Rate of hydrogen-iron redox exchange in silicate melts and glasses     

Fabrice Gaillard  Steven Mackwell 《Geochimica et cosmochimica acta》2003,67(13):2427-2441

A kinetic model for the rate of iron-hydrogen redox exchange in silicate glasses and melts has been derived from time-series experiments performed on natural rhyolitic obsidians. Cylinders of the starting glasses were exposed to reducing mixtures composed of H₂-Ar-CO₂-CO in 1-atm furnaces and H₂-Ar in a cold seal pressure vessel. Overall, runs covered the temperature range 300 to 1000°C. The progression of a front of ferric iron reduction within the quenched melt was observed optically through a change of color. For all run conditions, the advancement of the front (ξ) was proportional to the square root of time, revealing the reaction as a diffusion-limited process. Iso-fO₂ runs performed in CO₂-CO, H₂-Ar, and H₂-CO₂ gases have shown that fH₂ rather than fO₂ is the dominant parameter controlling the reaction rate. The fH₂ dependence of the rate constant was characterized in the range 0.02 to 70 bar. The growth of the reduced layer, which is accompanied by an increase in reaction-derived OH-group content, was fitted considering that the reaction rate is controlled by the migration of a free mobile species (H₂) immobilized in the form of OH subsequent to reaction with ferric iron. The reaction rate is thus a function of both solubility and diffusivity of H₂ weighted by the concentration of its sink (ferric iron). We extracted a single law for both solubility and diffusivity of H₂ in amorphous silicates that applies over a range of temperatures below and above the glass transition temperature. Melt/glass structure (degree of polymerization) does not seem to significantly affect both solubility and diffusivity of H₂. We therefore provide a model that allows the prediction of oxidation-reduction rates in the presence of hydrogen for a wide range of compositions of amorphous glasses and melts. Comparisons with previous work elucidating rate of redox exchange in dry systems allow us to anticipate the fH₂-T domains where different redox mechanisms may apply. We conclude that equilibration of redox potential in nature should be dominated by H₂ transfer at a rate controlled by both H₂ solubility and diffusion. Numerical applications of the model illustrate redox exchanges in natural magmas and in glasses exposed to weathering under near surface conditions. We show that crustal events such as magmas mixing should not modify the iron redox state of magmas. In the case of nuclear-waste-bearing glasses, the fH₂ conditions in the host terrain are clearly a parameter that must be taken into account to predict glass durability.  相似文献   

Experimental determination of activities of FeO and Fe₂O₃ components in hydrous silicic melts under oxidizing conditions     

Fabrice Gaillard  Michel Pichavant  Bruno Scaillet 《Geochimica et cosmochimica acta》2003,67(22):4389-4409

The critical role of iron on crystal-silicate liquid relationships and melt differentiation is mainly controlled by the redox conditions prevailing in magmas, but the presently available database merely constrains the thermodynamic properties of iron-bearing components in strongly reduced and anhydrous molten silicate where iron is in the ferrous form. This paper provides new standard states for pure ferrous (FeO^liq) and ferric (Fe₂O₃^liq) molten iron oxides and extends the experimental database towards oxidizing and water-bearing domains. Iron-iridium, iron-platinum alloys, magnetite or hematite were equilibrated with synthetic silicic liquids at high temperature and high pressure under controlled oxygen fugacity (fO₂) to determine activity-composition relationships for FeO^liq and Fe₂O₃^liq. Between 1000 and 1300°C, the fO₂ ranges from that in air to 3-log units below that of the nickel-nickel oxide buffer (NNO). Experiments were performed on both anhydrous and hydrous melts containing up to 6-wt.% water. Incorporation of water under reducing conditions increases the activity coefficient of FeO^liq but has an opposite effect on Fe₂O₃^liq. As calcium is added to system, the effect of water becomes weaker and is inverted for Fe₂O₃^liq. Under oxidizing conditions, water has a negligible effect on both activities of FeO^liq and Fe₂O₃^liq. In contrast, changes in redox conditions dominate the activity coefficients of both FeO^liq and Fe₂O₃^liq, which increase significantly with increasing fO₂. The present results combined with the previous work provide a specific database on the energetics of iron in silicate melts that cover most of the condition prevailing in natural magmas.  相似文献   

Palaeozoic (Silurian and Devonian) radiolarians and conodonts from chert olistoliths of the Volissos Turbidites, Chios island, Greece     

Christiano Larghi  Fabrice Cordey  Carlo Corradini  Maurizio Gaetani  Alda Nicora 《Eclogae Geologicae Helvetiae》2005,98(1):123-131

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Temperature dependence of shear wave attenuation in partially molten gabbronorite at seismic frequencies     

Fabrice R. Fontaine  Benoit Ildefonse  Nickolai S. Bagdassarov 《Geophysical Journal International》2005,163(3):1025-1038

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The statistical relationship between unconfined compressive strengths and the frequency distributions of slope gradients — A case study in northern Hungary     

Zoltn Püspki  Szilrd Szab  Gbor Demeter  Katalin Szalai  Richard W. McIntosh  Lszl Vincze  Gbor Nmeth  Imre Kovcs 《Geomorphology》2005,71(3-4):424-436

This paper focuses on the question of whether there is a deterministic connection between the slope gradient and unconfined compression strength (UCS) as a lithological factor and on describing the nature of this connection. Moreover, the authors determined the sensitive statistical parameters in the statistical surface analyses. The surface analyses were carried out in an area extending over 1500 km² in northern Hungary and containing parts of the uplifted Palaeozoic and Mesozoic basement and the semi-consolidated material of the Palaeogene and Neogene molasse sediments.The 67 geological formations of the area were grouped into 10 petrophysical categories characterised by unconfined compressive strength as a petrophysical parameter. The geological database was the digitalized geological map of North Hungarian Region (1 : 100 000). The digital topographic database was based on 10 m contour lines of 1 : 50 000 maps, the digital elevation model was generated by kriging interpolation. Three topographic models were created with resolutions of 25 × 25, 50 × 50 and 100 × 100 m per pixels.Evident correlation can be shown between the UCS and the relative frequency of the slope gradient. The adequate regression procedure is the power regression for low slope category values while logarithmic regression is applicable at high slope angles. Based on the characteristic of the relationship and the value of r², slope category intervals can be identified the relative frequency of which is proven to be determined by the UCS. These intervals are found to be between 4–10%, 10–16%, 16–22%, 22–44% and over 44%.Using the determination equations of slope gradient between 4–10% and over 44%, the UCS of the bedrock can be calculated approximately as the average value of the two calculated results. So the quotient of the frequency of these two category intervals can be regarded as an important morphometric index for a given bedrock.From the aspect of petrophysical characteristics, the rock with UCS between 6 and 86 MPa were proven to be deterministic for the slope development i.e. in the determination of the slope category frequencies. Applying the relative frequency of slope gradients a relative erosion resistance of the petrophysical categories can be calculated.Considering the determination coefficients, among the statistical parameters of the distribution of slope category values the standard deviation, mode, mean and median proved to be determined by the UCS of the bedrock.  相似文献   

Evolution of the REE mineralogy in HP–LT metapelites of the Sebtide complex, Rif, Morocco: Monazite stability and geochronology     

Emilie Janots  Franois Negro  Fabrice Brunet  Bruno Goff  Martin Engi  Mohamed L. Bouybaoune 《Lithos》2006,87(3-4):214-234

REE mineralogy was characterized at the micrometer scale (using scanning electron microscopy) in the four tectono-metamorphic units of the Beni Mzala window, Sebtide Complex, Internal Rif, Morocco, which sample a HP-LT metamorphic gradient from subgreenschist to blueschist/eclogite facies. These tectonic units are composed of garnet-free metapelites (and associated synmetamorphic veins) from the same homogeneous aluminium-rich and calcium-poor protolith. In that context, mineralogical differences between units are assumed to result from contrasted pressure and temperature histories. Light REE-bearing Al-phosphates of the crandallite group with florencite-rich composition and variable goyazite content are the dominant LREE minerals in the lowest-grade units, Tizgarine (300 °C, 3 kbar as peak conditions) and Boquete de Anjera (350 °C, 10 kbar). In the latter unit, metamorphic florencite is associated with MREE-rich monazite. In the schist of the highest-grade units, Beni Mzala 2 (420–450 °C, 12 kbar) and Beni Mzala 1 (550 °C, 16 kbar), associations of allanite-rich epidote and synchisite (a LREE-fluorocarbonate) are found whereas florencite and monazite are no longer observed. At the wall of the quartz–kyanite veins, retrograde monazite-(Ce) with grain size of 20 to 50 μm is the only LREE-bearing mineral. Th–Pb and U–Pb SIMS data on some of these grains yield ages of 21.3 ± 1.7 Ma and 20.9 ± 2.1 Ma, respectively, consistent with previous K–Ar ages obtained on retrograde clay–mica mixtures in rocks from the same locality. The identification of a stability field for monazite in high-pressure aluminous metapelites with an upper thermal-limit below 450 °C as well as the derivation of meaningful U–Pb and Th–Pb SIMS ages demonstrates the monazite potential for dating HP-LT metapelites. However, since, under these conditions, monazite growth occurs below its admitted closure-temperature, these ages must be interpreted on the basis of monazite textural relationships.  相似文献   

Long-term tropical morphogenesis of New Caledonia (Southwest Pacific): Importance of positive epeirogeny and climate change   总被引：1，自引：0，他引：1  

Violaine Chevillotte  Dominique Chardon  Anicet Beauvais  Pierre Maurizot  Fabrice Colin 《Geomorphology》2006,81(3-4):361-375

Extensive mapping of relict lateritic landsurfaces over the 1600 m of relief of mainland New Caledonia is used to evaluate long-term morphogenesis of the island since its emergence in the Oligocene. The results support the interpretation of the lateritic levels as abandoned, stepped remnant landsurfaces that may be grouped into eight island-scale erosion levels recording episodic lowering of base level. The separation of the erosion levels results from the combination of positive epeirogenic movements of the New Caledonia continental ridge and long-term Neogene climate variability, which is recorded by the contrasted erosion and weathering patterns of the three youngest levels and tentatively correlated with known global and regional paleoclimatic events.  相似文献   

Experimental study and modeling of fluid reaction paths in the quartz-kyanite ± muscovite-water system at 0.7 GPa in the 350-550 °C range: Implications for Al selective transfer during metamorphism     

Anne Verlaguet  Fabrice Brunet  William M. Murphy 《Geochimica et cosmochimica acta》2006,70(7):1772-1788

In order to quantify Al transfer in response to fluid-mineral equilibration under evolving metamorphic conditions, isobaric (0.7 GPa) experiments were conducted in the 350-550 °C range. Disequilibrium was induced (1) by holding initially pure water and natural minerals (kyanite + quartz ± muscovite enclosed in a perforated inner capsule) under isothermal conditions and (2) by stepwise temperature variations. In all experiments, secondary Al-bearing phases crystallized in the external tube of a “tube-in-tube” setup (SEM characterization); they are interpreted as witnesses of the evolution of the fluid composition (fluid reaction path). These reaction paths and the subsequent amount of secondary crystallizations were modeled using thermodynamic data from SUPCRT92 and estimates of both starting-mineral dissolution rates and elemental diffusion coefficients from the literature. A major result is that the amount of aluminum transferred to secondary phases is a thousand times larger than the calculated Al concentration in the fluid. Although the crystallization of Al-bearing phases was expected as a response to a temperature decrease, the stepwise temperature increase (20 °C/day) also led to aluminum transfer towards secondary phases. In the course of re-equilibration, the fluid first becomes saturated with respect to aluminosilicates and then reaches silica saturation, due to the low solubility of Al-minerals. Consequently, aluminosilicates partly recrystallize in response to a temperature increase. Crystallization of secondary Al-phases in the external tube implies that aqueous aluminum was efficiently transported from the inner capsule, even in the pure Al₂O₃-SiO₂-H₂O system. Therefore, mass balance calculations considering a constant Al reference frame, i.e., postulating Al immobility, should be regarded with caution.  相似文献   

Selection and ranking of ground motion models for seismic hazard analysis in the Pyrenees   总被引：1，自引：0，他引：1  

Stéphane Drouet  Frank Scherbaum  Fabrice Cotton  Annie Souriau 《Journal of Seismology》2007,11(1):87-100

The issue addressed in this paper is the objective selection of appropriate ground motion models for seismic hazard assessment in the Pyrenees. The method of Scherbaum et al. (2004a) is applied in order to rank eight published ground motion models relevant to intraplate or to low deformation rate contexts. This method is based on a transparent and data-driven process which quantifies the model fit and also measures how well the underlying model assumptions are met. The method is applied to 15 accelerometric records obtained in the Pyrenees for events of local magnitude between 4.8 and 5.1, corresponding to moment magnitudes ranging from 3.7 to 3.9. Only stations at rock sites are considered. A total of 720 spectral amplitudes are used to rank the selected ground motion models. Some control parameters of these models, such as magnitude and distance definitions, may vary from one model to the other. It is thus important to correct the selected models for their difference with respect to the magnitude and distance definitions used for the Pyrenean data. Our analysis shows that, with these corrections, some of the ground motion models successfully fit the data. These are the Lussou et al. (2001) and the Berge-Thierry et al. (2003) models. According to the selected ground motion models, a possible scenario of a magnitude 6 event is proposed; it predicts response spectra accelerations of 0.08–0.1 g at 1 Hz at a hypocentral distance of 10 km.  相似文献   

Impact of southeast Indian Ocean sea surface temperature anomalies on monsoon-ENSO-dipole variability in a coupled ocean–atmosphere model     

Pascal Terray  Fabrice Chauvin  Hervé Douville 《Climate Dynamics》2007,28(6):553-580

Recent studies show that SouthEast Indian Ocean (SEIO) SSTs are a highly significant precursor of transitions of the whole monsoon-El Niño-Southern Oscillation (ENSO) system during recent decades. However, the reasons for this specific interannual variability have not yet been identified unequivocally from the observations. Among these, the possibility of SEIO SST-driven variability in the monsoon-ENSO system is investigated here by inserting positive/negative SEIO temperature anomalies in the February’s restart files of a state-of-the-art coupled General Circulation Model (GCM) for 49 years of a control simulation. For each year of the control simulation, the model was then integrated for a 1-year period in fully coupled mode. These experiments show that Indian Summer Monsoon (ISM) and tropical Indian Ocean Dipole Mode (IODM) events are significantly influenced by the SEIO temperature perturbations inserted in the mixed layer of the coupled GCM several months before. A warm SEIO perturbation, inserted in late boreal winter, slowly propagates northward during the following seasons, implies enhanced ISM rainfall and finally triggers a negative IODM pattern during boreal fall in agreement with observations. A reversed evolution is simulated for a cold SEIO perturbation. It is shown that the life cycle of the simulated SEIO signal is driven by the positive wind-evaporation-SST, coastal upwelling and wind-thermocline-SST feedbacks. Further diagnosis of the sensitivity experiments suggests that stronger ISM and IODM variabilities are generated by excluding the El Niño years of the control simulation or when the initial background state in the SEIO is warmer. This finding confirms that IODM events may be triggered by multiple factors, other than ENSO, including subtropical SEIO SST anomalies. However, the ENSO mode does not react significantly to the SEIO temperature perturbation in the perturbed runs even though the simulated Pacific pattern agrees with the observations during boreal fall. These discrepancies with the observations may be linked to model biases in the Pacific and to the too strong ENSO simulated by this coupled GCM. These modeling evidences confirm that subtropical Indian Ocean SST anomalies generated by Mascarene high pulses during austral summer are a significant precursor of both ISM and IODM events occuring several months later.  相似文献