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2018
Pommier, A, Leinenweber KD.  2018.  Electrical cell assembly for reproducible conductivity experiments in the multi-anvil. American Mineralogist. 103:1298-1305.   10.2138/am-2018-6448   AbstractWebsite

Electrical conductivity experiments under pressure and temperature conditions relevant to planetary interiors are a powerful tool to probe the transport properties of Earth and planetary materials as well as to interpret field-based electrical data. To promote repeatability and reproducibility of electrical experiments among multi-anvil facilities that use this technique, we designed and developed an electrical conductivity cell for multi-anvil experiments based on the 14/8 assembly that was developed to allow access to high temperatures. Here we present the details of design and parts developed for this cell that is available via the Consortium for Material Properties Research in Earth Sciences (COMPRES). The electrical cell has been tested up to 10 GPa and about 2000 degrees C on different materials (silicates and metals, both in the solid and liquid state).

Pommier, A.  2018.  Influence of sulfur on the electrical resistivity of a crystallizing core in small terrestrial bodies. Earth and Planetary Science Letters. 496:37-46.   10.1016/j.epsl.2018.05.032   AbstractWebsite

Electrical experiments were performed on core analogues in the Fe-S system and on FeSi2 up to 8 GPa and 1850 degrees C in the multi-anvil apparatus. Electrical resistivity was measured using the four electrode method. For all samples, resistivity increases with increasing temperature. The higher the S content, the higher the resistivity and the resistivity increase upon melting. At 4.5 GPa, liquid FeS is up to >10 times more resistive than Fe-5 wt.% S and twice more resistive than FeSi2, suggesting a stronger influence of S than Si on liquid resistivity. Electrical results are used to develop crystallization resistivity paths considering both equilibrium and fractional crystallization in the Fe-S system. At 4.5 GPa, equilibrium crystallization, as expected locally in thin snow zones during top-down core crystallization, presents electrical resistivity variations from about 300 to 190 microhm-cm for a core analogue made of Fe-5 wt.%S, depending on temperature. Fractional crystallization, which is relevant to core-scale cooling, leads to more important electrical resistivity variations, depending on S distribution across the core, temperature, and pressure. Estimates of the lower bound of thermal resistivity are calculated using the Wiedemann-Franz law. Comparison with previous works indicates that the thermal conductivity of a metallic core in small terrestrial bodies is more sensitive to the abundance of alloying agents than that of the Earth's core. Application to Ganymede using core adiabat estimates from previous studies suggests important thermal resistivity variations with depth during cooling, with a lower bound value at the top of the core that can be as low as 3 Wim K. It is speculated that the generation and sustainability of a magnetic field in small terrestrial bodies might be favored in light element-depleted cores. (C) 2018 Elsevier B.V. All rights reserved.

Pommier, A, Laurenz V, Davies CJ, Frost DJ.  2018.  Melting phase relations in the Fe-S and Fe-S-O systems at core conditions in small terrestrial bodies. Icarus. 306:150-162.   10.1016/j.icarus.2018.01.021   AbstractWebsite

We report an experimental investigation of phase equilibria in the Fe-S and Fe-S-O systems. Experiments were performed at high temperatures (1400-1850 degrees C) and high pressures (14 and 20 GPa) using a multi anvil apparatus. The results of this study are used to understand the effect of sulfur and oxygen on core dynamics in small terrestrial bodies. We observe that the formation of solid FeO grains occurs at the FeS liquid - Fe solid interface at high temperature ( > 1400 degrees C at 20 GPa). Oxygen fugacities calculated for each O-bearing sample show that redox conditions vary from Delta 1W= 0.65 to 0. Considering the relative density of each phase and existing evolutionary models of terrestrial cores, we apply our experimental results to the cores of Mars and Ganymede. We suggest that the presence of FeO in small terrestrial bodies tends to contribute to outer-core compositional stratification. Depending on the redox and thermal history of the planet, FeO may also help form a transitional redox zone at the core-mantle boundary. (c) 2018 Elsevier Inc. All rights reserved.

Pommier, A, Kohlstedt DL, Hansen LN, Mackwell S, Tasaka M, Heidelbach F, Leinenweber K.  2018.  Transport properties of olivine grain boundaries from electrical conductivity experiments. Contributions to Mineralogy and Petrology. 173   10.1007/s00410-018-1468-z   AbstractWebsite

Grain boundary processes contribute significantly to electronic and ionic transports in materials within Earth's interior. We report a novel experimental study of grain boundary conductivity in highly strained olivine aggregates that demonstrates the importance of misorientation angle between adjacent grains on aggregate transport properties. We performed electrical conductivity measurements of melt-free polycrystalline olivine (Fo(90)) samples that had been previously deformed at 1200 degrees C and 0.3 GPa to shear strains up to gamma = 7.3. The electrical conductivity and anisotropy were measured at 2.8 GPa over the temperature range 700-1400 degrees C. We observed that (1) the electrical conductivity of samples with a small grain size (3-6 mu m) and strong crystallographic preferred orientation produced by dynamic recrystallization during large-strain shear deformation is a factor of 10 or more larger than that measured on coarse-grained samples, (2) the sample deformed to the highest strain is the most conductive even though it does not have the smallest grain size, and (3) conductivity is up to a factor of similar to 4 larger in the direction of shear than normal to the shear plane. Based on these results combined with electrical conductivity data for coarse-grained, polycrystalline olivine and for single crystals, we propose that the electrical conductivity of our fine-grained samples is dominated by grain boundary paths. In addition, the electrical anisotropy results from preferential alignment of higher-conductivity grain boundaries associated with the development of a strong crystallographic preferred orientation of the grains.

Davies, CJ, Pommier A.  2018.  Iron snow in the Martian core? Earth and Planetary Science Letters. 481:189-200.   10.1016/j.epsl.2017.10.026   AbstractWebsite

The decline of Mars' global magnetic field some 3.8-4.1 billion years ago is thought to reflect the demise of the dynamo that operated in its liquid core. The dynamo was probably powered by planetary cooling and so its termination is intimately tied to the thermochemical evolution and present-day physical state of the Martian core. Bottom-up growth of a solid inner core, the crystallization regime for Earth's core, has been found to produce a long-lived dynamo leading to the suggestion that the Martian core remains entirely liquid to this day. Motivated by the experimentally-determined increase in the Fe-S liquidus temperature with decreasing pressure at Martian core conditions, we investigate whether Mars' core could crystallize from the top down. We focus on the "iron snow" regime, where newly-formed solid consists of pure Fe and is therefore heavier than the liquid. We derive global energy and entropy equations that describe the long-timescale thermal and magnetic history of the core from a general theory for two-phase, two-component liquid mixtures, assuming that the snow zone is in phase equilibrium and that all solid falls out of the layer and remelts at each timestep. Formation of snow zones occurs for a wide range of interior and thermal properties and depends critically on the initial sulfur concentration, to. Release of gravitational energy and latent heat during growth of the snow zone do not generate sufficient entropy to restart the dynamo unless the snow zone occupies at least 400 km of the core. Snow zones can be 1.5-2 Gyrs old, though thermal stratification of the uppermost core, not included in our model, likely delays onset. Models that match the available magnetic and geodetic constraints have to xi(0) approximate to 10% and snow zones that occupy approximately the top 100 km of the present-day Martian core. (C) 2017 The Authors. Published by Elsevier B.V.

2017
Zhang, Z, Pommier A.  2017.  Electrical Investigation of Metal-Olivine Systems and Application to the Deep Interior of Mercury. Journal of Geophysical Research: Planets.   10.1002/2017JE005390   Abstract

We report electrical conductivity measurements on metal-olivine systems at about 5 and 6 GPa and up to 1,675°C in order to investigate the electrical properties of core-mantle boundary (CMB) systems. Electrical experiments were conducted in the multianvil apparatus using the impedance spectroscopy technique. The samples are composed of one metal layer (Fe, FeS, FeSi2, or Fe-Ni-S-Si) and one polycrystalline olivine layer, with the metal:olivine ratio ranging from 1:0.7 to 1:9.2. For all samples, we observe that the bulk electrical conductivity increases with temperature from 10−2.5 to 101.8 S/m, which is higher than the conductivity of polycrystalline olivine but lower than the conductivity of the pure metal phase at similar conditions. In some experiments, a conductivity jump is observed at the temperature corresponding to the melting temperature of the metallic phase. Both the metal:olivine ratio and the metal phase geometry control the electrical conductivity of the two-layer samples. By combining electrical results, textural analyses of the samples, and previous studies of the structure and composition of Mercury's interior, we propose an electrical profile of the deep interior of the planet that accounts for a layered CMB-outer core structure. The electrical model agrees with existing conductivity estimates of Mercury's lower mantle and CMB using magnetic observations and thermodynamic calculations, and thus, supports the hypothesis of a layered CMB-outermost core structure in the present-day interior of Mercury. We propose that the layered CMB-outer core structure is possibly electrically insulating, which may influence the planet's structure and cooling history.

Pommier, A, Evans RL.  2017.  Constraints on fluids in subduction zones from electromagnetic data. Geosphere. 13:1026-1041.   10.1130/ges01473.1   AbstractWebsite

Magnetotelluric data have been increasingly used to image subduction zones. Models of electrical resistivity commonly show features related to the release of fluids at several depths through the systems imaged, consistent with thermal and petrologic models of dehydration of the downgoing slab. Imaging the release of fluids from sediments and pore space in the crust requires controlled source electromagnetic techniques, which have to date only been used in one setting, offshore Nicaragua. The release of fluids related to the transition of basalt to eclogite is commonly imaged with magnetotelluric data. Deeper fluid release signals, from the breakdown of minerals like serpentine, are highly variable. We hypothesize that regions where very strong conductive anomalies are observed in the mantle wedge at depths of similar to 80-100 km are related to the subduction of anomalous seafloor, either related to excessive fracturing of the crust (e.g., fracture zones), subduction of seamounts, or other ridges and areas of high relief. These features deform the seafloor prior to entering the trench, permitting more widespread serpentinization of the mantle than would otherwise occur. An alternative explanation is that the large conductors represent melts with higher contents of crustal-derived volatiles (such as C and H), suggesting in particular locally higher fluxes of carbon into the mantle wedge, perhaps also associated with subduction of anomalous seafloor structures with greater degrees of hydrothermal alteration.

2015
Pommier, A, Leinenweber K, Tasaka M.  2015.  Experimental investigation of the electrical behavior of olivine during partial melting under pressure and application to the lunar mantle. Earth and Planetary Science Letters. 425:242-255.   10.1016/j.epsl.2015.05.052   AbstractWebsite

Electrical conductivity measurements were performed during melting experiments of olivine compacts (dry and hydrous Fo(77) and Fo(90)) at 4 and 6 GPa in order to investigate melt transport properties and quantify the effect of partial melting on electrical properties. Experiments were performed in the multi-anvil apparatus and electrical measurements were conducted using the impedance spectroscopy technique with the two-electrode method. Changes in impedance spectra were used to identify the transition from an electrical response controlled by the solid matrix to an electrical response controlled by the melt phase. This transition occurs slightly above the solidus temperature and lasts until T-solidus + 75 degrees C (+/- 25). At higher temperature, a significant increase in conductivity (corresponding to an increase in conductivity values by a factor ranging from similar to 30 to 100) is observed, consistent with the transition from a tube-dominated network to a structure in which melt films and pools become prominent features. This increase in conductivity corresponds to an abrupt jump for all dry samples and to a smoother increase for the hydrous sample. It is followed by a plateau at higher temperature, suggesting that the electrical response of the investigated samples lacks sensitivity to temperature at an advanced stage of partial melting. Electron microprobe analyses on quenched products indicated an increase in Mg# (molar Mg/(Mg + Fe)) of olivine during experiments (similar to 77-93 in the quenched samples with an initial Fo(77) composition and similar to 92-97 in the quenched samples with an initial Fo(90) composition) due to the partitioning of iron to the melt phase. Assuming a respective melt fraction of 0.10 and 0.20 before and after the phase of significant increase in conductivity, in agreement with previous electrical and permeability studies, our results can be reproduced satisfactorily by two-phase electrical models (the Hashin and Shtrikman bounds and the modified brick layer model), and provide a melt conductivity value of 78 (+/- 8) S/m for all Fo(77) samples and 45 (+/- 5) S/m for the Fo(90) sample. Comparison of our results with electromagnetic sounding data of the deep interior of the Moon supports the hypothesis of the presence of interconnected melt at the base of the lunar mantle. Our results underline that electrical conductivity can be used to investigate in situ melt nucleation and migration in the interior of terrestrial planets. (C) 2015 Elsevier B.V. All rights reserved.

Pommier, A, Leinenweber K, Kohlstedt DL, Qi C, Garnero EJ, Mackwell SJ, Tyburczy JA.  2015.  Experimental constraints on the electrical anisotropy of the lithosphere-asthenosphere system. Nature. 522:202-+.   10.1038/nature14502   AbstractWebsite

The relative motion of lithospheric plates and underlying mantle produces localized deformation near the lithosphere-asthenosphere boundary(1). The transition from rheologically stronger lithosphere to weaker asthenosphere may result from a small amount of melt or water in the asthenosphere, reducing viscosity(1-3). Either possibility may explain the seismic and electrical anomalies that extend to a depth of about 200 kilometres(4,5). However, the effect of melt on the physical properties of deformed materials at upper-mantle conditions remains poorly constrained(6). Here we present electrical anisotropy measurements at high temperatures and quasi-hydrostatic pressures of about three gigapascals on previously deformed olivine aggregates and sheared partially molten rocks. For all samples, electrical conductivity is highest when parallel to the direction of prior deformation. The conductivity of highly sheared olivine samples is ten times greater in the shear direction than for undeformed samples. At temperatures above 900 degrees Celsius, a deformed solid matrix with nearly isotropic melt distribution has an electrical anisotropy factor less than five. To obtain higher electrical anisotropy (up to a factor of 100), we propose an experimentally based model in which layers of sheared olivine are alternated with layers of sheared olivine plus MORB or of pure melt. Conductivities are up to 100 times greater in the shear direction than when perpendicular to the shear direction and reproduce stress-driven alignment of the melt. Our experimental results and the model reproduce mantle conductivity-depth profiles for melt-bearing geological contexts. The field data are best fitted by an electrically anisotropic asthenosphere overlain by an isotropic, high-conductivity lower most lithosphere. The high conductivity could arise from partial melting associated with localized deformation resulting from differential plate velocities relative to the mantle, with subsequent upward melt percolation from the asthenosphere.

2014
Pichavant, M, Scaillet B, Pommier A, Iacono-Marziano G, Cioni R.  2014.  Nature and evolution of primitive Vesuvius magmas: An experimental study. Journal of Petrology. 55:2281-2309.   10.1093/petrology/egu057   AbstractWebsite

Two mafic eruptive products from Vesuvius, a tephrite and a trachybasalt, have been crystallized in the laboratory to constrain the nature of primitive Vesuvius magmas and their crustal evolution. Experiments were performed at high temperatures (from 1000 to a parts per thousand yen1200A degrees C) and both at 0 center dot 1 MPa and at high pressures (from 50 to 200 MPa) under H2O-bearing fluid-absent and H2O- and CO2-bearing fluid-present conditions. Experiments started from glass except for a few that started from glass plus San Carlos olivine crystals to force olivine saturation. Melt H2O concentrations reached a maximum of 6 center dot 0 wt % and experimental fO(2) ranged from NNO - 0 center dot 1 to NNO + 3 center dot 4 (where NNO is nickel-nickel oxide buffer). Clinopyroxene (Mg# up to 93) is the liquidus phase for the two investigated samples; it is followed by leucite for H2O in melt < 3 wt %, and by phlogopite (Mg# up to 81) for H2O in melt > 3 wt %. Olivine (Fo(85)) crystallized spontaneously in only one experimental charge. Plagioclase was not found. Upon progressive crystallization of clinopyroxene, glass K2O and Al2O3 contents strongly increase whereas MgO, CaO and CaO/Al2O3 decrease; the residual melts follow the evolution of Vesuvius whole-rocks from trachybasalt to tephrite, phonotephrite and to tephriphonolite. Concentrations of H2O and CO2 in near-liquidus 200 MPa glasses and primitive melt inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was reconstructed by the olivine-added experiments. They show that the primitive Vesuvius melts are trachybasalts (K2O similar to 4 center dot 5-5 center dot 5 wt %, MgO = 8-9 wt %, Mg# = 75-80, CaO/Al2O3 = 0 center dot 9-0 center dot 95) that crystallize Fo-rich olivine (90-91) as the liquidus phase between 1150 and 1200A degrees C and from 300 to < 200 MPa. Primitive Vesuvius melts are volatile-rich (1 center dot 5-4 center dot 5 wt % H2O and 600-4500 ppm CO2 in primitive melt inclusions) and oxidized (from NNO + 0 center dot 4 to NNO + 1 center dot 2). Assimilation of carbonate wall-rocks by ascending primitive magmas can account for the disappearance of olivine from crystallization sequences and explains the lack of rocks representative of olivine-crystallizing magmas. A correlation between carbonate assimilation and the type of feeding system is proposed: carbonate assimilation is promoted for primitive magma batches of small volumes. In contrast, for longer-lived, large-volume, less frequently recharged, hence more evolved, cooler reservoirs, magma-carbonate interaction is limited. Primitive magmas from Vesuvius and other Campanian volcanoes have similar redox states. However, the Cr# of Vesuvius spinels is distinctive and therefore the peridotitic component in the mantle source of Vesuvius differs from that of the other Campanian magmas.

Khan, A, Connolly JAD, Pommier A, Noir J.  2014.  Geophysical evidence for melt in the deep lunar interior and implications for lunar evolution. Journal of Geophysical Research-Planets. 119:2197-2221.   10.1002/2014je004661   AbstractWebsite

Analysis of lunar laser ranging and seismic data has yielded evidence that has been interpreted to indicate a molten zone in the lowermost mantle overlying a fluid core. Such a zone provides strong constraints on models of lunar thermal evolution. Here we determine thermochemical and physical structure of the deep Moon by inverting lunar geophysical data (mean mass and moment of inertia, tidal Love number, and electromagnetic sounding data) in combination with phase-equilibrium computations. Specifically, we assess whether a molten layer is required by the geophysical data. The main conclusion drawn from this study is that a region with high dissipation located deep within the Moon is required to explain the geophysical data. This region is located within the mantle where the solidus is crossed at a depth of approximate to 1200 km (1600 degrees C). Inverted compositions for the partially molten layer (150-200 km thick) are enriched in FeO and TiO2 relative to the surrounding mantle. The melt phase is neutrally buoyant at pressures of similar to 4.5-4.6 GPa but contains less TiO2 (<15 wt %) than the Ti-rich (similar to 16 wt %) melts that produced a set of high-density primitive lunar magmas (density of 3.4 g/cm(3)). Melt densities computed here range from 3.25 to 3.45 g/cm(3) bracketing the density of lunar magmas with moderate-to-high TiO2 contents. Our results are consistent with a model of lunar evolution in which the cumulate pile formed from crystallization of the magma ocean as it overturned, trapping heat-producing elements in the lower mantle.

Pommier, A.  2014.  Interpretation of magnetotelluric results using laboratory measurements. Surveys in Geophysics. 35:41-84.   10.1007/S10712-013-9226-2   AbstractWebsite

Magnetotelluric (MT) surveying is a remote sensing technique of the crust and mantle based on electrical conductivity that provides constraints to our knowledge of the structure and composition of the Earth's interior. This paper presents a review of electrical measurements in the laboratory applied to the understanding of MT profiles. In particular, the purpose of such a review is to make the laboratory technique accessible to geophysicists by pointing out the main caveats regarding a careful use of laboratory data to interpret electromagnetic profiles. First, this paper addresses the main issues of cross-spatial-scale comparisons. For brevity, these issues are restricted to reproducing in the laboratory the texture, structure of the sample as well as conditions prevailing in the Earth's interior (pressure, temperature, redox conditions, time). Second, some critical scientific questions that have motivated laboratory-based interpretation of electromagnetic profiles are presented. This section will focus on the characterization of the presence and distribution of hydrogen in the Earth's crust and mantle, the investigation of electrical anisotropy in the asthenosphere and the interpretation of highly conductive field anomalies. In a last section, the current and future challenges to improve quantitative interpretation of MT profiles are discussed. These challenges correspond to technical improvements in the laboratory and the field as well as the integration of other disciplines, such as petrology, rheology and seismology.

2013
Khan, A, Pommier A, Neumann GA, Mosegaard K.  2013.  The lunar moho and the internal structure of the Moon: A geophysical perspective. Tectonophysics. 609:331-352.   10.1016/J.Tecto.2013.02.024   AbstractWebsite

Extraterrestrial seismology saw its advent with the deployment of seismometers during the Apollo missions that were undertaken from July 1969 to December 1972. The Apollo lunar seismic data constitute a unique resource being the only seismic data set which can be used to infer the interior structure of a planetary body besides the Earth. On-going analysis and interpretation of the seismic data continues to provide constraints that help refine lunar origin and evolution. In addition to this, lateral variations in crustal thickness (similar to 0-80 km) are being mapped out at increasing resolution from gravity and topography data that have and continue to be collected with a series of recent lunar orbiter missions. Many of these also carry onboard multi-spectral imaging equipment that is able to map out major-element concentration and surface mineralogy to high precision. These results coupled with improved laboratory-based petrological studies of lunar samples provide important constraints on models for lunar magma ocean evolution, which ultimately determines internal structure. Whereas existing constraints on initial depth of melting and differentiation from quantitative modeling suggested only partial Moon involvement (<500 km depth), more recent models tend to favor a completely molten Moon, although the former cannot be ruled out sensu stricto. Recent geophysical analysis coupled with thermodynamical computations of phase equilibria and physical properties of mantle minerals suggest that the Earth and Moon are compositionally distinct. Continued analysis of ground-based laser ranging data and recent discovery of possible core reflected phases in the Apollo lunar seismic data strengthens the case for a small dense lunar core with a radius of <400 km corresponding to 1-3% of lunar mass. (C) 2013 Elsevier B.V. All rights reserved.

Pommier, A, Evans RL, Key K, Tyburczy JA, Mackwell S, Elsenbeck J.  2013.  Prediction of silicate melt viscosity from electrical conductivity: A model and its geophysical implications. Geochemistry Geophysics Geosystems. 14:1685-1692.   10.1002/ggge.20103   AbstractWebsite

Our knowledge of magma dynamics would be improved if geophysical data could be used to infer rheological constraints in melt-bearing zones. Geophysical images of the Earth's interior provide frozen snapshots of a dynamical system. However, knowledge of a rheological parameter such as viscosity would constrain the time-dependent dynamics of melt bearing zones. We propose a model that relates melt viscosity to electrical conductivity for naturally occurring melt compositions (including H2O) and temperature. Based on laboratory measurements of melt conductivity and viscosity, our model provides a rheological dimension to the interpretation of electromagnetic anomalies caused by melt and partially molten rocks (melt fraction similar to >0.7).

Key, K, Constable S, Liu L, Pommier A.  2013.  Electrical image of passive mantle upwelling beneath the northern East Pacific Rise. Nature. 495:499-502.: Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.   10.1038/nature11932   AbstractWebsite

Melt generated by mantle upwelling is fundamental to the production of new oceanic crust at mid-ocean ridges, yet the forces controlling this process are debated1, 2. Passive-flow models predict symmetric upwelling due to viscous drag from the diverging tectonic plates, but have been challenged by geophysical observations of asymmetric upwelling3, 4, 5 that suggest anomalous mantle pressure and temperature gradients2, 6, 7, and by observations of concentrated upwelling centres8 consistent with active models where buoyancy forces give rise to focused convective flow2. Here we use sea-floor magnetotelluric soundings at the fast-spreading northern East Pacific Rise to image mantle electrical structure to a depth of about 160 kilometres. Our data reveal a symmetric, high-conductivity region at depths of 20–90 kilometres that is consistent with partial melting of passively upwelling mantle9, 10, 11. The triangular region of conductive partial melt matches passive-flow predictions, suggesting that melt focusing to the ridge occurs in the porous melting region rather than along the shallower base of the thermal lithosphere. A deeper conductor observed east of the ridge at a depth of more than 100 kilometres is explained by asymmetric upwelling due to viscous coupling across two nearby transform faults. Significant electrical anisotropy occurs only in the shallowest mantle east of the ridge axis, where high vertical conductivity at depths of 10–20 kilometres indicates localized porous conduits. This suggests that a coincident seismic-velocity anomaly12 is evidence of shallow magma transport channels13, 14 rather than deeper off-axis upwelling. We interpret the mantle electrical structure as evidence that plate-driven passive upwelling dominates this ridge segment, with dynamic forces being negligible.

2012
Pommier, A, Grove TL, Charlier B.  2012.  Water storage and early hydrous melting of the Martian mantle. Earth and Planetary Science Letters. 333:272-281.   Doi 10.1016/J.Epsl.2012.03.030   AbstractWebsite

We report an experimental investigation of the near-solidus phase equilibria of a water-saturated analog of the Martian mantle. Experiments were performed at low temperatures (700-920 degrees C) and high pressure (4-7 GPa) using multi-anvil apparatus and piston cylinder device (4 GPa). The results of this study are used to explore the role of water during early melting and chemical differentiation of Mars, and to further our understanding of the near-solidus behavior in planetary mantle compositions at high pressure. Water has a significant effect on the temperature of melting and, therefore, on accretion and subsequent differentiation processes. Experiments locate the wet solidus at similar to 800 degrees C, and is isothermal between 4 GPa and 7 GPa. The Martian primitive mantle can store significant amounts of water in hydrous minerals stable near the solidus. Humite-group minerals and phase E represent the most abundant hydrous minerals stable in the 4-7 GPa pressure range. The amount of water that can be stored in the mantle and mobilized during melting ranges from 1 to up to 4 wt% near the wet solidus. We discuss thermal models of Mars accretion where the planet formed very rapidly and early on in solar system history. We incorporate the time constraint of Dauphas and Pourmand (2011) that Mars had accreted to 50% of its present mass in 1.8 Myr and include the effects of Al-26 radioactive decay and heat supplied by rapid accretion. When accretion has reached 30% of Mars current mass (similar to 70% of its present size), melting starts, and extends from 100 to 720 km depth. Below this melt layer, water can still be bound in crystalline solids. The critical stage is at 50% accretion (similar to 80% of its size), where Mars is above the wet and dry solidi with most of its interior melted. This is earlier in the accretion process than what would be predicted from dry melting. We suggest that water may have promoted early core formation on Mars and rapidly extended melting over a large portion of Mars interior. (C) 2012 Elsevier B.V. All rights reserved.

Carporzen, L, Weiss BP, Gilder SA, Pommier A, Hart RJ.  2012.  Lightning remagnetization of the Vredefort impact crater: No evidence for impact-generated magnetic fields. Journal of Geophysical Research-Planets. 117   10.1029/2011je003919   AbstractWebsite

The Vredefort impact crater in South Africa is one of the oldest and largest craters on Earth, making it a unique analog for planetary basins. Intense and randomly oriented remanent magnetization observed in surface samples at Vredefort has been attributed to impact-generated magnetic fields. This possibility has major implications for extraterrestrial paleomagnetism since impact-generated fields have been proposed as a key alternative to the dynamo hypothesis for magnetization on the Moon and asteroids. Furthermore, the presence of single-domain magnetite found along shock-generated planar deformation features in Vredefort granites has been widely attributed to the 2.02 Ga impact event. An alternative hypothesis is that the unusual magnetization and/or rock magnetic properties of Vredefort rocks are the products of recent lightning strikes. Lightning and impact-generated fields can be distinguished by measuring samples collected from below the present surface. Here we present a paleomagnetic and rock magnetic study of samples from two 10 m deep vertical boreholes. We show that the magnetization at depth is consistent with a thermoremanent magnetization acquired in the local geomagnetic field following the impact, while random, intense magnetization and some of the unusual rock magnetic properties observed in surface rocks are superficial phenomena produced by lightning. Because Vredefort is the only terrestrial crater that has been proposed to contain records of impact-generated fields, this removes a key piece of evidence in support of the hypothesis that paleomagnetism of the Moon and other extraterrestrial bodies is the product of impacts rather than past core dynamos.

2011
Pommier, A, Le-Trong E.  2011.  "SIGMELTS": A web portal for electrical conductivity calculations in geosciences. Computers & Geosciences. 37:1450-1459.   10.1016/J.Cageo.2011.01.002   AbstractWebsite

Electrical conductivity measurements in the laboratory are critical for interpreting geoelectric and magnetotelluric profiles of the Earth's crust and mantle. In order to facilitate access to the current database on electrical conductivity of geomaterials, we have developed a freely available web application (SIGMELTS) dedicated to the calculation of electrical properties. Based on a compilation of previous studies, SIGMELTS computes the electrical conductivity of silicate melts, carbonatites, minerals, fluids, and mantle materials as a function of different parameters, such as composition, temperature, pressure, water content, and oxygen fugacity. Calculations on two-phase mixtures are also implemented using existing mixing models for different geometries. An illustration of the use of SIGMELTS is provided, in which calculations are applied to the subduction zone-related volcanic zone in the Central Andes. Along with petrological considerations, field and laboratory electrical data allow discrimination between the different hypotheses regarding the formation and rise from depth of melts and fluids and quantification of their storage conditions. (C) 2011 Elsevier Ltd. All rights reserved.

Caricchi, L, Pommier A, Pistone M, Castro J, Burgisser A, Perugini D.  2011.  Strain-induced magma degassing: insights from simple-shear experiments on bubble bearing melts. Bulletin of Volcanology. 73:1245-1257.   10.1007/S00445-011-0471-2   AbstractWebsite

Experiments have been performed to determine the effect of deformation on degassing of bubble-bearing melts. Cylindrical specimens of phonolitic composition, initial water content of 1.5 wt.% and 2 vol.% bubbles, have been deformed in simple-shear (torsional configuration) in an internally heated Paterson-type pressure vessel at temperatures of 798-848 K, 100-180 MPa confining pressure and different final strains. Micro-structural analyses of the samples before and after deformation have been performed in two and three dimensions using optical microscopy, a nanotomography machine and synchrotron tomography. The water content of the glasses before and after deformation has been measured using Fourier Transform Infrared Spectroscopy (FTIR). In samples strained up to a total of gamma similar to 2 the bubbles record accurately the total strain, whereas at higher strains (gamma similar to 10) the bubbles become very flattened and elongate in the direction of shear. The residual water content of the glasses remains constant up to a strain of gamma similar to 2 and then decreases to about 0.2 wt.% at gamma similar to 10. Results show that strain enhances bubble coalescence and degassing even at low bubble volume-fractions. Noticeably, deformation produced a strongly water under-saturated melt. This suggests that degassing may occur at great depths in the volcanic conduit and may force the magma to become super-cooled early during ascent to the Earth's surface potentially contributing to the genesis of obsidian.

2010
Pommier, A, Gaillard F, Pichavant M.  2010.  Time-dependent changes of the electrical conductivity of basaltic melts with redox state. Geochimica Et Cosmochimica Acta. 74:1653-1671.   10.1016/J.Gca.2009.12.005   AbstractWebsite

The electrical conductivity of basaltic melts has been measured in real-time after fO(2) step-changes in order to investigate redox kinetics. Experimental investigations were performed at 1 atm in a vertical furnace between 1200 and 1400 degrees C using air, pure CO2 or CO/CO2 gas mixtures to buffer oxygen fugacity in the range 10(-8) to 0.2 bars. Ferric/ferrous ratios were determined by wet chemical titrations. A small but detectable effect of fO(2) on the electrical conductivity is observed. The more reduced the melt, the higher the conductivity. A modified Arrhenian equation accounts for both T and fO(2) effects on the electrical conductivity. We show that time-dependent changes in electrical conductivity following fO(2) step-changes monitor the rate of Fe2+/Fe3+ changes. The conductivity change with time corresponds to a diffusion-limited process in the case of reduction in CO-CO2 gas mixtures and oxidation in air. However, a reaction at the gas-melt interface probably rate limits oxidation of the melt under pure CO2. Reduction and oxidation rates are similar and both increase with temperature. Those rates range from 10(-9) to 10(-8) m(2)/s for the temperature interval 1200-1400 degrees C and show activation energy of about 200 kJ/mol. The redox mechanism that best explains our results involves a cooperative motion of cations and oxygen, allowing such fast oxidation-reduction rates. (c) 2009 Elsevier Ltd. All rights reserved.

Pommier, A, Tarits P, Hautot S, Pichavant M, Scaillet B, Gaillard F.  2010.  A new petrological and geophysical investigation of the present-day plumbing system of Mount Vesuvius. Geochemistry Geophysics Geosystems. 11   10.1029/2010gc003059   AbstractWebsite

A model of the electrical resistivity of Mt. Vesuvius has been elaborated to investigate the present structure of the volcanic edifice. The model is based on electrical conductivity measurements in the laboratory, on geophysical information, in particular, magnetotelluric (MT) data, and on petrological and geochemical constraints. Both 1-D and 3-D simulations explored the effect of depth, volume and resistivity of either one or two reservoirs in the structure. For each configuration tested, modeled MT transfer functions were compared to field transfer functions from field magnetotelluric studies. The field electrical data are reproduced with a shallow and very conductive layer (similar to 0.5 km depth, 1.2 km thick, 5 ohm. m resistive) that most likely corresponds to a saline brine present beneath the volcano. Our results are also compatible with the presence of cooling magma batches at shallow depths (<3-4 km depth). The presence of a deeper body at similar to 8 km depth, as suggested by seismic studies, is consistent with the observed field transfer functions if such a body has an electrical resistivity > similar to 100 ohm. m. According to a petro-physical conductivity model, such a resistivity value is in agreement either with a low-temperature, crystal-rich magma chamber or with a small quantity of hotter magma interconnected in the resistive surrounding carbonates. However, the low quality of MT field data at long periods prevent from placing strong constraints on a potential deep magma reservoir. A comparison with seismic velocity values tends to support the second hypothesis. Our findings would be consistent with a deep structure (8-10 km depth) made of a tephriphonolitic magma at 1000 degrees C, containing 3.5 wt%H2O, 30 vol.% crystals, and interconnected in carbonates in proportions similar to 45% melt -55% carbonates.

Pommier, A, Gaillard F, Malki M, Pichavant M.  2010.  Methodological re-evaluation of the electrical conductivity of silicate melts. American Mineralogist. 95:284-291.   10.2138/Am.2010.3314   AbstractWebsite

Electrical impedance measurements in the laboratory oil silicate melts are used to interpret magnetotelluric anomalies. On the basis of 2- and 4-electrode measurements, we show that the influence of the electrodes of the 2-electrode system Oil the measured resistivity call be of significant importance for low-resistivity melts and increases with temperature. At 1400 degrees C, the resistivity of very conductive melts measured with two electrodes call reach six times the resistivity value measured with four electrodes. A short-circuit experiment is needed to correct the 2-electrode data. Electrodes contribution is also estimated for samples from other studies, for which the resistance of the electrical cell call be as high as the resistance of the sample. A correction of the resistivity data from the literature is proposed and Values of the corresponding Arrhenian parameters are recommended.

2008
Pommier, A, Gaillard F, Pichavant M, Scaillet B.  2008.  Laboratory measurements of electrical conductivities of hydrous and dry Mount Vesuvius melts under pressure. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005269   AbstractWebsite

[1] 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 Mount Vesuvius (Italy) have been measured using an impedance spectrometer. Experiments were conducted on both glasses and melts between 400 and 1300 degrees C, at both ambient pressure in air and 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 investigated compositions, increasing pressure decreases the conductivity, although the effect of pressure is relatively small. The three investigated compositions have similar activation volumes (Delta V= 16-24 cm(3) mol(-1)). 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 sigma(0). A semiempirical method allowing the determination of the electrical conductivity of natural magmatic liquids is proposed, in which the activation energy is modeled on the basis of the Anderson-Stuart model, sigma(0) being obtained from the compensation law and Delta V being fitted from our experimental data. The model enables the electrical conductivity to be calculated for the entire range of melt compositions at Mount Vesuvius and also satisfactorily predicts the electrical response of other melt compositions. Electrical conductivity data for Mount Vesuvius melts and magmas are slightly lower than the electrical anomaly revealed by MT studies.