Al Duba

The electrical conductivity of an isotropic olivine mantle

In order to extend the useful temperature range of interpretation of olivine electrical conductivity σ we have used the nonlinear iterative Marquardt technique to fit experimental data over the range 720°–1500°C to the parametric form σ = σ1e−A1/kT + σ2e−A2/kT, where k is Boltzmanns constant and T is absolute temperature. The model describes conduction by migration of two different thermally activated defect populations with activation energies A1 and A2, and preexponential terms σ1 and σ2 that depend on number of charge carriers and their mobility and that may be different for each crystallographic direction. A combined interpretation of recent high (San Carlos olivine) and low (Jackson County dunite) temperature measurements has been made that demonstrates that a single activation energy A1 for all three crystallographic directions adequately fits the data. The parametric fits show that the high-temperature conduction mechanism has far greater anisotropy than the low-temperature mechanism, consistent with previous assignments to ionic and electronic conduction, respectively. The geometric mean of the conductivity in the three directions is approximately –¯=102.402e-1.60eV/kT+109.17e-4.25eV/kT S/m and is presented as a model for isotropic olivine, SO2, appropriate from 720°C to above 1500°C, at oxygen fugacities near the center of the olivine stability field. It is observed that the magnitudes of σ1 for the three crystal directions are similar to the ratios of the inter-ionic distances between the M1 magnesium sites in olivine, to within 5%, consistent with Fe3+ preferring the M1 site below 1200°C.

The influence of oxidation state on the electrical conductivity of olivine

Abstract The electrical conductivity of a single crystal of San Carlos olivine (Fo 92 , 0.16 wt.% Fe 2 O 3 ) has been measured as a function of temperature and oxygen fugacity (fO 2 ). After heating to 1338°C at fO 2 = 10 −12 atm., the conductivity at 950°C was 10 −5 (ohm-m) −1 , almost 3 orders of magnitude less than that measured in air. This decrease is due to the reduction of Fe 3+ to Fe 2+ . Further heating to 1500°C at fO 2 = 10 −14 atm., decreased the electrical conductivity at 950°C to 10 −6 (ohm-m) −1 . When recovered at room temperature, the speciment had Fo 96 composition and contained small, opaque blebs distributed throughout the crystal. Derivations of temperature profiles for the earths mantle from conductivity-depth models must take account of the important role played by iron oxidation state in the electrical conductivity of olivine.

Electrical conductivity of San Carlos Olivine along [100] under oxygen‐ and pyroxene‐buffered conditions and implications for defect equilibria

The electrical conductivity along [100] of single crystal San Carlos olivine was measured as a function of temperature between 1100° and 1200°C and oxygen fugacity between 10−6 and 10+0.5 Pa (at 1200°C), and either with (“pyroxene-buffered”) or without (“self-buffered”) an added natural pyroxene buffer from a San Carlos lherzolite. Under these temperature and ƒO2 conditions, electrical conduction in the self-buffered sample is attributed to polarons (Fe*) and electrons (e′) and in the pyroxene-buffered sample is attributed to polarons(Fe*) and magnesium vacancies(V″Mg). Over the range of temperature and ƒO2 investigated, the electrical conductivity of the self-buffered sample is given by σsb[100] = 2.27(S/m)e−0.55(eV)/kT ƒO20.18+306.3(S/m)e−2.25(eV)/kT ƒO2−0.18 and for the pyroxene-buffered sample by σpb[100] = 0.18(S/m)e−0.34(eV)/kT ƒO20.17+15.2(S/m)e−1.3(eV)/kT where k is Boltzmanns constant, T is in Kelvin, and ƒO2 is in atmospheres. The conductivity of the pyroxene-buffered sample is lower than that of the self-buffered sample, primarily as a result of a decrease in the polaron concentration. the electrical conductivity of both samples was found to decrease irreversibly once the samples experienced an oxygen fugacity more reducing than approximately the wustite-magnetite buffer. Electron microprobe analyses indicate that this effect results from loss of iron from the olivine samples to the iridium electrodes. A series conduction model based on the observed compositional gradient adequately accounts for the magnitude of the irreversible conductivity decrease and limits the thickness of any surficial pyroxene phase to <0.1 μm. Mantle temperature profiles based on laboratory measurements of self-buffered samples predict temperatures of the order of between 25° and 150°C colder, depending on the ambient oxygen fugacity, than those based on measurements of pyroxene-buffered samples.

Increase of electrical conductivity with pressure as an indicator of conduction through a solid phase in midcrustal rocks

Rocks freshly cored from depth at the German continental scientific drilling site (KTB) offer an opportunity to study transport properties in relatively unaltered samples resembling material in situ. Electrical conductivity σ was measured to 250 MPa pressure, and room temperature on 1 M NaCl-saturated amphibolites from 4 to 5 km depth. An unexpected feature was an increase of σ with pressure P that appeared (anisotropically) in most samples. To characterize this behavior, we fitted the linear portion of log σ versus P to obtain two parameters: the slope dlogσ/dP (of order 10−3 MPa−1) and the zero-pressure intercept σ0. Samples of positive and negative slopes behave differently. Those having negative slopes show strong correlation of σ0 with a fluid property (permeability). This behavior indicates that fluids exert the dominant control on σ0 at low pressure when σ0 is greatest, which is typical behavior observed in previous studies. In contrast, samples with positive slopes lack a correlation of σ0 with permeability, indicating that fluids are less important to positive pressure behavior. Another result is that samples of negative dlogσ/dP have uncorrelated slopes and initial conductivities. In significant contrast, samples of positive slopes have the greatest P dependence for lowest initial conductivity σ0, that is, the less fluid, the more positive dlogσ/dP. Hence positive dlogσ/dP is consistent with reconnection of solid phases into a conductive texture better resembling that of rock at depth. Detailed examination of one sample by electron probe and scanning electron microscope reveals the presence of carbon on internal cleavage surfaces in amphibole, the most abundant mineral present. Thus carbon probably dominates the reconnection, but total σ still involves fluids as well as Fe-Ti oxides. For the KTB location it is inferred that the reason mid to deep crustal electrical conductivities modeled from geophysical measurements are so much higher than conductivities of silicates is the presence of interconnected good conductors involving films of carbon on surfaces and other solid phases.

Experimental melting curve of iron revisited

With new experimental data presented in the last 2 years, it becomes possible to resolve conflicts in the data sets used in constructing the melting curve of iron, Tm(P). On the basis of these new data, several data sets have been dropped: the Williams et al. [1987] melting curve up to 100 GPa and the Bass et al [1987] and Yoo et al. [1993] shock-wave-derived Tm(P) in the 200–300 GPa range based on light emissivity measurements. The Boehler [1993] Tm(P) curve to 200 GPa and the Brown and McQueen [1986] shock-wave-determined Tm(240) remain, leaving a gap between 240 and 330 GPa. We fill this gap using the Lindemann law of melting. The Lindemann law and the temperature values along the Brown and McQueen [1986] Hugoniot require the value of the Gruneisen ratio, γ; thus γ connects Tm at 330 GPa with Tm found for values of the Hugoniot. It is further shown that the heat of crystallization, ΔHm, is dependent on γ. Thus, through 7, a connection is made between the melting curve and the power generated within the inner core. The effect of all these connections of physical properties through γ leads us to recommend 5600–6500 K as the Tm of iron at inner-outer core boundary conditions. Though argument continues concerning the amount and nature of alloying elements, there remains little ground for doubting that both inner and outer cores consist mainly of iron.

THE ELECTRICAL CONDUCTIVITY OF POLYCRYSTALLINE OLIVINE AND PYROXENE UNDER PRESSURE

Abstract Electrical conductivity (σ) measurements on polycrystalline olivine and pyroxene were made at pressures up to 5.0 GPa and at temperatures up to 1200°C to examine the effect of grain boundaries on conduction. Values of σ were observed within one-half order of magnitude of those for single crystals at lower pressures and under controlled oxygen fugacity (fo2). if this difference prevails over the entire temperature range, these values result in temperature profiles for the earth and moon that are lower than profiles from single-crystal data by no more than 200°C. However the small σ differences between single and polycrystals most probably are due to a higher fo2 in the experimental apparatus rather than the presence of grain boundaries. The data indicate pressure has a small effect on conductivity and activation enthalpies associated with conduction.

Electrical conductivity and carbon in metamorphic rocks of the Yukon-Tanana Terrane, Alaska

Electrical conductivity of a water-saturated quartz-mica-garnet-schist, collected from a surface outcrop near the Denali Fault Zone in the Yukon-Tanana terrane of east central Alaska, increases slightly with pressure to about 200 MPa. This behavior is unlike that exhibited by other Yukon-Tanana samples or by most rocks from other locations. Detailed petrographic examination of the sample revealed the presence of a stringer of carbonaceous material generally less than 10 μm thick enclosed in and intergrown with one of the muscovite layers and extending for about 2 cm along the foliation. The stringer is probably responsible for the anomalous conductivity change with pressure, making the sample the first for which anomalous electrical conductivity behavior can be attributed to carbon associated with a specific feature. The carbonaceous stringer together with its host muscovite layer are deformed and broken around a rotated garnet porphyroclast. The deformation was accommodated by plastic deformation of quartz and therefore occurred in the ductile regime under conditions at least equivalent to greenschist facies metamorphism. We interpret the textural relations to indicate that the carbonaceous material was formed by fluid deposition in a fracture formed within the muscovite layer, possibly during the main phase of metamorphism and deformation, and that the mica and carbon stringer were then deformed by the noncoaxial deformation responsible for rotation of the garnet porphyroclasts. Together the facts that the deformation resulting in garnet rotation was ductile and that the garnet rotation disrupted the stringer demonstrate that the carbonaceous stringer was present at depth (i.e., >10 km). Brittle deformation on the microscopic scale is observed to have broken the connectivity of the carbon stringer, explaining in part why the rock does not exhibit anomalously high conductivity at 0.1 MPa (1 atm) pressure. The brittle deformation is interpreted to have been caused by unloading due to uplift. The observations indicate that carbonaceous material may exert a primary control on crustal electrical conductivity because it may be present as interconnected arrays in grain boundaries or microfractures or in megascopic, throughgoing fractures.

Carbon‐enhanced electrical conductivity during fracture of rocks

Changes in electrical resistance during rock fracture in the presence of a carbonaceous atmosphere have been investigated using Nugget sandstone and Westerly granite. The experiments were performed in an internally heated, gas-pressure vessel with a load train that produced strain rates between 10−6 and 10−5 s−1. Samples were deformed at temperatures of 354° to 502°C and pressures of 100 to 170 MPa in atmospheres of Ar or mixtures of 95% CO2 with 5% CO or 5% CH4, compositions that are well within the field of graphite stability at the run conditions. In experiments using Nugget sandstone, resistance reached a minimum value when the maximum temperature was achieved and good electrode contact was made. The resistance then increased as the experiment continued, probably due to dry out of the sample, a change in the oxidation state of the Fe-oxide associated with the cement, or destruction of current-bearing pathways. At approximately 200-MPa end load, the rock sample failed. Plots of load and resistance versus time show several interesting features. In one experiment, for example, as the end load reached about 175 MPa, resistance stopped increasing and remained fairly constant for a period of approximately 0.5 hour. During loading, the end load displayed small decreases that were simultaneous with small decreases in resistance; when the end load (and the displacement) indicated rock failure, resistance decreased dramatically, from ∼150 MΩ to 100 MΩ. In a single experiment, the Westerly granite also showed a decrease in resistance during dilatancy. The nature and distribution of carbon in the run products were studied by electron microprobe and time-of-flight secondary-ion mass spectroscopy (TOP-SIMS). Carbon observed by mapping with the former is clearly observed on micro-cracks that, based on the microtexture, are interpreted to have formed during the deformation. The TOF-SIMS data confirm the electron-probe observations that carbon is present on fracture surfaces. These observations and experimental results lead to the hypothesis that as microfractures open in the time leading up to failure along a fracture, carbon is deposited as a continuous film on the new, reactive mineral surfaces, and this produces a decrease in resistance. Subsequent changes in resistance occur as connectivity of the initial fracture network is altered by continued deformation. Such a process may explain some electromagnetic effects associated with earthquakes.

Is the asthenosphere electrically anisotropic

Abstract Two techniques for resolving the conductance of an electrically conductive asthenosphere are presented. The first technique combines observatory electromagnetic data in the period range of the daily variation and longer, which provides penetration depths of 400 km and deeper, with laboratory data of the conductivity of upper mantle materials. The conductance can be estimated by this technique, but the geometry - e.g. the exact depth - is not resolved. The second technique extends the classical magnetotelluric (MT) tensor estimation and decomposition to the period range 1000–30 000 s. If an electrical asthenosphere exists, then in this period range the electromagnetic field penetrates into it (1000 s) but also through it (30 000 s). The concept of regional strike directions which has been successfully used to obtain crustal anisotropy directions is again employed at these long periods. The surprising result is that in the depth range of the asthenosphere a strong directional dependence of the conductance again occurs. Although large arrays will be necessary in order to prove that this is really conductivity anisotropy in the asthenosphere, first field results from arrays 100–200 km in extent are encouraging. The concept of a superposition of crustal and upper mantle anisotropy explains to some extent why earlier magnetotelluric determinations of the asthenosphere had little success: only in the fortunate case where a MT experiment was performed not parallel to the direction of the crustal conductor, but parallel to the asthenospheric one, is the second structure resolved. Electrical anisotropy in the uppermost mantle supports the concept of intracrystalline water, which allows for a contribution of hydrogen diffusity to the conductivity of olivine: this diffusity is highly anisotropic with respect to the crystal axis, and if the crystals are partly aligned, the conductivity measured with MT is also direction-dependent.

The effect of pressure on the electrical conductivity of KTB rocks

Complex electrical resistivity and permeability were measured on two gneiss samples and nine amphibolites (originally located at a depth of 4150 m to 5012 m) from the main drilling of the German deep drilling project (KTB). Measurements were performed as a function of hydrostatic pressures up to 240 MPa on core samples (30 mm in diameter and 10–20 mm high). For each measurement, two samples were used, one being parallel, and one perpendicular to the borehole axis. At low pressures and again at maximum pressure the frequency dispersion (1 kHz up to 1 MHz) of the complex resistivity was measured using a two electrode device. An unusual pressure effect was detected on some of the samples and was established to be due to the oriented deposition of good conducting phases in the foliation. Rock fabric and the orientation of ore mineralization was measured on thin sections and polished sections prepared from the same samples.