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

International audienceMount 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

Carbonatite Melts and Electrical Conductivity in the Asthenosphere

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

Extremely reducing conditions reached during basaltic intrusion in organic matter-bearing sediments

International audienceRedox conditions in magma are widely interpreted as internally buffered and closely related to that of their mantle source regions. We use thermodynamic calculations to show that high-temperature interaction between magma and organic matter can lead to a dramatic reduction of the magma redox state, and significant departure from that of the original source. Field studies provide direct evidence of the process that we describe, with reported occurrences of graphite and native iron in igneous mafic rocks, implying very reducing conditions that are almost unknown in average terrestrial magmas. We calculate that the addition of 0.6 wt% organic matter (in the form of CH or CH2) to a standard basalt triggers graphite and native iron crystallisation at depths of few hundred meters. Interaction with organic matter also profoundly affects the abundance and the redox state of the gases in equilibrium with the magma, which are CO-dominated with H2 as the second most abundant species on a molar basis, H2O and CO2 being minor constituents. The assimilation of only 0.1 wt% organic matter by a basalt causes a decrease in its oxygen fugacity of 2-orders of magnitude. The assimilation of 0.6 wt% organic matter at depths < 500 m implies minimum CO content in the magma of 1 wt%, other gas components being less than 0.1 wt%. In the light of our calculations, we suggest that the production of native iron-bearing lava flows and associated intrusions was most likely accompanied by degassing of CO-rich gases, whose fluxes depended on the magma production rates

Melt inclusions track changes in chemistry and oxidation state of Etnean magmas

Mount Etna (Italy) is a stratovolcano, located near the convergent boundary between African and European plates. Since its appearance, it was characterized by continuous variability of eruptive style and magma composition, though more subtle. Currently, its volcanic activity consists of effusive and explosive eruptions marked by high gas fluxes. Olivine hosted melt inclusions (MIs), belonging to products of the last 15 ky, were analysed for their chemical composition, volatiles contents and Fe speciation, in order to interpret the chemical variability and to evaluate the oxidation state of Etnean magmas and its eventual evolution. Olivine phenocrysts were selected from the most primitive Fall Stratified (FS) eruption of picritic composition (Fo91), from the oldest Mt. Spagnolo and from more recent eruptions: 2002-2003, 2006, 2008-2009, and 2013; the MIs of some of these eruptions (Mt Spagnolo, 2008-2009 and 2013) are here investigated for the first time. The variability of the major elements contents in the MIs designates a continuous differentiation trend, marked by the decrease of MgO and CaO/Al2O3 ratio and the increase of alkalis. The volatiles content in etnean magmas is extremely variable. The highest H2O (5-6 wt.%) and CO2 (~0.5 wt.%) contents are found in FS magma entrapped at depth of 16-18 km (below crater level). S content achieves 4150 ppm in the older Mt. Spagnolo inclusions, completely H2O and CO2\u2013free. Fe3+/\u3a3Fe ratios obtained from XANES spectra for some melt inclusions, generally decrease from the most primitive and volatile-rich FS to the most evolved and degassed melts, suggesting changing in the oxidation state of etnean magmas. Petrological arguments coupled to modelling of fractional crystallization and degassing processes concur to suggest that the magmas of Mt. Spagnolo and of the recent eruptions may be produced by differentiation from the most oxidized and hydrous pristine FS magma along highly variable P-T paths, occasionally accompanied by mixing processes

Economic geology: Volatile destruction

International audienceDirect evidence for the role of volatiles in magmatic ore formation has been elusive. Magma degassing at Merapi volcano in Indonesia is found to be directly linked to the selective leaching of metals from sulphide melts that ultimately form ore deposits

Upward migration of Vesuvius magma chamber over the past 20 thousand years

International audienceForecasting future eruptions of Vesuvius is an important challenge for volcanologists, as its reawakening could threaten the lives of 700,000 people living near the volcano1,2. 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 16km (refs 3– 7). Geophysical surveys have imaged some levels of seismic attenuation, the shallowest of which lies at 8–9km depth8, 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 7–8km to 3–4km depth between the AD 79 (Pompeii) and AD 472 (Pollena) events. If data from the Pomici di Base event 18.5 kyr ago9 and the 1944 Vesuvius eruption7 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 formation10 or volcano spreading11. Reservoir migration, and the possible influence on feeding rates12, should be integrated into the parameters used for defining expected eruptive scenarios at Vesuvius

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

International audienceThe low-viscosity layer in the upper mantle, the asthenosphere, is a requirement for plate tectonics1. The seismic low velocities and the high electrical conductivities of the asthenosphere are attributed either to subsolidus, water-related defects in olivine minerals2, 3, 4 or to a few volume per cent of partial melt5, 6, 7, 8, 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 phases9 (including pargasite amphibole at moderate temperatures10) and partial melting at high temperatures9. 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 segregation11, 12. 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 asthenosphere10, 13, 14, 15, 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 plates4, where seamount volcanism occurs6, a higher degree of melting is expected

The degassing of magma and planetary redox dynamics

International audienceIn this talk, I review how volcanic degassing can be modeled from solubility laws in multicomponent silicate melts and how such models can be used to broadly capture the key role of planetary degassing on the redox bio-geo-dynamics. Applications of such degassing model are relevant to the early Earth as well as other planetary systems. After this introduction, I will here focus on two applications related to small planetary bodies: (i) the degassing from Jupiter's moon Io, that is a differentiated body, and (ii) the degassing of small undifferentiated primitive terrestrial bodies. In both cases, degassing triggers an important decrease in oxygen fugacity as it has been noted for volcanic system on Earth. However, because degassing in such systems occurs under very low pressure conditions (<0.01 bar), this phenomena is more extreme: degassing on Io produces conditions close to FMQ-4, that is, close to the domain of metal iron saturation, while degassing of carbonaceous material from un-differentiated bodies leads to much more reduced conditions, where iron exclusively exists in the metal form, like in enstatite chondrites

The degassing of magma and planetary redox dynamics

International audienceIn this talk, I review how volcanic degassing can be modeled from solubility laws in multicomponent silicate melts and how such models can be used to broadly capture the key role of planetary degassing on the redox bio-geo-dynamics. Applications of such degassing model are relevant to the early Earth as well as other planetary systems. After this introduction, I will here focus on two applications related to small planetary bodies: (i) the degassing from Jupiter's moon Io, that is a differentiated body, and (ii) the degassing of small undifferentiated primitive terrestrial bodies. In both cases, degassing triggers an important decrease in oxygen fugacity as it has been noted for volcanic system on Earth. However, because degassing in such systems occurs under very low pressure conditions (<0.01 bar), this phenomena is more extreme: degassing on Io produces conditions close to FMQ-4, that is, close to the domain of metal iron saturation, while degassing of carbonaceous material from un-differentiated bodies leads to much more reduced conditions, where iron exclusively exists in the metal form, like in enstatite chondrites