Scaillet, Bruno

To testmechanisms of basalticmagma degassing, continuous decompressions of volatile-bearing (2.7–3.8 wt% H2O, 600–1,300 ppm CO2) Stromboli melts were performed from 250–200 to 50–25 MPa at 1,180–1,140 C.Ascent rates were varied from 0.25 to *1.5 m/s. Glasses after decompression show a wide range of textures, from totally bubblefree to bubble-rich, the latter with bubble number densities from 104 to 106 cm-3, similar to Stromboli pumices. Vesicularities range from 0 to *20 vol%. Final melt H2O concentrations are homogeneous and always close to solubilities. In contrast, the rate of vesiculation controls the finalmelt CO2 concentration. High vesicularity charges have glass CO2 concentrations that follow theoretical equilibrium degassing paths, whereas glasses from low vesicularity charges show marked deviations from equilibrium, with CO2 concentrations up to one order of magnitude higher than solubilities. FTIR profiles and maps reveal glass CO2 concentration gradients near the gas–melt interface. Our results stress the importance of bubble nucleation and growth, and of volatile diffusivities, for basaltic melt degassing. Two characteristic distances, the gas interface distance (distance either between bubbles or to gas–melt interfaces) and the volatile diffusion distance, control the degassing process. Melts containing numerous and large bubbles have gas interface distances shorter than volatile diffusion distances, and degassing proceeds by equilibrium partitioning of CO2 and H2O between melt and gas bubbles. For melts where either bubble nucleation is inhibited or bubble growth is limited, gas interface distances are longer than volatile diffusion distances. Degassing proceeds by diffusive volatile transfer at the gas– melt interface and is kinetically limited by the diffusivities of volatiles in the melt. Our experiments show that CO2-oversaturated melts can be generated as a result of magma decompression. They provide a new explanation for the occurrence of CO2-rich natural basaltic glasses and open new perspectives for understanding explosive basaltic volcanism

Crystallization experiments on two pantellerites from Pantelleria, Italy, provide new evidence for the relationships between mineral phases in pantelleritic rocks aswell as the influence of temperature and redox conditions on mineral assemblages. Experiments were performed at 1 kbar with temperature ranging between 750–900°C, and fluid saturation conditionswith XH2O (=H2O/H2O+CO2) between 0 and 1. Redox conditionswere fixed at, or slightly below, the FMQbuffer. Results showthat at temperature of 900 °C pantelleriticmagmas arewell above the liquidus regardless their water content; we also observed a decrease in liquidus temperature (800°C) with increasingly reducing conditions. Mineral assemblages of the natural rocks have been successfully reproduced, particularly the relationship between fayalite and aenigmatite, which appear to be strongly controlled by melt peralkalinity, temperature and redox conditions. This is the first time that fayalitic olivine have been synthetized in experimental studies on pantellerites, which was found to be stable only for temperatures ≥750°C while amphibole can be stable at temperatures as high as 800°C at high fF2. Experimental results have been compared with the composition of mineral phases as well as with the results obtained from mineral equilibrium, geothermometry, and oxygen barometry studies on pantelleritic lava and tuffs from Pantelleria (Italy), Eburru (Kenya) and Menengai (Kenya). Petrological characteristics appear similar at different locations worldwide, with typical assemblages of anorthoclase and sodian clinopyroxene with variable fayalite, aenigmatite, Fe-Ti oxides and amphibole.

The relationships between trachytes and peralkaline rhyolites (i.e. pantellerites and comendites), which occur in many continental rift systems, oceanic islands and continental intraplate settings, is unclear. To fill this gap, we have performed phase equilibrium experiments on two representative metaluminous trachytes from Pantelleria to determine both their pre-eruptive equilibration conditions (pressure, temperature, H2O content and redox state) and liquid lines of descent. Experiments were performed in the temperature range 750–950 C, pressure 0 5–1 5 kbar and fluid saturation conditions with XH2O [¼H2O/(H2OþCO2)] ranging between zero and unity. Redox conditions were fixed below the nickel–nickel oxide buffer (NNO). The results show that at 950 C and melt water contents (H2Omelt) close to saturation, trachytes are at liquidus conditions at all pressures. Clinopyroxene is the liquidus phase, being followed by iron-rich olivine and alkali feldspar. Comparison of experimental and natural phases (abundances and compositions) yields the following pre-eruptive conditions: P¼160 5 kbar, T¼925625 C, H2Omelt¼261wt %, and fO2 between NNO– 0 5 and NNO– 2. A decrease in temperature from 950 C to 750 C, as well as of H2Omelt, promotes a massive crystallization of alkali feldspar to over 80 wt %. Iron-bearing minerals show gradual iron enrichment when T and fO2 decrease, trending towards the compositions of the phenocrysts of natural pantellerites. Despite the metaluminous character of the bulk-rock compositions, residual glasses obtained after 80 wt % crystallization evolve toward comenditic compositions, owing to profuse alkali feldspar crystallization, which decreases the Al2O3 of the melt, leading to a consequent increase in the peralkalinity index [PI¼molar (Na2OþK2O)/Al2O3]. This is the first experimental demonstration that peralkaline felsic derivatives can be produced by low-pressure fractional crystallization of metaluminous mafic magmas. Our results show that the pantelleritic magmas of basalt–trachyte–rhyolite igneous suites require at least 95 wt % of parental basalt crystallization, consistent with trace element evidence. Redox conditions, through their effect on Fe–Ti oxide stabilities, control the final iron content of the evolving melt.

New experimental data of Ar and Ne solubility at pressures up to 360 MPa in alkali-basaltic (Mt. Etna, Italy) and rhyolitic (Vulcano Island, Italy) melts are presented. Solubility experiments have been conducted in internally heated pressure vessels at 1200 °C under nominally anhydrous conditions. Ar and Ne contents dissolved in the experimental glasses were then measured by quadrupole mass spectrometry. Over the pressure range investigated, Ar and Ne solubilities vary linearly with Ar and Ne pressures and can be described by Henry's constant (kAr,Ne=PAr, Ne /xAr, Ne, where PAr, Ne is the partial pressure of Ar or Ne and xAr, Ne is the molar fraction of Ar or Ne in the melt) of 7.6±0.8×105 and 1.9±0.4×105 MPa, respectively for Ar and Ne in the basaltic melt and 1.5±0.2×105 and 3.8±0.2×104 MPa, respectively for Ar and Ne in the rhyolitic melt. In accordance with existing models, rhyolitic melts show higher noble gas solubilities than basaltic melts, Ne solubility being higher than that of Ar in a given composition. We propose a semi-empirical model of noble gas (Ar, Ne and He) solubility calibrated on a very large set of measurements in natural and synthetic silicate melts. The model expands the concept of ionic porosity in terms of porosity accessible for noble gas dissolution in melt, taking into account the large-scale structural effects of cations, as well as temperature and pressure. The model is valid over a wide range of temperatures (800–1600 °C), pressures (up to 3 GPa) and compositions, being useful for both geological and physico-chemical studies.

Electrically conductive regions in the Earth 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 3 orders of magnitude higher than those of molten silicate and 5 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 % of carbonatite melts on average, which agrees with the CO2 content of Mid Ocean Ridge Basalts.

characterization of ultra-calcic arc melts, equilibrium phase relations have been determined experimentally for the La Sommata basalt (Som-1, Vulcano, Aeolian arc). Som-1 (Na2O + K2O = 4.46 wt.%, CaO = 12.97 wt.%, MgO = 8.78 wt.%, CaO/Al2O3 = 1.03) is a reference primitive ne-normative arc basalt with a strong ultra-calcic affinity. The experiments have been performed between 44 and 154 MPa, 1050 and 1150 °C and from NNO + 0.2 to NNO + 1.9. Fluid-present conditions were imposed with H2O–CO2 mixtures yielding melt H2O concentrations from0.7 to 3.5wt.%. Phases encountered include clinopyroxene, olivine, plagioclase and Fe-oxide. Clinopyroxene is slightly earlier than olivine in the crystallization sequence. It is the liquidus phase at 150 MPa, being joined by olivine on the liquidus between 44 and 88MPa. Plagioclase is the third phase to appear in the crystallization sequence and orthopyroxene was not found. Experimental clinopyroxenes (Fs7–16) and olivines (Fo78–92) partially reproduce the natural phenocryst compositions (respectively Fs5–7 and Fo87–91). Upon progressive crystallization, experimental liquids shift towards higher SiO2 (up to ~55 wt.%), Al2O3 (up to ~18 wt.%) and K2O (up to ~5.5wt.%) and lower CaO,MgO and CaO/Al2O3. Experimental glasses and natural whole-rock compositions overlap, indicating that progressive crystallization of Som-1 type melts can generate differentiated compositions such as those encountered at Vulcano. The lowpressure cotectic experimental glasses reproduce glass inclusions in La Sommata clinopyroxene but contrast with glass inclusions in olivine which preserve basaltic melts more primitive than Som-1. Phase relations for the La Sommata basalt are identical in all critical aspects to those obtained previously on a synthetic ultra-calcic arc composition. In particular, clinopyroxene+olivine co-saturation occurs at very low pressures (≤100 MPa). Ultra-calcic arc compositions do not represent primary mantle melts but result from the interaction between a primary mantle melt and clinopyroxene-bearing rocks in the arc crust. At Vulcano, primitive ultra-calcic end-member melts were generated between 250 and 350 MPa in the lower magma accumulation zone by reaction between hot primitive melts and wehrlitic or gabbroic lithologies. At Stromboli, golden pumices and glass inclusions with an ultra-calcic affinity were also generated at shallow pressures, between 150 and 250 MPa, suggesting that the interaction model is of general significance in the Aeolian arc.

From oxic atmosphere to metallic core, the Earth’s components are broadly stratified with respect to oxygen fugacity. A simple picture of reducing oxygen fugacity with depth may be disrupted by the accumulation of oxidised crustal material in the deep lower mantle, entrained there as a result of subduction. While hotspot volcanoes are fed by regions of the mantle likely to have incorporated such recycled material, the oxygen fugacity of erupted hotspot basalts had long been considered comparable to or slightly more oxidised than that of mid-ocean ridge basalt (MORB) and more reduced than subduction zone basalts. Here we report measurements of the redox state of glassy crystal-hosted melt inclusions from tephra and quenched lava samples from the Canary and Cape Verde Islands, that we can independently show were entrapped prior to extensive sulphurdegassing. We find high ferric iron to total iron ratios (Fe3+/ Fe) of up to 0.27–0.30, indicating that mantle plume primary melts are significantly more oxidised than those associated with mid-ocean ridges and even subduction zone. These results, together with previous investigations from the Erebus, Hawaiian and Icelandic hotspots, confirm that mantle upwelling provides a return flow from the deep Earth for components of oxidised subducted lithosphere and suggest that highly oxidised material accumulates or is generated in the lower mantle. The oxidation state of the Earth’s interior must therefore be highly heterogeneous and potentially locally inversely stratified.

Forecasting future eruptions of Vesuvius is an important challenge for volcanologists, as its reawakening could threaten the lives of 700,000 people living near the volcano. Critical to the evaluation of hazards associated with the next eruption is the estimation of the depth of the magma reservoir, one of the main parameters controlling magma properties and eruptive style. Petrological studies have indicated that during past activity, magma chambers were at depths between 3 and 16 km. Geophysical surveys have imaged some levels of seismic attenuation, the shallowest of which lies at 8–9 km depth, and these have been tentatively interpreted as levels of preferential magma accumulation. By using experimental phase equilibria, carried out on material from four main explosive events at Vesuvius, we show here that the reservoirs that fed the eruptive activity migrated from 7–8 km to 3–4 km depth between the AD 79 (Pompeii) and AD 472 (Pollena) events. If data from the Pomici di Base event 18.5 kyr ago and the 1944 Vesuvius eruption are included, the total upward migration of the reservoir amounts to 9–11 km. The change of preferential magma ponding levels in the upper crust can be attributed to differences in the volatile content and buoyancy of ascending magmas, as well as to changes in local stress field following either caldera formation or volcano spreading. Reservoir migration, and the possible influence on feeding rates, should be integrated into the parameters used for defining expected eruptive scenarios at Vesuvius.

We present an empirical model of sulphur solubility that allows us to calculate f S2 if P, T, fO2 and the melt composition, including H2O and S, are known. The model is calibrated against three main experimental data bases consisting in both dry and hydrous silicate melts. Its prime goal is to calculate the f S2 of hydrous basalts that currently lack experimental constraints of their sulphur solubility behaviour. Application of the model to Stromboli, Vesuvius, Vulcano and Etna eruptive products shows that the primitive magmas found at these volcanoes record f S2 in the range 0.1-1 bar. In contrast, at all volcanoes the magmatic evolution is marked by dramatic variations in f S2 that spreads over up to 9 orders of magnitude. The f S2 can either increase during differentiation or decrease during decompression to shallow reservoirs, and seems to be related to closed versus open conduit conditions, respectively. The calculated f S2 shows that the Italian magmas are undersaturated in a FeS melt, except during closed conduit conditions, in which case differentiation may eventually reach conditions of sulphide melt saturation. The knowledge of f S2, fO2 and fH2O allows us to calculate the fluid phase composition coexisting with magmas at depth in the C-O-H-S system. Calculated fluids show a wide range in composition, with CO2 mole fractions of up to 0.97. Except at shallow levels, the fluid phase is generally dominated by CO2 and H2O species, the mole fractions of SO2 and H2S rarely exceeding 0.05 each. The comparison between calculated fluid compositions and volcanic gases shows that such an approach should provide constraints on both the depth and mode of degassing, as well as on the amount of free fluid in magma reservoirs. Under the assumption of a single step separation of the gas phase in a closed-system condition, the application to Stromboli and Etna suggests that the main reservoirs feeding the eruptions and persistent volcanic plumes at these volcanoes might contain as much as 5 wt% of a free fluid phase. Consideration of the magma budget needed to balance the amounts of volatiles emitted in the light of these results shows that the amount of nonerupted magma could be overestimated by as much as one order of magnitude.

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.