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Zhang, Z, Qin T, Pommier A, Hirschmann MM.  2019.  Carbon storage in Fe-Ni-S liquids in the deep upper mantle and its relation to diamond and Fe-Ni alloy precipitation. Earth and Planetary Science Letters. 520:164-174.   10.1016/j.epsl.2019.05.039   AbstractWebsite

To better understand the role of sulfide in C storage in the upper mantle, we construct a thermodynamic model for Fe-Ni-S-C sulfide melts and consider equilibrium between sulfide melts, mantle silicates, Fe-Ni alloy, and diamond. The sulfide melt model is based upon previous parameterization of Fe-Ni-S melts calibrated at 100 kPa, which we have extended to high pressure based on volumetric properties of end-member components. We calculate the behavior of C in the sulfide melt from empirical parameterization of experimental C solubility data. We calculate the continuous compositional evolution of Fe-Ni sulfide liquid and associated effects on carbon storage at pressure and redox conditions corresponding to mantle depths of 60 to 410 km. Equilibrium and mass balance conditions were solved for coexisting Fe-Ni-S melt and silicate minerals (olivine [(Mg,Fe,Ni)(2)SiO4], pyroxene [(Mg,Fe)SiO3]) in a mantle with 200 ppmw S. With increasing depth and decreasing oxygen fugacity ( f(02)), the calculated melt (Fe+Ni)/S atomic ratio increases from 0.8-1.5 in the shallow oxidized mantle to 2.0-10.5 in the reduced deep upper mantle (>8 GPa), with Fe-Ni alloy saturation occurring at >10 GPa. Compared to previous calculations for the reduced deep upper mantle, alloy saturation occurs at greater depth owing to the capacity of sulfide melt to dissolve metal species, thereby attenuating the rise of Fe and Ni metal activities. The corresponding carbon storage capacity in the metal-rich sulfide liquid rises from negligible below 6 GPa to 8-20 ppmw at 9 GPa, and thence increases sharply to 90-110 ppmw at the point of alloy saturation at 10-12 GPa. The combined C storage capacity of liquid and solid alloy reaches 110-170 ppmw at 14 GPa. Thus, in the deep upper mantle, all carbon in depleted sources (10-30 ppmw C) can be stored in the sulfide liquid, and alloy and sulfide liquids host a significant fraction of the C in enriched sources (30-500 ppmw C). Application of these results to the occurrences of inferred metal-rich sulfide melts in the Fe-Ni-S-C system and inclusions in diamonds from the mantle transition zone suggests that oxidization of a reduced metal-rich sulfide melt is an efficient mechanism for deep-mantle diamond precipitation, owing to the strong effect of (Fe+Ni)/S ratio on carbon solubility in Fe-Ni-S melts. This redox reaction likely occurs near the boundary between oxidized subducted slabs and the reduced ambient peridotitic mantle. (C) 2019 Elsevier B.V. All rights reserved.

Pommier, A, Williams Q, Evans RL, Pal I, Zhang Z.  2019.  Electrical investigation of natural lawsonite and application to subduction contexts. Journal of Geophysical Research-Solid Earth. 124:1430-1442.   10.1029/2018jb016899   AbstractWebsite

We report an experimental investigation of the electrical properties of natural polycrystalline lawsonite from Reed Station, CA. Lawsonite represents a particularly efficient water reservoir in subduction contexts, as it can carry about 12wt% water and is stable over a wide pressure range. Experiments were performed from 300 to about 1325 degrees C and under pressure from 1 to 10GPa using a multi-anvil apparatus. We observe that temperature increases lawsonite conductivity until fluids escape the cell after dehydration occurs. At a fixed temperature of 500 degrees C, conductivity measurements during compression indicate electrical transitions at about 4.0 and 9.7GPa that are consistent with crystallographic transitions from orthorhombic C to P and from orthorhombic to monoclinic systems, respectively. Comparison with lawsonite structure studies indicates an insignificant temperature dependence of these crystallographic transitions. We suggest that lawsonite dehydration could contribute to (but not solely explain) high conductivity anomalies observed in the Cascades by releasing aqueous fluid at a depth (similar to 50km) consistent with the basalt-eclogite transition. In subduction settings where the incoming plate is older and cooler (e.g., Japan), lawsonite remains stable to great depth. In these cooler settings, lawsonite could represent a vehicle for deep water transport and the subsequent triggering of melt that would appear electrically conductive, though it is difficult to uniquely identify the contributions from lawsonite on field electrical profiles in these more deep-seated domains.

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, 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.

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.

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.

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.