Fabrice Gaillard

Rapid magma ascent recorded by water diffusion profiles in mantle olivine

Mechanisms and rates of magma ascent play a critical role in eruption dynamics but remain poorly constrained phenomena. Water, dissolved in mantle minerals as hydrogen and partitioned into the magma during ascent, may provide clues to quantifying magma ascent rates prior to eruption. We determined the dehydration profiles in olivine crystals from peridotite mantle xenoliths within the Pali-Aike alkali basalt from Patagonia, Chile. The results demonstrate that the amount of water stored in the uppermost mantle has likely been underestimated due to water loss during transport. Using experimental diffusion data for hydrogen, we estimate that the xenoliths reached the surface from 60–70 km depth in several hours, a surprisingly rapid rise comparable to ascent rates for kimberlite magmas.

Carbonatite Melts and Electrical Conductivity in the Asthenosphere

Electrically conductive regions in Earths mantle have been interpreted to reflect the presence of either silicate melt or water dissolved in olivine. On the basis of laboratory measurements, we show that molten carbonates have electrical conductivities that are three orders of magnitude higher than those of molten silicate and five orders of magnitude higher than those of hydrated olivine. High conductivities in the asthenosphere probably indicate the presence of small amounts of carbonate melt in peridotite and can therefore be interpreted in terms of carbon concentration in the upper mantle. We show that the conductivity of the oceanic asthenosphere can be explained by 0.1 volume percent of carbonatite melts on average, which agrees with the carbon dioxide content of mid-ocean ridge basalts.

Atmospheric oxygenation caused by a change in volcanic degassing pressure

The Precambrian history of our planet is marked by two major events: a pulse of continental crust formation at the end of the Archaean eon and a weak oxygenation of the atmosphere (the Great Oxidation Event) that followed, at 2.45 billion years ago. This oxygenation has been linked to the emergence of oxygenic cyanobacteria and to changes in the compositions of volcanic gases, but not to the composition of erupting lavas—geochemical constraints indicate that the oxidation state of basalts and their mantle sources has remained constant since 3.5 billion years ago. Here we propose that a decrease in the average pressure of volcanic degassing changed the oxidation state of sulphur in volcanic gases, initiating the modern biogeochemical sulphur cycle and triggering atmospheric oxygenation. Using thermodynamic calculations simulating gas–melt equilibria in erupting magmas, we suggest that mostly submarine Archaean volcanoes produced gases with SO2/H2S < 1 and low sulphur content. Emergence of the continents due to a global decrease in sea level and growth of the continental crust in the late Archaean then led to widespread subaerial volcanism, which in turn yielded gases much richer in sulphur and dominated by SO2. Dissolution of sulphur in sea water and the onset of sulphate reduction processes could then oxidize the atmosphere.

Laboratory measurements of electrical conductivity of hydrous and dry silicic melts under pressure

An interpretation of the electrical signature of molten rocks within the Earth’s interior in terms of nature and temperature conditions of magma requires additional laboratory data on the electrical conductivity of silicate melts. This paper describes an experimental setup and presents measurements of the electrical impedance of dry and hydrous (1–3 wt% H2O) metaluminous obsidians determined in an internally heated pressure vessel, in the range of 50–400 MPa and 350–1325°C. It is shown that the temperature and pressure dependences of electrical conductivity of both hydrous and dry obsidian can be fitted by an Arrhenius law applying in the melt and the glass regions. This suggests that a similar transport mechanism operates in both melt and glass. The determined activation energies are 70 kJ/mol for the dry obsidian and 65 and 61 kJ/mol for the 1 and 3 wt% H2O melts, respectively. The activation volume is 20 cm3/mol. Combination of tracer diffusion and electrical conductivity reveals that sodium is the dominant charge carrier in the hydrous and dry obsidians. The temperature and pressure effects on conductivity are therefore interpreted in terms of activation energy and activation volume for Na mobility in dry and hydrous rhyolites. An increase in conductivity associated with addition of water was observed and is shown to reflect the effect of water incorporation in melts on the mobility of sodium. As a prospective, it is anticipated that both the mobility and content of sodium could control the electrical conductivity of most terrestrial silicate melts. Magma under differentiation becomes more conductive due to sodium and water enrichment associated with fractional crystallization; therefore, its electrical signature must reveal its nature and maturity.

Electrical conductivity during incipient melting in the oceanic low-velocity zone

The low-viscosity layer in the upper mantle, the asthenosphere, is a requirement for plate tectonics. The seismic low velocities and the high electrical conductivities of the asthenosphere are attributed either to subsolidus, water-related defects in olivine minerals or to a few volume per cent of partial melt, but these two interpretations have two shortcomings. First, the amount of water stored in olivine is not expected to be higher than 50 parts per million owing to partitioning with other mantle phases (including pargasite amphibole at moderate temperatures) and partial melting at high temperatures. Second, elevated melt volume fractions are impeded by the temperatures prevailing in the asthenosphere, which are too low, and by the melt mobility, which is high and can lead to gravitational segregation. Here we determine the electrical conductivity of carbon-dioxide-rich and water-rich melts, typically produced at the onset of mantle melting. Electrical conductivity increases modestly with moderate amounts of water and carbon dioxide, but it increases drastically once the carbon dioxide content exceeds six weight per cent in the melt. Incipient melts, long-expected to prevail in the asthenosphere, can therefore produce high electrical conductivities there. Taking into account variable degrees of depletion of the mantle in water and carbon dioxide, and their effect on the petrology of incipient melting, we calculated conductivity profiles across the asthenosphere for various tectonic plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (more than five million years old), incipient melts probably trigger both the seismic low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas in young plates, where seamount volcanism occurs, a higher degree of melting is expected.

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 fO2 NNO+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.

Laboratory measurements of electrical conductivities of hydrous and dry Mount Vesuvius melts under pressure

Quantitative interpretation of MT anomalies in volcanic regions requires laboratory measurements of electrical conductivities of natural magma compositions. The electrical conductivities of three lava compositions from Mt. Vesuvius (Italy) have been measured using an impedance spectrometer. Experiments were conducted on both glasses and melts between 400 and 1300°C, and both at ambient pressure in air and at high pressures (up to 400 MPa). Both dry and hydrous (up to 5.6 wt% H2O) melt compositions were investigated. A change of the conduction mechanism corresponding to the glass transition was systematically observed. The conductivity data were fitted by sample-specific Arrhenius laws on either side of Tg. The electrical conductivity increases with temperature and is higher in the order tephrite, phonotephrite to phonolite. For the three compositions investigated, increasing pressure decreases the conductivity, although the effect of pressure is relatively small. The three compositions investigated have similar activation volumes (ΔV=16-24 cm3/mol). Increasing the water content of the melt increases the conductivity. Comparison of activation energies (Ea) from conductivity and sodium diffusion, and use of the Nernst-Einstein relation allow sodium to be identified as the main charge carrier in our melts and presumably also in the corresponding glasses. Our data and those of previous studies highlight the correlation between the Arrhenius parameters Ea and σ0. A semi-empirical method allowing the determination of the electrical conductivity of natural magmatic liquids is proposed, in which the activation energy is modelled on the basis of the Anderson-Stuart model, σ0 being obtained from the compensation law and ΔV fitted from our experimental data. The model enables the electrical conductivity to be calculated for the entire range of melt compositions at Mt. Vesuvius and also predicts satisfactorily the electrical response of other melt compositions. Electrical conductivity data for Mt. Vesuvius melts and magmas are slightly lower than the electrical anomaly revealed by MT studies.

Role of non-mantle CO2 in the dynamics of volcano degassing: The Mount Vesuvius example

Mount Vesuvius, Italy, quiescent since A.D. 1944, is a dangerous volcano currently characterized by elevated CO2 emissions of debated origin. We show that such emissions are most likely the surface manifestation of the deep intrusion of alkalic-basaltic magma into the sedimentary carbonate basement, accompanied by sidewall assimilation and CO2 volatilization. During the last eruptive period (1631–1944), the carbonate-sourced CO2 made up 4.7–5.3 wt% of the vented magma. On a yearly basis, the resulting CO2 production rate is comparable to CO2 emissions currently measured in the volcanic area. The chemical and isotopic composition of the fumaroles supports the predominance of this crust-derived CO2 in volatile emissions at Mount Vesuvius.

Electrical conductivity of magma in the course of crystallization controlled by their residual liquid composition

[1] The electrical conductivity of a magma in the course of crystallization was experimentally investigated in the temperature range of 1350–1018� C. Large samples of basaltic composition with a homogeneous crystal content were synthesized in a gas mixing furnace at 1 atm pressure. The samples were analyzed by electron microprobe. The relative proportions of the phases as a function of temperature were determined. Depending on temperature, the phase assemblies included quenched silicate liquid, ±plagioclase, ±pyroxene, ±Fe-Ti oxides. The crystal content varied from 0 to 80 wt %. In response to partial crystallization, the residual liquid changed composition from basalt, to andesite, to dacite liquid. The electrical conductivity of the partially crystallized basaltic samples was measured. In addition, above liquidus conductivity measurements were conducted on compositions matching the residual liquid at different temperature. These supplemental electrical measurements allowed us to discriminate the effect of crystal content from the effect of changing liquid composition associated with partial crystallization. Combining with the modified Archie’s law a set of constraints describing the conductivity of the residual liquid versus chemical composition and temperature, we propose an equation to calculate changes in conductivity associated with partial magma crystallization. We showed how the composition of the residual liquid is critical on the electrical behavior of crystal-liquid system. The model overcomes the previous difficulties in finding a robust model for describing the electrical behavior of crystal-liquid systems. The effect of liquid composition on the electrical conductivity is related to diffusion mechanisms and transport properties in molten silicate. Combining known constraints on Na tracer diffusion and our conductivity results confirms the statements that sodium is the dominant charge carrier silicate liquids from basalt to rhyolite. These findings revealed that we need a comprehensive model that can predict the conductivity of molten silicate as a function of chemical composition.

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

Abstract 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.