MR13C-1395
What was the cooling rate of cometary dust particles when they were collected by the Stardust spacecraft ?
Cometary dust particles are considered as relicts of pristine materials that accreted to form primitive meteorites and eventually planets. Compared to other small primitive objects of the solar system, cometary grains are poorly known. In this respect, the first sample return of cometary materials, the Stardust mission, raised considerable expectations from astrophysicists, geologists and cosmochemists. Indeed, these dusts are the only undisputable comet samples available so far. The mission was a plain success but still had to face inherent problems related to the collect of samples itself. The grains were captured from the 81P/Wild 2 comet tail at a relative velocity of 6.1 km/s in a low density silica aerogel medium. During this hypervelocity impact, flash heating of dusts could not be avoided. This thermal event may hinder detailed understanding of dust mineralogy and geochemistry because the parameters of this flash heating are not well constrained. Models and experiments tend to indicate a high peak temperature, strong thermal gradients and a very fast quench (within a few microseconds). In this study we estimate the parameters of the flash heating based on the interdiffusion of MgO and SiO2 between molten MgO-rich cometary dust particles and the embedding molten aerogel. The presence of a miscibility gap in the MgO-SiO2 binary system precludes full mixing of the Mg-rich melt and the surrounding melted aerogel. The persistence of the two coexisting molten domains, the extend of the MgO contamination of the silica matrix and the shape of the inter-diffusion profile are then used to shed light on the thermal history of cometary dusts particles collected by the spacecraft. Comparison between the models and experimental work will be detailed, together with potential implications on the thermal alteration (intensity of heating and kinetics) of cometary grains during their capture.
MR13C-1396
Li Diffusion in Olivine
We have carried out experiments to study the diffusion of Li along the [001] direction of oriented, polished single crystals of San Carlos olivine at 800 °C and atmospheric pressure under a controlled fO2 of 10- 17 bars (~ WM buffer). Cubes of the crystals were annealed surrounded by a powder mix of the same olivine and 6Li enriched lithium silicate. After the experiments, the crystals were sectioned parallel to the diffusion direction and concentration profiles were measured using a Cameca IMS4f ion microprobe in the step scanning mode. The concentration profiles show (i) complex shapes with plateaus in between that are not amenable to treatment using standard solutions to the diffusion equation, and (ii) isotopic fractionation. The shapes of the profiles clearly indicate that at least two different species of Li are involved in the diffusion process. However, a time series of anneals (10 min, 5 h, 10 h, 40 h and 100 h) demonstrate that the shapes are reproducible and evolve systematically with time. We find that the concentration evolution can be described by the following set of processes: (i) incorporation of Li in olivine primarily in octahedral metal sites, (ii) a fast, homogeneous reaction involving metal vacancies (V''Me) that partitions Li between the octahedral site (Li'Me) and interstitial positions (Li\bulleti): Li'Me = V''Me + Li\bulleti, and (iii) diffusion of total Li as Li'Me as well as Li\bulleti, at very different rates. The concentration of Li'Me is at least 10 times higher than the concentration of Li\bulleti but the diffusion rate of Li\bulleti is about two orders of magnitude faster than of Li'Me. At the experimental conditions, we can fit the profiles using known values of metal vacancy diffusion coefficients in olivine D(V''Me) = 10-14 m2/s, D(Li'Me) = 10-15 m2/s and D(Li\bulleti) = 2.5 · 10-13 m2/s. Once these parameters are obtained by fitting any one profile, calculated profiles for any other run duration are an excellent fit to the observed profile shapes, confirming the inferred mechanism. Experiments are in progress to study the temperature dependence of these processes.
MR13C-1397
REE Diffusion in Olivine
Diffusion of rare earth elements has been characterized in synthetic forsterite and natural olivine (Fo90) under dry conditions. The source of diffusant was a mixture of REE aluminate garnet powder and synthetic forsterite powder in 3:1 proportions. Experiments were prepared by enclosing source material and polished forsterite in platinum capsules, which were crimped shut. For experiments on natural olivine, samples were placed with the source in AgPd capsules, which were sealed under vacuum in silica glass ampoules with a solid buffer to buffer at NNO. Prepared capsules were then annealed in 1 atm furnaces for times ranging from an hour to several weeks, at temperatures from 850 to 1300„aC. REE distributions in the olivine were profiled by Rutherford Backscattering Spectrometry (RBS). The following Arrhenius relation is obtained for Dy diffusion in forsterite: D = 2.3x10-10 exp(-273 kJ mol-1/RT) m2sec-1. Diffusivities of Yb and La in forsterite appear similar, and preliminary results for Dy diffusion in natural olivine also suggest similar diffusivities. REE diffusivities in forsterite are about an order of magnitude faster than those of REE+3 in enstatite (Cherniak and Liang, 2007). However, if the Arrhenius relation above is extrapolated up to 1450C, it yields diffusivities about three orders of magnitude smaller than those determined for REE diffusion in olivine by Spandler et al. (2007). Work is currently underway in further characterizing diffusion in Fe-bearing olivine. If an Arrhenius relation similar to that for forsterite is found to apply, timescales for preservation of REE signatures in olivine-hosted melt inclusions may be relatively long. Cherniak and Liang (2007) GCA 71, 1324-1340; Spandler et al. (2007) Nature 447, 303-306.
MR13C-1398
Point defects in (Mg,Fe)O at high pressures: where does hydrogen dominate over ferric iron?
The point defects play an important role in transport processes of minerals including diffusion, electrical conduction and plastic deformation. Point defects caused by ferric iron and/or hydrogen (proton) are dominant defects in most of the iron-bearing minerals including olivine and (Mg,Fe)O. In many upper-mantle minerals such as olivine, the concentration of ferric iron is much smaller than that of hydrogen, and therefore the small amount of hydrogen changes their transport properties dramatically. However, the situation is very different for lower- mantle minerals such as (Mg,Fe)O. In this presentation, we will review the available experimental data on point defects in (Mg,Fe)O and discuss the relative importance of ferric iron and hydrogen at high pressures based on atomic models. The existing low-pressure data indicate that the maximum solubility of ferric iron in (Mg,Fe)O is on the order of 0.1 (atomic fraction in the total iron), which is much higher than that of hydrogen. However, experimental studies by Bolfan-Casanova et al (2002, 2006) indicate that the solubility of ferric iron decreases while that of hydrogen increases with pressure. This suggests that the dominant impurity to generate point defects in (Mg,Fe)O may change from ferric iron to hydrogen at high pressure. Therefore it is important to quantify the pressure dependence of the solubility of ferric iron and hydrogen. We have explored two models of ferric iron- related defects and found that the existing experimental data suggest that ferric iron may occur at two lattice sites: the tetrahedral site as interstitial atoms as well as the octahedral site. The pressure dependence of the solubility of hydrogen in (Mg,Fe)O are also estimated based on the experimental data and defect models. The cross-over of defect solubility likely occurs in the lower mantle, but the exact depth is poorly constrained because of large uncertainties in the hydrogen solubility and the mechanisms of hydrogen dissolution in (Mg,Fe)O. Possible implications of this cross-over on some physical properties of (Mg,Fe)O will be discussed.
MR13C-1399
Grain Boundary Transport of Siderophile Elements in MgO at High Pressure
The extent of interaction between the Earth's core and mantle remains an actively debated question. Siderophile element signatures in rocks that can be observed at the surface indicate that the mantle and core may have exchanged material over the history of the Earth. Here, a potential physical mechanism to facilitate this communication is considered. It has recently been shown that grain boundaries in lower mantle analog materials at 2.5 GPa act as reservoirs and fast transport pathways for incompatible elements, specifically siderophile elements [1]. In the present study, we conducted multi-anvil experiments held at 10 GPa and 1600oC for 5 hours to examine the persistence of fast grain boundary transport at higher pressures. Thin layers of Os and Au powders were loaded in a standard 10/5 multi-anvil assembly and separated from a Pt foil by a cylindrical MgO plug approximately 1mm long. These two elements were expected to be among the slowest and fastest diffusers respectively. The final composition of the Pt foil was measured by electron microprobe. The presence of measurable siderophile element "blebs" in the Pt foil indicates substantial grain boundary diffusion. Our preliminary results suggest that siderophile element mobility and presence on grain boundaries may be affected slightly with increased pressure, but could remain a viable mechanism for transport on length scales applicable to communication within the deeper Earth over it's history. The effect of pressure and grain size on grain boundary diffusion, and potential reasons for a large variation between diffusivities of different siderophile elements will be discussed. [1] Hayden, L., and Watson, E.B., 2006. GCA Supp., v. 70, iss. 18, p. 238
MR13C-1400
Structural Properties of MgSiO3 Perovskite, Twin Walls and Their Binding to Oxygen Vacancies at High Pressures from Force Field Simulations
Aiming to understand possible origins of seismic wave attenuation and variations in the quality factor (Q) of minerals at the Earth's interior, it has been accepted that the friction in the motion of twin domain walls under applied stress leads to a reduction in Q [1]. To interpret this behaviour one must first appreciate the influence of pressure on wall motion and dynamics. Experimental observations of twin wall motion at high pressure are difficult at present. For this reason, we have undertaken a computational study of wall structure and dynamics in MgSiO3 perovskite, in the absence or presence of defects, as a function of pressure between 0 and 140 GPa. This work is based on previous studies on CaTiO3 perovskite [2,3] (symmetry Pbnm) with 7800 atoms and orthorhombic periodic boundary conditions. It has been geometrically adapted to lower mantle's MgSiO3 perovskite. The system comprises two (100) ferroelastic twin walls separated by 34.2 Å, relaxed within a variable supercell of 26x10x6 unit cells, with the rigid ion set of interatomic potentials with partial ionic charges employed by Alfredsson et al., 2005 [4]. The degree of distortion in the system is manifest in the octahedral tilting angles along each of the crystallographic directions. The octahedral tilting orientations close to the walls are well described by the behaviour of two order parameters: the main one, defining the wall, goes as α \hspace{1mm} tanh(2(x - x')/w) , where α = 11.14° is the bulk value for the tilt, and w = 11.11 Å is the wall width. The secondary order parameters describe a slight breather at the wall. As wall dynamics are influenced by the presence of oxygen vacancies, we have calculated their binding energies to the described twin walls as a function of pressure. We find that this binding strongly depends on the vacancy disposition. For a vacancy between Si atoms in an axis perpendicular to the wall the binding energy is 1.09 eV at zero pressure. For vacancies along Si-Si axes parallel to the wall, the binding is reduced by 43%. This behaviour is analogous to what reported for CaTiO3 [3]. The dependence on pressure of the binding, however, is quite different, with a clear increase in binding for higher pressures, reaching a binding energy of 1.22 eV (perpendicular) and 0.94 eV (parallel) at 140 GPa. \noindent {\small [1] Harrison RJ and Redfern SAT, Phys. Earth Planet. Inter. 134, 253 (2002)}. \noindent {\small [2] Calleja M, Dove M and Salje EKH, J. Phys.: Condens. Matter 15, 2301 (2003)}. \noindent {\small [3] Goncalves-Ferreira L, Redfern SAT and Artacho E, American Geophysical Union, Fall Meeting 2006, abstract #MR11A-0095 (2006) and to be published}. \noindent {\small [4] Alfredsson M, Brodholt JP, Dobson DP, Oganov AR, Catlow CRA, Parker SC and Price GD, Phys. Chem. Miner. 31, 671 (2005)}.
MR13C-1401
Anelasticity Arising From Intra-crystalline Cation Exchange in Spinel
The dynamic Young's modulus and anelastic properties of spinel have been measured by forced pendulum techniques as a function of frequency and temperature between room temperature and 1400 K and at frequencies between 10 Hz and 0.01 Hz. In this temperature range it is known that MgAl2O4 spinel displays variations in Mg-Al order disorder across the octahedral and tetrahedral sites of the structure, with increasing disorder towards an inverse spinel arrangement of cations at the highest temperatures of our experiments [1]. Our measurements of the mechanical properties across the same temperature interval of interest reveal a strong variation in the phase angle difference (?) between applied strain and stress, which corresponds to a Debye-like peak in the internal friction or anelastic attenuation, Q-1. This peak in the high temperature internal friction (IF) spectrum occurs at ~ 1080 K when the stress is cycled at 2 Hz. The peak appears to show no hysteresis between heating and cooling cycles and moves to higher temperatures with increasing frequency; consistent with a thermally activated process associated with point defect movements. Since the point defects must act as an elastic dipole, and the temperature the maximum in resonance occurs is at such an elevated temperature, this peak may be attributed to Mg-Al exchange between the tetrahedral and octahedral sites of the structure. Indeed, the temperature at which the resonance peak appears corresponds to the same temperature range that cation relaxation was observed in the kinetic experiments given in [1]. This is the first such observation of cation- exchange-related anelastic attenuation in a silicate, and points to the possibility of cation ordering significantly modifying the anelastic properties of minerals at high temperatures. [1] S A T Redfern, R J Harrison, H StC O'Neill, D R R Wood (1999) Thermodynamics and kinetics of cation ordering in MgAl2O4 spinel up to 1600 °C from in situ neutron diffraction. Am Mineral 84: 299–310
MR13C-1402
A Finite Element Study of Elastically-Accommodated Grain Boundary Sliding
Grain--boundary sliding is an important, but still poorly understood aspect of high--temperature viscoelastic behaviour. As a first step in the development of a numerical model of diffusionally--assisted grain boundary sliding in a polycrystalline material, we report on the numerical solution of the Raj--Ashby model. In that model, two identical elastic grains of rigidity μ and mean thickness d are separated by a spatially-periodic boundary having prescribed location. Two constitutive equations relate the normal n and tangential s components of the discontinuity in velocity [∂ u /∂ t] to appropriate components of the stress tensor. The first of these, ℓ σns = η [∂ us/ ∂ t], confers Newtonian viscosity η upon the boundary region of thickness ℓ, and the associated sliding timescale is tη = η d / μ ℓ. Secondly, because we do not include diffusion in the model (at this point), the normal component of velocity is continuous across the interface, i.e, [∂ un/∂ t ]=0. The deformation of the sample is driven by a sinusoidally time-varying shear displacement imposed at the distal boundaries of the two grains. The grain boundary is represented by N terms of the Fourier series for a sawtooth (piecewise linear) boundary whose linear segments make angles ± θ with the direction of that imposed displacement. Our numerical results show that the model behaves like a standard anelastic solid, characterized by a Debye dissipation peak. For small θ, our results agree quantitatively with the predictions of the perturbation analysis reported by two of us at this meeting last year. As θ is increased, the sliding amplitude decreases and, as a result, the relaxed shear modulus GR increases whereas the mechanical loss (i.e. L = \mbox{tan arg } G) decreases. As θ increases from 5° to 45°, L decreases by a factor of about 1000, and for θ > 45°, L increases weakly. In fact, due to the non-monotonic behaviour of L with θ, we predict a value of L ≈ 0.1 for θ=60° and θ=30°; close to the value 0.09 found by Ghahremani (1980) in his finite element study of elastically--accommodated grain boundary sliding in an array of hexagonal crystals. As suggested by Faul et al. (2002), we also find that because sharp corners inhibit sliding by inducing stress concentrations, increasing N inhibits sliding, and so causes the maximum value of L to decrease; increasing N from 1 to 100 reduces L by about 4--fold. We are now adding diffusion to our numerical solution of the Raj-Ashby model.
MR13C-1403
Analysis of micro-fabricated, controlled geometry samples for determination of material transport properties of core materials
Chemical diffusion rates in iron and nickel provide key constraints on the behavior of planetary core materials. Samples were created for use in the laser-heated or externally-heated diamond anvil cell by sputtering 300 nm of Ni onto the surface of a 10-micron thick Fe64Ni36 alloy foil under ultra-high vacuum, after ion-etching to remove surface oxidation. Samples heated under pressure up to 1200 K show that a depth profile through the sample using secondary ionization mass spectrometry (SIMS) can accurately constrain the diffusivity of iron and nickel in these samples. Diffusivities over 10-19 m2/s can be determined with 0.1 log unit accuracy. Initial experiments at 1063K give a diffusivity of 1.2x10-17 m2/s, consistent with Yunker and van Orman (2004). Issues of the pressure medium, initial surface roughness, and knock-on effects will also be discussed. The diffusivity of hcp iron and iron-nickel alloys as determined in the laser-heated diamond anvil cell can be used to infer the solid-state viscosity of the inner-core, which may govern the mechanism to develop inner-core anisotropy.
MR13C-1404
Preliminary Results for Fluorine Diffusion in Biotite
Recent experiments evaluated the diffusion of F in biotite at 650, 700, 750 °C, 0.4 GPa of pressure. Experiments assembled a mixture of two naturally occurring, compositionally homogenous biotites separated from schists of the Seridó Fold Belt in northeastern Brazil. A number of ~0.1 mm long PPPL 18 biotite grains, with F=2.5wt% and an Mg-number of 0.36, were mixed with ~0.1 mm long St. Andre biotite grains, with F=0.3wt% and an Mg-number of 0.38. These were loaded into silver capsules with a powdered anorthite-fluorite-sillimanite- quartz (AFSQ) buffer and 4M HF acid. The capsules and their silver covers were positioned into piston-cylinder aparati, pressurized to 0.4 GPa, and held at that pressure for 30 minutes to insure a pressure weld between the capsule and its cover. The materials were then heated to either 650, 700, 750 °C at 70 °C/min and held for 74, 78, and 24 hours, respectively. The materials were quenched to room temperature in less than 1 minute, removed, sectioned, and polished. These were evaluated by electron microprobe analysis (EMPA) using a 1 μm spot size at 15 keV and 100 nA for mapping and 20 nA for spot analyses. X-ray mapping revealed that high-F PPPL 18 biotites were largely unaltered. However, the low-F St. Andre biotites were enriched in fluorine along edges adjacent to pore spaces. There were additional linear zones of F enrichment following defects perpendicular to the c axis. Presumably these defects are related to pre-run cleavage that only partially annealed at run conditions. Diffusion profiles were obtained from crystal edge-core EMPA traverses in areas significantly away from these linear defects. Profiles were fitted with a complementary error function to determine diffusivities. The diffusivities for biotite at 650, 700, and 750 °C are 1.31 ± 0.5 E-17, 1.70 ± 0.5 E-16, 2.71 ± 0.3 E- 15 m2/s, respectively. This yields an Arrhenius relationship with an activation energy (Ea) of 314.5 kJ and an intercept (Do) of 22.7 m2/s. The diffusivities and activation energy are much higher than those for F in tremolite over this temperature range. The diffusivities are also higher than those determined from down- temperature extrapolation of F-diffusion in apatite, while the Ea for apatite appears to be comparable to that for biotite.
MR13C-1405
Effect of fCO2 on the Diffusion of C and O in Calcite at 700 °C, 100 MPa
We conducted a series of experiments to determine how decreasing the fugacity of CO2 affects the diffusivity of C and O in calcite. Previous experimental studies have shown that the diffusivity of O in calcite is ~ 2 orders of magnitude greater in the presence of nearly pure H2O than in a pure-CO2 environment. We reduced the fCO2 at constant T and p by dilution with either N2 or H2O to determine whether merely reducing fCO2 or adding H increases the rate of diffusion. Single cleavage fragments of calcite were enclosed in gold capsules with 13C18O2 and various amounts of H218O or Cu3N, a source of N2. The capsules were heated in cold-seal hydrothermal vessels to 700 °C at 100 MPa for periods up to 79 d. After experimentation, the 18O/(18O + 16O) and 13C/(13C + 12C) were measured by depth-profiling with SIMS ion probes. The results indicate that even a small amount of H2O (xCO2 = 0.8) caused an increase in DO from 10- 15.8 to 10-13.8 cm2/s. Greater proportions of H2O did not further increase DO appreciably. DC changed little from 10-17 cm2/s at xCO2 = 1.0, although the data are scattered. The effect of N2 on DO is similar to that for H2O. The effect on DC is more pronounced, though, with DC increasing apparently linearly from 10-17 cm2/s at xCO2 = 1.0 to 10-15.3 cm2/s at xCO2 = 0.2. That result is consistent with a model for C isotope exchange and diffusion controlled by a reaction at the surface of calcite for which the diffusivity is inversely proportional to fCO2. The diffusion of O is clearly enhanced by the presence of H2O, but the effect is similar for N2.
MR13C-1406
Grain boundary diffusion in garnet and fast-path diffusion along diffusion channels in plagioclase: Growth of garnet reaction rims at granulite facies conditions
We explored corona structures with the succession plagioclase/garnet/pyroxene, which were formed by the reaction plagioclase + pyroxene = garnet + quartz, in order to derive information about diffusion paths that allowed for material redistribution during reaction progress. While quartz formed disconnected single grains along the garnet/hedenbergite boundaries, garnet formed about twenty μm wide continuous polycrystalline rims along former plagioclase/hedenbergite phase boundaries. Individual garnet crystals are separated by low angle grain boundaries, which often form a direct link between the reaction interfaces of the plagioclase/garnet/hedenbergite succession. Compositional variations in garnet involve i) an overall asymmetric compositional zoning in Ca, Fe2+, Fe3+ and Al across the garnet layer and ii) micron-scale compositional variations in the near grain boundary regions and along plagioclase/garnet phase boundaries. These compositional variations formed during garnet rim growth and evaluation of the corona structure using diffusion modeling suggests that the material transport rates between sites of local reaction are regulated by local equilibrium with the minerals surrounding the reaction sites and by effective transport rates across the growing reaction rim. Transport in the garnet polycrystal occurred partly by fast-path diffusion along grain boundaries and slow volume diffusion, which in combination determine effective bulk diffusion in garnet. In the studied example effective diffusion in garnet was too slow to allow for chemical equilibration of the two reaction fronts. This resulted in the formation of a characteristic asymmetric growth zoning. Because the characteristic of such an asymmetric growth zoning involves information of the relative contribution of fast-path and slow volume diffusion the evaluation of the observed growth zoning by diffusion modeling allows derivation of a set Dgb/Dvol ratios for Ca, Fe2+ and Al in garnet. Based on the observation of nano-scale diffusion channels in plagioclase and on the base of our diffusion modeling effective material transport in plagioclase towards reaction sites is suggested to be determined by a combination of fast-path diffusion along diffusion channels and slow volume diffusion.
MR13C-1407
Stress Activation and Propagation of Electronic Charge Carriers in Igneous Rocks
Igneous and high-grade metamorphic rocks in the Earth's crust generate electric currents when subjected to deviatoric stresses. The reason is that these rocks contain dormant electronic charge carriers in the form of peroxy links. Peroxy links are sites in the crystal structures of the constituent minerals where oxygen anions have converted from their common 2- valence state to the 1- valence state, O3X-OO-XO3 with X=Si4+, Al3+ etc. As rocks are stressed and dislocations sweep through the mineral grains, the peroxy links break up, activating electrons and pholes ("phole" is an abbreviation for "positive hole", a defect electron on the oxygen sublattice, chemically O– in a matrix of O2–). The pholes are mobile electronic charge carriers that can spread out of the stressed rock into the surrounding unstressed rock. They travel via energy levels at the upper edge of the valence bands, cross grain boundaries and achieve a phase velocity on the order of 200±50 m/sec, consistent with phonon-assisted electron hopping. Due to mutual repulsion inside the rock volume the pholes spread to the surface, where they build up a positive surface charge. The surface charge can be measured with a non-contact capacitive sensor. If a Cu contact is applied to the surface of the rock, electrons are injected from ground into the rock in response to the evolving positive charge on the rock surface. We modeled surface potentials and burst-like electron injections following low and medium velocity impact experiments, 100 m/sec and 1.5 km/sec respectively.
MR13C-1408
Shear Attenuation and Dispersion in Harzburgite
The shear attenuation, and the associated dispersion in the shear modulus, of fine-grained, texturally mature, synthetic vacuum-sintered aggregates of olivine (Fo91) + orthopyroxene (En92) was measured over a frequency range of 10-2.25 to 1 Hz at temperatures between 1000 and 1300°C in a 1-atm reciprocating torsion apparatus. Experiments were conducted on samples with a range of orthopyroxene contents (0-40 wt%) and a range of average grain sizes (2-10μm). All samples contain a small fraction (φ < 0.01) of silicate melt. In experiments with little or no pyroxene (<20%), high-temperature and/or low frequency measurements show a monotonic decrease in QG-1 with an increase in frequency where QG-1 \propto f-0.35, consistent with diffusion-limited relaxation of normal tractions along grain boundaries. At lower temperature and/or higher frequencies this behavior gives way to a broad dissipation plateau consistent with a contribution of grain boundary sliding to the dissipation. In experiments with larger amounts of orthopyroxene (>20%) the high-temperature, low-frequency response follows QG-1 \propto f- 0.6. Moreover, in all pyroxene-bearing samples, a pronounced peak in dissipation occurs at f~1 Hz at T=1300°C that shifts to lower frequencies at lower temperatures. Additionally, this peak shifts to lower frequency with higher orthopyroxene contents. These attenuation data are compared critically to similar experimental studies on dunite and our preliminary results suggest that fine-grained harzburgite displays both higher attenuation and less modulus dispersion than a dunite with similar microstructure over the range of experimental conditions tested.
MR13C-1409
Thermophysical peculiarities in rocks of Earth's crust
This paper is devoted to a study of thermophysical properties of rocks uncovered by SG-10 deep borehole in the large gold ore deposit Muruntau /Western Uzbekistan/. The results of determination of thermal conductivity (ë,W/mK) and thermal capacity (c,J/kgK), based on 1528 bore specimens taken each 10 meters within the interval of 0-4296m, served as a basis for division of the section of Muruntau SG-10 deep hole rocks into thermophysical complexes. If division of the section is considered from a point of view of a specific character of its thermophysical features, there are three distinct complexes: the first one occupies a interval from the hole's mouth to a depth of 1000m, and is characterized by increased mean values of thermal conductivity within 2.9-3.2 W/mK, and gradient temperature raise of thermal capacity with depth from 560 to 750 J/kgK. The second one is within the interval of 1000-2500m. It is remarkable for its decrease in thermal conductivity level, especially in roof and foot parts (up to 1.30-1.45 W/mK), and for mean values of thermal capacity within 750-780 J/kgK, as well as for direct correlation between thermal conductivity and thermal capacity in some local anomalies, while, on the whole, correlations of these parameters are inverse for the section. The third complex is below 2500m. It is characterized by the lowest mean thermal conductivity (1.6 W/mK) and the highest thermal capacity (1250 J/kgK). What is thermophysical features distribution zoning explained by? There was a thermal field – one of basic factors in ore components activization and redistribution, their concentration and zoning formation within the intrusion area of the Muruntau granitoid intrusive (C3-P1). Magmatic activization produced a strong heat impulse, which resulted in the appearance of zones of permeability with high thermal and temperature conductive features favourable for energy transfer and removal of some active chemical components. The result of the influence of the Muruntau intrusive thermal field is redistribution of thermophysical parameters.
MR13C-1410
Influence Of The Redox State On The Electrical Conductivity Of Basaltic Melts
The electrical conductivity is an efficient probe of mass transfer processes within silicate melts and magmas. Previous studies have established that the electrical conductivity is sensitive to parameters such as temperature, melt composition and pressure. In contrast to what is known for Fe-bearing minerals, little attention has been given to the influence of redox state on the electrical conductivity of melts. Experiments were performed on tephritic and basaltic compositions respectively from Mt. Vesuvius and Pu'u ‘O'o. Measurements were carried out on cylindrical glass samples (OD: 6 mm, ID: 1 mm, L: 8 mm) drilled from glass obtained by fusing each rock sample at 1400°C in air. A two-electrode configuration was adopted, with the electrical impedance being radially measured. A Pt wire was used as the internal electrode whereas a Pt tube served as the external electrode. Experiments were conducted at 1 atm in a vertical furnace between 1200°C and 1300°C, both in air and in a CO-CO2 atmosphere at a fO2 corresponding to NNO+1. Both reduction and oxidation experiments were performed. In reduction experiments (pure CO2 then CO-CO2 gas mixture), electrical conductivities progressively increase with time. The reverse is observed in oxidation experiments (CO-CO2 gas mixture then pure CO2). These variations of electrical conductivities are correlated with modifications of the Fe2+/Fe3+ ratio in the melt, and are consistent with the respective structural roles of Fe2+ and Fe3+. In both types of experiments, a minimum of about 400 mn is necessary before a plateau is reached, interpreted to reflect the kinetics of attainment of the equilibrium Fe2+/Fe3+ ratio in the melt. Differences between plateau and initial values are typically of a few ohms, much higher than the sensitivity of our measurements (better than 0.1 ohm). When increasing temperature, the time required for reaching plateau values decreases. At NNO+1, the electrical activation energy (Ea) was determined for both compositions: Ea=137 kJ/mol (tephrite) and 73 kJ/mol (basalt). Further experiments are necessary to quantify the influence of redox state on electrical conductivity, especially at fO2 below NNO+1.
MR13C-1411
Electrical conductivity variations of hydrous mineral and rocks associate with dehydration process
The electrical conductivity of rocks and minerals is an important parameter, because its value can help understanding subsurface conductivity structures. In particular, conductivity of hydrous mineral and rock contains hydrous mineral vary drastically due to the effect of water content and influence of ionic water. To detect electrical conductivity variations associate with dehydration process, we have conducted different types of laboratory measurements. Firstly, to observe dehydration reaction of brucite by electrical conductivity variation, we have developed a technique in a sealed condition. Electrical conductivity measurement of brucite was performed as a function of temperature while the confining pressure was kept at 1 GPa. As a result, two types of remarkable electrical conductivity variation of brucite could be seen. Below 700 K, conductivity of the sample seems to stabilize and show linear variation as expected from the Arrhenius equation. Once temperature was increased around dehydration boundary, brucite showed high conductivity. This may be because, coexisting solid and fluid phases, a mixed electronic and ionic conduction mechanism may operate in the sample. Secondly, electrical conductivity measurements of gneiss, basic rock and amphibolite were conducted using the conventional cell. The rock samples were not sealed in the high insulation assembly. The conductivity hysteresis of various rocks was plotted and examined. The systematic increase in conductivity up to 1000 K was observed. Above 1000 K, the conductivity of amphibolites does not vary linearly. As amphibolites contain much hydrous minerals, conductivity of the sample may be high after dehydration. From the successful experimental results, we found remarkable electrical conductivity variations associate with dehydrations. When temperature exceeds metamorphic condition or is near phase boundary condition, electrical conductivity of the sample increase drastically by 2 to 3 order of magnitude. For this phenomenon, we positively evaluate the effect of water and the influence of ionic fluid after dehydration, quantitatively. Even though small amount of H2O is formed after dehydration, bulk conductivity of the sample varies drastically. This increase of the conductivity associate with dehydration is often observed by EM soundings in and around the subduction zones in the Earth.
MR13C-1412
Physicochemical states of water at high temperatures in the microcrystalline silica (chalcedony) by in-situ infrared spectroscopy
Water in rocks is present mainly as H2O in grain boundaries and pores, and as hydroxyl species in crystal structures. At higher temperatures, physicochemical states of water in rocks would be changed and dehydration is also expected. One of the representative materials in the crust including both H2O and hydroxyl is the microcrystalline silica, chalcedony. Water in chalcedony from Oshamanbe, Hokkaido, Japan was investigated at high temperatures by in-situ infrared (IR) spectroscopy. First, distribution of water in the sample was investigated by polarized optical microscope (POM) observation and IR measurement. Under the POM, the sample exhibits typical aspects of chalcedony, showing length-fast and rhythmic extinctions. The sizes of optical domains caused by the preferred orientation of crystallites are ranging from one micron (fine) to several microns (coarse) and heterogeneous in millimeter to centimeter scales. IR spectra of these domains show a broad band from 3800 to 2500cm-1 due to O-H stretching of H2O, with a shoulder at 3585cm-1 due to Si-OH species. Less water was contained by IR measurement in coarser grain domains. Second, the sample was heated from room temperature to 400° C at 50° C intervals under the IR microscope. Every 50° C, the peak position of H2O (3430cm-1 at 25° C) shifted to higher frequency by about 20cm-1 and that of Si-OH (3585cm-1 at 25° C) shifted to higher frequency by about 2.5cm-1. The peak shift of H2O can be explained by the decrease of average coordination numbers of H2O. The peak shift of Si-OH can be due to the increase of hydrogen bond distance of Si-OH--- O. Third, the sample was heated isothermally from 400° C to 700° C under the IR microscope. By analyzing the OH band area, the dehydration kinetics of chalcedony was quantitatively investigated.
MR13C-1413
High Pressure Electrochemistry: Application to silver halides
Electron and ion charge transfer processes help govern electrical conductivity and diffusive mass and heat transport properties in deep Earth minerals. In an attempt to understand how pressure influences charge transfer behavior, the halide silver bromide (AgBr) was studied under the influence of an electric potential difference applied across two electrodes in a diamond anvil cell. This study follows our previous work on AgI, which was found to dissociate to molecular iodine and silver metal due to pressure and voltage influences. We performed two sets of experiments on AgBr at high pressure in a diamond anvil cell: electrochemical dissociation and electrical resistance measurements. In our study, we were able to electrochemically dissociate AgBr at pressures of 0.25-1.6 GPa by applying a voltage across the electrodes in the diamond cell sample chamber. Ag metal grew visibly on the negatively-charged electrode when voltages varying from 0.1 V to 5 V were applied. Additionally, a dark blue color appeared in low pressure areas of the diamond cell and grew darker from both voltage application and light exposure, indicating photochemical effects. We found that the reaction area and growth rate of both metal and dark blue color strongly increased as voltage increased, but tended to decrease with greater pressure. The resistance across the cell was observed to be influenced by both pressure and light exposure. As the AgBr sample was exposed to visible light, the resistance dropped instantaneously, and after the light was turned off, the resistance increased on a timescale of 10's of seconds to minutes. Notably, at higher pressures, the AgBr showed less photosensitivity. Exploration of these metal halide systems has many potential applications. First, these experiments explore the pressure-dependence of photochemical and photovoltaic processes, and may spur development of pressure-tuned microscale electronic devices. Second, these experimental results can be used to constrain thermodynamic models of pressure-dependent electrochemical behavior of materials, which may then be applied to the high temperature, high pressure mineral phases of the deep Earth and planets.