De l'eau dans les magmas

La présence de concentrations importantes en eau dissoute (jusqu'à environ 10 % du poids) dans les magmas terrestres est une des caractéristiques spécifiques de notre planète. Plus que tout autre constituant, l'eau modifie en profondeur les propriétés physiques et chimiques des magmas. Elle affecte des processus géologiques fondamentaux comme la fusion partielle, l'ascension, l'éruption, le dégazage et la cristallisation des magmas. C'est un agent essentiel dans le transport et la redistribution des métaux dans les gisements magmatiques-hydrothermaux

Gold Solubility in Arc Magmas: Experimental Determination of the Effect of Sulfur at 1000°C and 0.4 GPa

International audienceTo investigate the behaviour of gold in sulfur-bearing hydrous intermediate calcalkaline melts under different redox states typical of subduction-zone settings, we have determined the solubility of Au at 0.4 GPa and 1000°C for three dacitic magmas (two adakites and one calc-alkaline composition) from the North-Luzon Arc (Philippines). The experiments were performed over an oxygen fugacity (fO2) range corresponding to reducing (~NNO-1), moderately oxidizing (~NNO+1.5) and strongly oxidizing ( NNO+3) conditions as measured by solid Ni-Pd-O sensors. They were carried out in gold containers, serving also as the source of gold, in presence of variable amounts of H2O and ~1 wt% of elemental sulfur (S). Concentrations of Au in glasses were determined by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). Gold solubility in S-bearing melts is drastically enhanced compared to S-free melts, by up to two orders of magnitude. In addition, very high gold solubilities are reached under reducing conditions (< NNO-1) in Fe-poor, S-rich, sulfide-saturated melts probably as a result of an increase of fH2S, and a strong increase of gold solubility is observed at the sulfide/sulfate transition (from ~NNO+1.25 to NNO+1.6) due to the destabilization of sulfides and the increase of melt S2- concentration. Thermodynamic modelling of the experimental results suggest that the dissolution of gold in silicate melt is the result of a combination of various gold species (Au0, Au2O, Au2O3, Au2S3, Au2(SO4)3, and Au2FeS2) present in the melt in variable proportions, depending on the three parameters considered in the model - log fO2, log fS2 and log XFeS - which are the main variables controlling the dissolution of gold in melt under our experimental PTX conditions. Our modelling shows that Au2FeS2 is the main gold species dissolved under reducing conditions (i.e., S6+/Stotal ~ 0), whereas at sulfate saturation gold is mainly dissolved as Au metal and Au2O. The present study shows that sulfide undersaturation of primary mantle magmas or/and highly oxidizing conditions are not required for metal mobilization from the source, since gold enrichment in evolving arc magmas and exsolving fluid phases is likely to occur over a wide range of fO2 at sulfide saturation, from NNO < -1 to the sulfide-sulfate transition (i.e., NNO+0.5-2.0); nevertheless it is critically controlled by variations of fS2 and fH2S. The role of Au-enriched slab partial melts and slab-derived aqueous fluids, and the importance of the abundance of sulfur in the source for an early gold enrichment in the melt, are emphasized

A thermodynamic model for hydrous silicate melts in the system NaAlSi3O8–KAlSi3O8–Si4O8–H2O

Computation of crystal–liquid equilibria in hydrous silicate systems requires a model of the free energy of the hydrous liquid that defines the activity of the melt components at given temperature, pressure and composition. We present in this study a parametrization of the free energy of the liquid in the haplogranite system NaAlSi3O8–KAlSi3O8–Si4O8–H2O based on the Margules approach. The excess free energy of the multicomponent melt is approximated from the binaries with the Kohler extrapolation method. Model parameters have been fitted to phase equilibrium data by mathematical programming techniques. A small but complex excess function of the anhydrous melt composition is necessary to reproduce reported liquidus phase relations. Using partial molar Cp data from the literature for the H2O melt component and a simple polynomial approximation for the molar volume, standard state enthalpy and entropy were refined close to −287 kJ/mol and 67.2 J/K mol, respectively. Calculated crystal–liquid phase relations are in good agreement with measurements to 5 kbar, and the modelled melt–fluid coexistence surface yields a valuable first order approximation of the H2O solubility at near liquidus temperatures. Thermodynamic assessment of solubility and liquidus data suggests that H2O mixing differs considerably in feldspar melts and in silica melts. Si4O8–H2O mixing contributes to a very minor degree to the haplogranite system

Crystallization of primitive basaltic magmas at crustal pressures and genesis of the calc-alkaline igneous suite: experimental evidence from St Vincent, Lesser Antilles arc

International audienceNear-liquidus crystallization experiments have been carried out on two basalts (12.5 and 7.8 wt% MgO) from Soufriere, St Vincent (Lesser Antilles arc) to document the early stages of differentiation in calc-alkaline magmas. The water-undersaturated experiments were performed mostly at 4 kbar, with 1.6 to 7.7 wt% H2O in the melt, and under oxidizing conditions (ΔNNO = −0.8 to +2.4). A few 10 kbar experiments were also performed. Early differentiation of primitive, hydrous, high-magnesia basalts (HMB) is controlled by ol + cpx + sp fractionation. Residual melts of typical high-alumina basalt (HAB) composition are obtained after 30–40% crystallization. The role of H2O in depressing plagioclase crystallization leads to a direct relation between the Al2O3 content of the residual melt and its H2O concentration, calibrated as a geohygrometer. The most primitive phenocryst assemblage in the Soufriere suite (Fo89.6 olivine, Mg-, Al- and Ti-rich clinopyroxene, Cr–Al spinel) crystallized from near-primary (Mg# = 73.5), hydrous (∼5 wt% H2O) and very oxidized (ΔNNO = +1.5–2.0) HMB liquids at middle crustal pressures and temperatures from ∼1,160 to ∼1,060°C. Hornblende played no role in the early petrogenetic evolution. Derivative HAB melts may contain up to 7–8 wt% dissolved H2O. Primitive basaltic liquids at Soufriere, St Vincent, have a wide range of H2O concentrations (2–5 wt%)

Physical conditions of primitive tephritic magmas from vesuvius : first experimental results.

Volatile-rich tephritic melts represent the most primitive compositions at Vesuvius. Recent eruptions (1906, 1944) record the multiple injections of such compositions at shallow levels, and their crystallization and mixing. Plinian magma chambers (eg, the 79 AD Pompei eruption) grow from the periodic supply of K-rich mafic melts

The effect of B2O3 on the viscosity of haplogranitic liquids

The effect of B2O3 on the viscosity of a haplogranitic liquid (KrO-Na,O-AlrOr-SiO,) has been determined at I atm pressure in the temperature interval of 600-1600 °C. Viscosity measurementso f a haplogranite, haplogranite + 4.35 wt% B2O3 and haplogranite + 8.92 wt% B2O3 have been performed using the concentric cylinder and micropenetration methods. The viscosity of a B-enriched natural rhyolite obsidian, macusanite from Macusani, Peru, has also been determined. The viscosity of haplogranite liquid decreases with the addition of B2O3 at all temperatures investigated. The viscosity decrease is nonlinear, with the strongestd ecreasee xhibited at low B2O3 concentration. The temperature dependence of the viscosity of all the investigated liquids is Arrhenian, in strong contrast to the case for B2O3 Iiquid. The Arrhenian activation energy is much lower in the B2O3-bearing liquids than in the B2O3-free haplogranite, with the result that the effect of B2O3 on viscosity is a strong function of temperature. At temperatures corresponding to the crystallization of B-rich granitic and pegmatitic systems the addition of I wt% of B2O3 decreases the viscosity 2 orders of magnitude. The macusanite liquid exhibits a reduced viscosity compared with B-free rhyolite that is consistent with the synthetic liquid systematics. B must be considered as a fluxing agent in B-rich granitic and pegmatitlc systems

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

PhD Financed by the Région Centre and by the EC TMR network "Hydrous Silicate Melts"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 (FeOliq) and ferric (Fe2O3liq) 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 FeOliq and Fe2O3liq. 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 FeOliq but has an opposite effect on Fe2O3liq. As calcium is added to system, the effect of water becomes weaker and is inverted for Fe2O3liq. Under oxidizing conditions, water has a negligible effect on both activities of FeOliq and Fe2O3liq. In contrast, changes in redox conditions dominate the activity coefficients of both FeOliq and Fe2O3liq, 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

Experimental Phase Equilibria: Data and Models

Equilibria between silicate melts, crystalline and vapour phases essentially control major and trace element compositions of natural magmas and play an essential role in partial melting as well as in differentiation processes. The development of models of phase equilibria is a prerequisite for assessing the dynamics of magmatic processes, such as magma storage in a reservoir or magma ascent in a conduit. The interest of experimental phase equilibria is essentially twofold. (1) Experimental phase equilibrium results can put precise constraints on the magmatic conditions, either in the magma storage region or in the conduit. Knowledge of pre-eruptive parameters (P, T, H2O in melt, fO2, ...) is necessary for eruption models and consequently for the evaluation of volcanic risk. (2) Experimental phase equilibria constitute our main source of information for the calibration of the mixing properties of multicomponent silicate melts, and for the construction of thermodynamic models which are our tomorrow's tools for the simulation of magmatic processes

The effect of water and fO2 on the ferric–ferrous ratio of silicic melts

New experiments on the effect of dissolved water on the ferric–ferrous ratio of silicic melts have been performed at 200 MPa, between 800°C and 1000°C and for fO2 between NNO−1.35 and NNO+6.6. Water-saturated conditions were investigated. Compositions studied include six metaluminous synthetic melts, with FeOtot progressively increasing from 0.47 to 4.25 wt.%, two natural obsidian glasses (one peraluminous and the other peralkaline) and a synthetic rhyolitic glass having the composition of the matrix glass of the June 15, 1991 Pinatubo dacite. Ferrous iron was analyzed by titration and FeOtot by electron microprobe. Variation of quench rate was found to have no detectable effect on the ferric–ferrous ratio of the hydrous silicic melts investigated. At NNO, no dependence of the ferric–ferrous ratio with temperature is observed. At fO2NNO+1, the experimental ferric–ferrous ratios are equal or lower than calculated. The peralkaline samples show the same type of behaviour. A non-linear relationship between XFe2O3/XFeO and fO2 implies that a term for dissolved water must be added to the KC equation if it is to be applied to the calculation of ferric–ferrous ratios of hydrous silicic melts. Above NNO+1, the ferric–ferrous ratio is essentially controlled by the anhydrous melt composition and fO2. However, differences exist between measured and calculated ferric–ferrous ratios of silicic melts that are not all attributable to the effect of dissolved water. Additional work is needed to describe more precisely the dependence of the ferric–ferrous ratio on anhydrous melt composition. The oxidizing effect of water is restricted to relatively reduced magmatic liquids. In oxidized calk-alkaline magma series, the presence of dissolved water will not largely influence melt ferric–ferrous ratios

Time-dependent changes of the electrical conductivity of basaltic melts with redox state

International audienceThe electrical conductivity of basaltic melts has been measured in real-time after fO2 step-changes in order to investigate redox kinetics. Experimental investigations were performed at 1 atm in a vertical furnace between 1200°C and 1400°C using air, pure CO2 or CO/CO2 gas mixtures to buffer oxygen fugacity in the range 10-8 to 0.2 bars. Ferric/ferrous ratios were determined by wet chemical titrations. A small but detectable effect of fO2 on the electrical conductivity is observed. The more reduced the melt, the higher the conductivity. A modified Arrhenian equation accounts for both T and fO2 effects on the electrical conductivity. We show that time-dependent changes in electrical conductivity following fO2 step-changes monitor the rate of Fe2+/Fe3+ changes. The conductivity change with time corresponds to a diffusion-limited process in the case of reduction in CO-CO2 gas mixtures and oxidation in air. However, a reaction at the gas-melt interface probably rate limits oxidation of the melt under pure CO2. Reduction and oxidation rates are similar and both increase with temperature. Those rates range from 10-9 to 10-8m2/s for the temperature interval 1200-1400°C and show activation energy of about 200kJ/mol. The redox mechanism that best explains our results involves a cooperative motion of cations and oxygen, allowing such fast oxidation-reduction rates