitants of Dieng Plateau (Indonesia) were asphyxiated by poisonous gases during a mild phreatic eruption. From later fields gas collection and analysis, the casualties are considered to be due to CO₂ rich volcanic gases.     

Correspondence analysis applied to volcanic aerosols

A statistical analysis of volcanic aerosol based on 1,083 variables including concentration, grain-size and site variabilities establishes correlations between sites and among chemical elements.Variation curves of element concentration show parallel behaviour. High coefficients of correlation between anions and cations confirm associations between elements.