The effect of pressure and water concentration on the electrical conductivity of dacitic melts: Implication for magnetotelluric imaging in subduction areas

International audienceSilica-rich hydrous magmas are commonly stored in crustal reservoirs, but are also present at mantle depths in subduction contexts as a result of slab melting in the presence of considerable amounts of water and other vol-atile species. Magnetotelluric surveys frequently identify highly conductive zones at crustal or mantle depths possibly revealing the presence of such silica-rich melts and this can be used to trace the cycling of water in sub-duction zones and its relationship with arc-magmatism. The achievement of such a purpose is impeded by poor knowledge of the electrical conductivity of both dry and hydrous silica-rich melts at pressure. To fill this gap, we performed in situ electrical conductivity measurements on a dacitic melt using a 4-wire set up to 1300 °C, 3.0 GPa and H 2 O content up to 12 wt.%. Melt conductivity is strongly correlated with its water content, and we reveal a complex effect of pressure being relatively small at low water contents and major at high water contents: with increasing water content, the activation volume ranges between 4 (dry) and 25 cm 3 /mol (H 2 O = 12 wt.%) and the activation energy decreases from 96 kJ (dry) to 62 kJ (12 wt.% H 2 O). By comparison with diffusivity data, so-dium appears to be the main charge carrier, even at high (12 wt.%) water content. A T–P–[H 2 O] model predicting the conductivity of dacitic melts shows that crustal and mantle wedge conductive bodies can be interpreted by the presence of silica-rich, hydrous, partially crystallized magma

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

La conductivité électrique des liquides riches en volatils (C-O-H) produits lors de la fusion partielle du manteau terrestre

Electromagnetic data images mantle regions more conductive than that of dry olivine. There is no doubt that melt is thermodynamically stable and present in the asthenosphere, but how they can impact on mantle electrical conductivity remains debated. Petrological studies realized some 30 years ago have shown that peridotites exposed at the P-T-fO₂ conditions of the asthenosphere produced H₂O and CO₂ rich-melts, but electrical conductivities of these melts are poorly known. Therefore, electrical conductivity experiments have been performed in piston cylinder on H₂O-CO₂ rich melts. Different melt compositions have been explored, from carbonated melts to basalts. The effects of chemical compositions and volatiles on these melts have been determined. The electrical conductivity measurements have shown that hydrous carbonated melts are very conductive, and the incorporation of basalt decreases the conductivity. With these new data, a semi-empirical law predicting the conductivity as a function of H₂O and CO₂ contents has been produced. Based on this law and the electrical conductivity of olivine, 1D conductivity profiles were constructed. With these profiles, the effect of volatile contents (partitioned between the melt and in the solids), melt fractions (mixing law and interconnection of the melt) and different temperature regimes on conductivity are discussed. These calculations are conducted on oceanic and continental settings with different ages. The electrical conductivities of the mantle is thus a powerful tool to track the fundamental process of mantle partial melting, which is in turn narrowly associated to the cycling of H₂O and CO₂ in the upper mantle.Les données électromagnétiques imagent des zones du manteau plus conductrice que l’olivine sèche. Il y a peu d’ambiguïté sur le fait qu’un liquide est thermodynamiquement stable et présent au niveau de l’asthénosphère, mais son impact sur la conductivité électrique du manteau reste débattu. Les études pétrologiques réalisées ces 30 dernières années ont montré qu’une péridotite exposée aux conditions the P-T-fO₂ de l’asthénosphère produisait des liquides riches en H₂O and CO₂, mais les conductivités électriques de ces liquides sont mal connues. Pour cette raison, des expériences de conductivité électrique ont été réalisées en piston cylindre sur des liquides riches en H₂O and CO₂. Différentes compositions de liquides ont été explorées, des liquides carbonatés aux basaltes. Les effets de la composition chimique et des volatiles sur ces liquides ont été déterminés. Les mesures de conductivités électriques ont montré que les liquides hydratés et carbonatés sont très conducteurs, et que l’incorporation de basalte décroit la conductivité. Avec ces nouvelles données, un modèle semi-empirique calculant la conductivité en fonction des teneurs en H₂O and CO₂ a été produit. Sur la base de ce modèle et de la conductivité électrique de l’olivine, des profils 1D de conductivité ont été construits. Avec ces profils, l’effet des teneurs en volatiles (partagé entre le liquide et le solide), les fractions de liquides (loi de mélange et interconnexion du liquide) et les différents régimes de température sur la conductivité ont été discutés. Ces calculs ont été considérés en milieu océanique et continental pour différents âges. La conductivité électrique du manteau est donc un outil puissant pour suivre les processus fondamentaux de la fusion du manteau, qui est à son tour étroitement liée aux cycles de H₂O and CO₂ dans le manteau supérieur

High electrical conductivities in the mantle: an experimental study on low melt fractions

International audienceMany regions in the Earth's mantle show elevated electrical conductivities. Under mid-oceanic ridges, electrical conductivities are in the range 0.1 S.m1 to 0.3 S.m1whereas off-axis conductivities show slightly lower values, which, however, remains well above expected conductivity for conventional peridotitic rocks. A correct interpretation of the petrological nature of the conductive mantle is critical for our understanding of mantle geodynamics. Conductive mantle regions most likely reflect partial melting. The presence of melts in the Earth's mantle has long been proved by geochemical observations and experimental petrology on peridotite rocks by the presence of volatile species (water, carbon dioxide, halogens) which produce small melt fractions (either silicated or carbonated in composition). Hydrated basalts show elevated conductivities, but high melt fractions that carbonated melts are very conductive: 100 to 300 S.m1. Pressure and chemical compositions have little effect on their electrical conductivities. Using different mixing laws (Hashin-Shtrikman upper bound (HS+) or tube law) between carbonated melts and olivine, it is possible to anticipate that we need less than 1% of carbonated melts in the mantle to explain high electrical conductivities. However the electrical conductivity depends on the melt configuration in grain boundaries (sheets, tube, ...). So it is very important to perform new electrical conductivity measurements on peridotite samples containing carbonated melt fraction less to 1%. Since it is very difficult to observe small melt fractions, we can only compare predicted conductivities using different mixing laws with the electrical response of partially molten peridotitic samples. We will report here new measurements of electrical conductivities of peridotitic rocks impregnated with low fractions of carbonated melts. Measurements of sample containing 0.1 to 1% will be presented. The solid matrix will be constituted of dry forsterite and multi-component synthetic peridotites. With these experiments, we will be able to deduce the melt configuration in grain boundaries and to provide a more quantitative interpretation of elevated electrical conductivities imaged in Earth's mantle

Effects of temperature, pressure and chemical compositions on the electrical conductivity of carbonated melts and its relationship with viscosity

International audienceCarbonated melts constitute a key medium in the global deep carbon cycle: their impact on the geochemical signature of deep rocks is well studied because of their role as metasomatic agents in the deep mantle. However, their physical properties and in particular their electrical conductivity at high temperature and high pressure remain poorly constrained. In this study, we investigated the effect of chemical composition on the electrical conductivity of carbonated melts. We characterized this effect for various temperatures (1000–1700 °C) and pressures (1 to 4 GPa). Measurements show a very high electrical conductivity (N 100 S·m −1) with weak temperature, pressure and chemical composition dependence. Carbonated melts are five orders of magnitude more conductive than mantle olivine, and up to two orders of magnitude more conductive than basalts at similar T and P. The electrical conductivity of molten carbonates follows an Arrhenius law and the different parameters were determined. A common activation volume was defined with ΔV = 0.275 J/bar. As a result, we are able to calculate the electrical conductivity for larger temperature and pressure ranges for the melt compositions considered here. By combining the Nernst–Einstein and Eyring equations, a remarkably simple correlation was established between electrical conductivity and viscosity. The viscosity of carbonated melts, which is a key parameter defining the rate of metasomatic fluids flowing in the earth's mantle, can therefore be calculated as a function of pressure and temperature. We used these new data to interpret the high electrical conductivity recently observed in the mantle under the Brazilian craton. The anomalously elevated conductivity most likely images the process of lithospheric rejuvenation involving 0.03 to 0.2% of carbonated melt

Electrical Propertiesof hydrous magmas

International audienceVolatiles strongly affect physical and chemical properties of magmas which are major vectors of mass and heat transfer in the Earth’s. In subduction zones, hydrated melts prevail during the entire course of differentiation from basalts, andesites, dacites to rhyolites. Several electrical surveys obtained by magneto telluric investigations are currently deployed at subduction zones. The electrical conductivity of hydrous melts is however poorly constrained: so far only three studies have experimentally addressed this topic. Here, we show in situ electrical impedance of natural dacites, andesites (from Uturuncu Volcano, Bolivia) and basaltic magmas obtained with a 4-wire set up in a piston cylinder and internally heated pressure vessel. The range of temperature (500 to 1300°C), pressure (0.3 to 2 Gpa), and the various water contents and crystal fractions covers the respective ranges occurring at natural conditions. First results show that the conductivity increases with the temperature, the melt fraction, and a slightly decreases with the pressure and the crystal fraction. The compilation of these results with previous studies (rhyolitic, phonolitic and basaltic compositions) will lead to a general model of the electrical properties of magmas. Such a model will help in (i) interpreting the electrical signature of natural magmas and (ii) constraining their conditions (chemical composition, temperature, pressure, water content, melt fraction) from the source to the storage location

Petrologically-based Electrical Profiles vs. Geophysical Observations through the Upper Mantle (Invited)

International audienceMineralogical transformations in the up-welling mantle play a critical role on the dynamics of mass and heat transfers at mid-ocean-ridgeS. The melting event producing ridge basalts occur at 60 km depth below the ridge axis, but because of small amounts of H2O and CO2 in the source region of MOR-basalts, incipient melting can initiate at much greater depth. Such incipient melts concentrate incompatible elements, and are particularly rich in volatile species. These juices evolve from carbonatites, carbonated basalts, to CO2-H2O-rich basalts as recently exposed by petrological surveys; the passage from carbonate to silicate melts is a complex pathway that is strongly non-linear. This picture has recently been complicated further by studies showing that oxygen increasingly partitions into garnet as pressure increases; this implies that incipient melting may be prevented at depth exceeding 200 km because not enough oxygen is available in the system to stabilize carbonate melts. The aim of this work is twofold: - We modelled the complex pathway of mantle melting in presence of C-O-H volatiles by adjusting the thermodynamic properties of mixing in the multi-component C-O-H-melt system. This allows us to calculate the change in melt composition vs. depth following any sortS of adiabat. - We modelled the continuous change in electrical properties from carbonatites, carbonated basalts, to CO2-H2O-rich basalts. We then successfully converted this petrological evolution along a ridge adiabat into electrical conductivity vs. depth signal. The discussion that follows is about comparison of this petrologically-based conductivity profile with the recent profiles obtained by inversion of the long-period electromagnetic signals from the East-Pacific-Rise. These geophysically-based profiles reveal the electrical conductivity structure down to 400 km depth and they show some intriguing highly conductive sections. We will discuss heterogeneity in electrical conductivity of the upper mantle underneath the ridge in terms of melting processes. Our prime conclusion is that the redox melting process, universally predicted by petrological models, might not be universal and that incipient melting can extend down to the transition zone

Liquid–liquid transition and critical point in sulfur

International audienceThe liquid-liquid transition (LLT), in which a single-component liquid transforms into another one via a first-order phase transition, is an intriguing phenomenon that has changed our perception of the liquid state. LLTs have been predicted from computer simulations of water 1,2 , silicon 3 , carbon dioxide 4 , carbon 5 , hydrogen 6 and nitrogen 7. Experimental evidence has been found mostly in supercooled (that is, metastable) liquids such as Y 2 O 3-Al 2 O 3 mixtures 8 , water 9 and other molecular liquids 10-12. However, the LLT in supercooled liquids often occurs simultaneously with crystallization, making it difficult to separate the two phenomena 13. A liquid-liquid critical point (LLCP), similar to the gas-liquid critical point, has been predicted at the end of the LLT line that separates the low-and high-density liquids in some cases, but has not yet been experimentally observed for any materials. This putative LLCP has been invoked to explain the thermodynamic anomalies of water 1. Here we report combined in situ density, X-ray diffraction and Raman scattering measurements that provide direct evidence for a first-order LLT and an LLCP in sulfur. The transformation manifests itself as a sharp density jump between the low-and high-density liquids and by distinct features in the pair distribution function. We observe a non-monotonic variation of the density jump with increasing temperature: it first increases and then decreases when moving away from the critical point. This behaviour is linked to the competing effects of density and entropy in driving the transition. The existence of a first-order LLT and a critical point in sulfur could provide insight into the anomalous behaviour of important liquids such as water

The effect of Mg concentration in silicate glasses on CO 2 solubility and solution mechanism: Implication for natural magmatic systems

International audienceFollowing an experimental approach conducted between 0.5 and 1.5 GPa, we investigated the change in CO2 solubility as a function of the XMg (MgO/(MgO+CaO)) for a range of silicate glasses. The synthesised CO2-bearing glasses have XMg up to 0.72, stoichiometric NBO/T (degree of polymerization) up to 2.6 corresponding to highly depolymerized compositions analogues to kimberlites. Several samples were synthesised with 17O enrichment to investigate the CO2 dissolution mechanism via the change in O species environments by NMR spectroscopy.The experimental results show that CO2 solubility increases with NBO/T in agreement with previous works. In addition, increasing XMg strongly decreases CO2 solubility: from 18 to 7 wt.% CO2 as XMg ranges from 0 to 0.6 (1.5 GPa and NBO/T ~ 2).17O NMR results demonstrate that CO2 molecules dissolve as CO3^2- groups showing a signal at +146 ppm for which the intensity is linearly correlated to the wt.% CO2 determined by Raman. The analysis of the oxygen environments as a function of CO2 content for MgO^NBO (+62 ppm) and CaO^NBO (+103 ppm) show that CO2 dissolves preferentially in the vicinity of Ca2+ atoms. The difference in CO2 solubility is explained by the ability for Mg2+ cations to act as a weak network former and to be present in four-fold coordination or by the stronger affinity of CO2 molecules for Ca2+ rather than for Mg2+. We show that the CO2 solubility is negatively correlated to the melt ionic field strength which reflects the variation in the affinity of CO2 molecules for one cation or another.Strongly depolymerized mantle melts, such as kimberlites, melilitites, nephelinites and basanites will exhibit lower CO2 solubility than currently assumed due to their high MgO content which must imply degassing at greater depth, potentially in the sub-lithospheric mantle