The carbon dioxide solubility in alkali basalts: an experimental study

International audienceExperiments 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

The H2O solubility of alkali basaltic melts: an experimental study

International audienceExperiments were conducted to determine the water solubility of alkali basalts from Etna, Stromboli and Vesuvius volcanoes, Italy. The basaltic melts were equilibrated at 1,200°C with pure water, under oxidized conditions, and at pressures ranging from 163 to 3,842 bars. Our results show that at pressures above 1 kbar, alkali basalts dissolve more water than typical mid-ocean ridge basalts (MORB). Combination of our data with those from previous studies allows the following simple empirical model for the water solubility of basalts of varying alkalinity and fO2 to be derived: {\text{H}}_{ 2} {\text{O}}\left( {{\text{wt}}\% } \right) = {\text{ H}}_{ 2} {\text{O}}_{\text{MORB}} \left( {{\text{wt}}\% } \right) + \left( {5.84 \times 10^{ - 5} *{\text{P}} - 2.29 \times 10^{ - 2} } \right) \times \left( {{\text{Na}}_{2} {\text{O}} + {\text{K}}_{2} {\text{O}}} \right)\left( {{\text{wt}}\% } \right) + 4.67 \times 10^{ - 2} \times \Updelta {\text{NNO}} - 2.29 \times 10^{ - 1} where H2OMORB is the water solubility at the calculated P, using the model of Dixon et al. (1995). This equation reproduces the existing database on water solubilities in basaltic melts to within 5%. Interpretation of the speciation data in the context of the glass transition theory shows that water speciation in basalt melts is severely modified during quench. At magmatic temperatures, more than 90% of dissolved water forms hydroxyl groups at all water contents, whilst in natural or synthetic glasses, the amount of molecular water is much larger. A regular solution model with an explicit temperature dependence reproduces well-observed water species. Derivation of the partial molar volume of molecular water using standard thermodynamic considerations yields values close to previous findings if room temperature water species are used. When high temperature species proportions are used, a negative partial molar volume is obtained for molecular water. Calculation of the partial molar volume of total water using H2O solubility data on basaltic melts at pressures above 1 kbar yields a value of 19 cm3/mol in reasonable agreement with estimates obtained from density measurements

Influence of glass polymerisation and oxidation on micro-Raman water analysis in alumino-silicate glasses

International audienceThe development of an accurate analytical procedure for determination of dissolved water in complex alumino-silicate glasses via micro-Raman analysis requires the assessment of the spectra topology dependence on glass composition. We report here a detailed study of the respective influence of bulk composition, iron oxidation state and total water content on the absolute and relative intensities of the main Raman bands related to glass network vibrations (LF: not, vert, similar490 cm−1; HF: not, vert, similar960 cm−1) and total water stretching (H2OT: not, vert, similar3550 cm−1) in natural glasses. The evolution of spectra topology was examined in (i) 33 anhydrous glasses produced by the re-melting of natural rock samples, which span a very large range of polymerisation degree (NBO/T from 0.00 to 1.16), (ii) 2 sets of synthetic anhydrous basaltic glasses with variable iron oxidation state (Fe3+/FeT from 0.05 to 0.87), and (iii) 6 sets of natural hydrous glasses (CH2OT from 0.4 to 7.0 wt%) with NBO/T varying from 0.01 to 0.76. In the explored domain of water concentration, external calibration procedure based on the H2OT band height is matrix-independent but its accuracy relies on precise control of the focusing depth and beam energy on the sample. Matrix-dependence strongly affects the internal calibrations based on H2OT height scaled to that of LF or HF bands but its effect decreases from acid (low NBO/T, SM) to basic (high NBO/T, SM) glasses. Structural parameters such as NBO/T (non-bridging oxygen per tetrahedron) and SM (sum of structural modifiers) describe the matrix-dependence better than simple compositional parameters (e.g. SiO2, Na2O + K2O). Iron oxidation state has only a minor influence on band topology in basalts and is thus not expected to significantly affect the Raman determinations of water in mafic (e.g. low SiO2, iron-rich) glasses. Modelling the evolution of the relative band height with polymerisation degree allows us to propose a general equation to predict the dissolved water content in natural glasses: View the MathML source where CH2OT is the total water content (in wt%) dissolved in glass; TOTN represents the computed ILF/IHF variation as a function of the calculated NBO/T and SM parameters; IH2ON is the H2O band height scaled to ratio of the reference bands; k is the linearity spectrometer response on the H2OT band in function of water content. The water concentrations of the reference glasses are reproduced using this equation with a standard deviation of 0.06 wt%. The adopted parameterisation provides a useful tool towards the characterisation of composition dependence of micro-Raman procedures for silicate glasses. We show, based on the widest range of glass compositions so far investigated, that accurate evaluation of dissolved water content is achieved by micro-Raman spectroscopy

Etude expérimentale de la solubilité des volatils C-H-O-S dans les basaltes alcalins italiens. Simulations numériques du dégazage chimique : application à l'Etna

Arc volcanism is known for his dangerousness because of its high volatiles contents. Major volatiles C, H, O, S, present in magmatic systems give important information on the chemical and physical properties of volcanic systems. Those information are important for a correct assessment of volcanic hazards. In this PhD report, the solubility laws of major volatiles H2O, CO2 and S (SO2, H2S) were defined for three alkali basalts from Italian volcanoes. Equilibrium experiments between a fluid and a silicate melt phases, at high temperature, at pressures up to 3000 bar and under varying fO2, were conducted in an internal heated pressure vessel equipped with a rapid quench. Dissolved volatiles in the synthesised glasses were analyzed by using classical methods (FTIR, EMPA, KFT) and allow us to derive the solubility laws for each volatile species for the three basaltic glasses from Vesuvius, Etna and Stromboli. This experimental work shows that alkalis play a significant role on water solubility at pressures above 1000 bar, and an important one on CO2 solubility. Then, the use of the solubility laws obtained for the Etna basalt in a numerical model of magma degassing during ascent, contributes to the knowledge of degassing phenomena, when compared to available natural data basing on volatile behaviour at Etna.Le volcanisme d'arc est connu pour ses fortes teneurs en volatiles, qui lui confèrent un degré de dangerosité élevé. L'étude des volatils majeurs C, H, O, S, présents dans les systèmes magmatiques apporte des informations importantes à la compréhension du fonctionnement des systèmes volcaniques, du point de vue chimique et physique. Ces informations sont primordiales pour la prévention du risque volcanique. Dans ce travail, nous avons déterminé de façon expérimentale les lois de solubilité des volatils majeurs, H2O, CO2 et S (SO2, H2S) présents dans les systèmes volcaniques pour trois basaltes alcalins de trois volcans italiens. L'utilisation d'un autoclave à chauffage interne, équipé d'un système de trempe rapide nous a permis de réaliser des expériences d'équilibre entre un liquide silicaté et une phase fluide en excès à haute température, et à des pressions allant jusqu'à 3000 bars. L'utilisation de méthodes courantes (FTIR, KFT, EMPA) pour l'analyse des volatils dissous dans ces verres basaltiques synthétisés, nous a permis d'obtenir des résultats à partir desquels les lois de solubilité de chacune des espèces volatiles ont été définies pour les verres basaltiques alcalins issus du Vésuve, de l'Etna et du Stromboli. L'étude expérimentale a permis de montrer l'importance des alcalins sur la solubilité de H2O à des pressions supérieures à 1000 bars, mais surtout sur la solubilité du CO2. L'introduction des lois de solubilité des volatils majeurs dans un modèle numérique, appliqué à l'Etna, permet de mieux comprendre les phénomènes de dégazage, en se référant aux données naturelles disponibles (inclusions vitreuses et chimie des gaz en sortie de conduit)

An Experimental Study to Determine the Solubility of C-H-O-S Volatiles in Basaltic Melts.

International audienceArc volcanism is known for his dangerousness, because of his high content of volatiles. Major volatiles present in magmatic liquids are C, H, O, S, forming the major gases emitted by volcanoes: H2O, CO2, and SO2 and/or H2S, depending on fO2. A significant amount of work has been done to define the solubility laws of H2O, CO2 and S in silicic melts. However, such data are still scarce for basaltic liquids. To remedy this gap, we are conducting experiments on basaltic liquids at 1050°C and 1200°C, at pressures varying between 250 and 2000 bar in oxidized (NNO+2) or reduced (NNO-1) conditions, using an IHPV equipped with a system of rapid quench. Basaltic compositions from Vesuvius, Etna and Stromboli are equilibrated with H2O, H2O+ CO2, H2O+S and H2O+CO2+S rich fluid phase. After a rapid quench, H2O and CO2 dissolved in the glasses are analyzed using both KFT and FTIR. Major elements and sulphur contents are determined by electron microprobe analyses. The comparison of our results with studies carried out on MORBs (Dixon et al. 1995) or on other basaltic compositions (Berndt et al., 2002), shows that there is no significant effect of composition on water solubilities under these experimental conditions. In contrast, the CO2 content of basaltic melts is strongly dependent on its composition, and this dependence increases with pressure (at 2kbar basalt from Vesuvius (7,39% Na2O+K2O) dissolves 3900 ppm of total carbon whereas a basalt from Etna (5.38% Na2O+K2O) or from Stromboli (4.20% Na2O+K2O) dissolves less than 2000 ppm). Microprobe analyses show that, at near H2O saturation, sulphur contents increase strongly with pressure (from 2500 ppm at 250 bar, to 6700 ppm at 2000 bar for Etna and Stromboli compositions at NNO+2). Basaltic melts dissolve more S under oxidized conditions than under reduced conditions. First results obtained on basaltic liquids equilibrated with an H2O+CO2+S rich fluid phase show that the C/S ratio increases strongly with pressure and alkalies content of the melt

Solubility of C-H-O-S Volatiles in Basaltic Melts.

Arc volcanism is known for his dangerousness because of the high contents H$_{2}$O, CO$_{2}$ and S (H$_{2}$S or SO$_{2}$ depending on the fO$_{2}$ of the system). The behaviour of the volatiles C, H, O and S in basaltic melts is poorly known, yet the knowledge of the solubility of these volatiles is critical to understand volcanic degassing. A significant amount of work has been done to define the solubility laws of H$_{2}$O, CO$_{2}$ an S in silicic melts. However, such data are still scarce for basaltic liquids. To remedy this gap, we are conducting experiments on basaltic liquids at $1200\deg$C, at pressures varying between 250 and 2000 bar and at high fO$_{2}$ (NNO+2). Basaltic compositions from Vesuvius, Etna and Stromboli are equilibrated with an H$_{2}$O+S and H$_{2}$O+CO$_{2}$+S rich fluid phase. After rapid quench, contents of H$_{2}$O and CO$_{2}$ in glasses are determined by using KFT and near/mid infrared spectroscopic measurements. Major elements and sulphur contents are determined by electron microprobe analyses. Microprobe analyses show that, at near H$_{2}$O saturation, sulphur contents increase with pressure (from 2500 ppm at 250 bar, to 6700 ppm at 2000 bar for Etna and Stromboli compositions). The melt composition has an influence on the sulphur contents, in particular iron, as shown in previous work, and alkalies. In particular, potassium seems to have a negative role on sulphur: for similar pressure, temperature, oxygen fugacity, and bulk S content, a difference of 3% in alkalinity induces a difference of 1000 to 2000 ppm of S contents. Results for basaltic melts equilibrated with H$_{2}$O+CO$_{2}$+S fluid phase will be presented at the meeting. Experimental results on H$_{2}$O, CO$_{2}$ and S solubilities in basaltic liquids will constitute a data base for subsequent experiments on degassing processes

The solubility of sulfur in hydrous basaltic melts

International audienceExperiments were performed to determine the sulfur solubilities of hydrous basalts from Vesuvius, Etna and Stromboli (Italy). The melts were equilibrated at 1050 and 1200 degrees C with H2O and sulfur (added as pyrrhotite), and at pressures ranging from 250 to 2000 bar. Most experiments were performed under oxidising conditions (NNO + 2), and a few under reducing conditions (NNO - 1), with melt water contents of 0.5-3.5 wt.%. Sulfur contents in glasses were determined by electron microprobe and range from 860 up to 6700 ppm. No compositional effect is found between the three alkali basaltic melts. The fugacities of S-bearing species were derived using an MRK equation of state applied to an O-H-S fluid, knowing H-2 and H2O fugacities, and range from 50 up to 3000 bar. A thermodynamic species-based model is derived from our results along with available data in the literature, assuming that sulfur dissolution results from the additive contributions of both H2S and SO2 dissolution reactions. Compared to similar models developed for silicic melts, basalt compositions requires the incorporation of an Fe term, which accounts for the strong association between Fe and S in silicate melts, and considers the elevated Fe content of mafic melts. The model shows that, at any fixed fS(2), the sulfur solubility in hydrous basalt displays a pronounced minimum around NNO, the position of which depends on temperature. The minimum in sulfur solubility coincides with the redox range were the abundance of S-2 in the fluid reaches its maximum compared to either H2S or SO2 species. Such a minimum in solubility is in agreement with experimental constraints at 1 bar under carefully controlled fO(2) and fS(2). Calculated proportions of dissolved species in the melt depend on the prevailing fS(2) and fO(2), being in general agreement with available spectroscopic models. Calculations of gas saturation pressures, which classically consider only H2O and CO2 dissolved volatiles, are strongly affected by S-bearing species. At fO(2) close to, or higher than, NNO + 1, omission of sulfur species may result in underestimates of gas saturation pressures of 1 kbar or more. The same happens at fO(2) below NNO - 1

Experimental parametrization of magma mixing : application to the 1530 AD eruption of La Soufrière, Guadeloupe (Lesser Antilles)

International audienceNew petrological data on eruption products and experimental results are integrated and a model for the evolution of the La Soufrière (Guadeloupe, Lesser Antilles arc) magma reservoir prior to the 1530 AD eruption is presented. In comparison with recent volcanic crises in the Antilles, the 1530 AD eruption is distinctive. The eruptive pyroclastic sequence shows a continuous zonation in whole-rock composition from silicic (∼ 62 wt% SiO2) to mafic andesite (∼ 55 wt% SiO2). Mafic products are estimated to be 80% of the total eruption volume. All juvenile clasts are crystal-rich (46-60 vol.% phenocrysts), the crystallinity being inversely correlated with the bulk-rock SiO2 content. The phenocryst assemblage (plagioclase, orthopyroxene, clinopyroxene, magnetite) is constant throughout the sequence. Complexly zoned crystals are encountered, but An60-65, En56-59 and Mt66-68 compositions occur in all samples. Glass inclusions are rhyolitic with up to 5-5.5 wt % H2O. Matrix glasses are strongly heterogeneous, from ∼64 to > 76 wt % SiO2. The pre-eruptive evolution of the reservoir is dominated by the remobilization of a resident andesitic body following the arrival of a basaltic magma batch. Conditions of early remobilization are constrained from experiments on a basalt from the L’Echelle scoria cone. The arrival magma is a crystal-poor, moderately hot (975-1025 °C), wet (> 5 wt % H2O) and oxidized (NNO+1) low-MgO high alumina basalt similar to those involved in other Antilles volcanic centers. Geothermometry and experiments on a silicic andesite product of the eruption show that, for melt H2O contents between 5 and 5.5 wt %, phenocrysts and interstitial melt in the resident magma were in mutual equilibrium at ∼875 °C and NNO+0.8. However, matrix glass and glass inclusion compositions show that, locally, the andesite body was as cold as 825 °C. Melt volatile concentrations imply a minimum depth for the magma reservoir between 5.6 and 7.1 km, and the absence of amphibole phenocrysts indicates a maximum depth at 8.5 km. The 1530 AD eruption tapped an hybrid magma assembled by mixing approximately equal proportions of resident andesite and arrival basalt. Mineralogical indicators of the mixing event include An-rich layers in plagioclase, En-rich rims on orthopyroxene and core-rim zonation in magnetite, but overall phenocrysts were little modified during assembly of the hybrid magma. In comparison, matrix glasses were more severely affected. Mixing proceeded essentially by the addition of a mafic melt to the andesite body. The continuous chemical zonation observed in 1530 AD eruption products reflects mixing between three components (mafic melt, silicic melt, phenocryst assemblage). Timescales measured on different eruptive products range from several thousand years (U-Th-Ra disequilibria), 10s days (diffusion modelling in orthopyroxenes) to 10s hours (heterogeneous matrix glasses). Short timescales since mafic recharge, lack of extensive transformations of phenocrysts, continuous whole-rock chemical zonation and predominance of mafic products are all consistent with triggering of the 1530 AD eruption by a major mafic recharge event which originated in the middle to lower Lesser Antilles arc crust

SolEx: A Model for Mixed COHSCl-volatile Solubilities and Exsolved Gas Compositions in Basalt

We present a software application, SolEx, to calculate basaltic melt and coexisting vapour compositions in the system C–O–H–S–Cl. Such a model has great utility in interpreting emitted gas and melt inclusion compositions, especially through the incorporation of sulphur and chlorine, the most commonly measured volcanic gas species. We assume that the behaviour of the fluid phase is controlled by the volumetrically dominant volatile species, H2O and CO2, whereas sulphur and chlorine partition between the melt and fluid phases. Melt–fluid partition coefficients for S and Cl were parameterised from measurements by Lesne et al. (2011a, p. 1737). The model of Churakov and Gottschalk (2003a, p. 2415) was applied to calculate fugacity coefficients and the equilibrium constants for the reaction imelt → ifluid were thereby deduced. SO2 dominates at oxidation states of ΔNNO \u3e 0.5 (Jugo et al., 2010, p. 5926), where this model is applicable. In the forward model, total volatile inventories and melt composition are specified by the user. The parameterisation of Dixon (1997, p. 368) is used to predict the partitioning of CO2 and H2O between vapour and melt phases. An iterative procedure is employed to predict the partitioning of S and Cl components between fluid and melt phases. Melt and gas compositions and gas volume fraction are thereby modelled over pressures in the range 5–4000 bar. This approach satisfactorily reproduces independent literature data on S and Cl behaviour in basalt. SolEx is a user-friendly software package available for OS X and Windows, facilitating modelling of closed- and open-system C–O–H–S–Cl degassing in basalts