Laboratory Measurements Of Electrical Conductivity Research Articles

Laboratory measurements of electrical conductivity in a gabbro of the Oman ophiolite at high–pressures and high–temperatures: implications for interpretation of resistivity structures of lower oceanic crust

Laboratory measurements of electrical conductivity in a gabbro of the Oman ophiolite at high–pressures and high–temperatures: implications for interpretation of resistivity structures of lower oceanic crust

Electrical conductivity of hydrous andesitic melts pertinent to subduction zones

AbstractAndesitic magmatism and rocks are widespread at convergent plate boundaries. Electrically conductive bodies beneath subduction zone arc volcanoes, such as the Uturuncu Volcano, Bolivia, may correspond to active reservoirs of H2O‐bearing andesitic magma. Laboratory measurements of electrical conductivity of hydrous andesitic melts are required to constrain the physicochemical conditions of these magma reservoirs in combination with magnetotelluric data. This experimental study investigates electrical conductivity of andesitic melts with 0.01–5.9 wt % of H2O at 1164–1573 K and 0.5–1.0 GPa in a piston cylinder apparatus using sweeping‐frequency impedance spectroscopy. Electrical conductivity of andesitic melt increases with increasing temperature and H2O concentration but decreases with pressure. Across the investigated range of H2O concentration, electrical conductivity varies by 1.2–2.4 log units, indicating stronger influence of H2O for andesitic melt than for rhyolitic and dacitic melts. Using the Nernst‐Einstein equation, the principal charge carrier is inferred to be Na in anhydrous melt but divalent cations in hydrous andesitic melts. The experimental data are regressed into a general electrical conductivity model for andesitic melt accounting for the pressure‐temperature‐H2O dependences altogether. Modeling results show that the conductive layer at >20 km depths beneath the surface of the Uturuncu Volcano could be interpreted by the presence of less than 20 vol % of H2O‐rich andesitic melt (with 6–9 wt % H2O).

Electrical conductivity of hydrous silicate melts and aqueous fluids: Measurement and applications

Abstract The combination of magnetotelluric survey and laboratory measurements of electrical conductivity is a powerful approach for exploring the conditions of Earth’s deep interior. Electrical conductivity of hydrous silicate melts and aqueous fluids is sensitive to composition, temperature, and pressure, making it useful for understanding partial melting and fluid activity at great depths. This study presents a review on the experimental studies of electrical conductivity of silicate melts and aqueous fluids, and introduces some important applications of experimental results. For silicate melts, electrical conductivity increases with increasing temperature but decreases with pressure. With a similar Na+ concentration, along the calc-alkaline series electrical conductivity generally increases from basaltic to rhyolitic melt, accompanied by a decreasing activation enthalpy. Electrical conductivity of silicate melts is strongly enhanced with the incorporation of water due to promoted cation mobility. For aqueous fluids, research is focused on dilute electrolyte solutions. Electrical conductivity typically first increases and then decreases with increasing temperature, and increases with pressure before approaching a plateau value. The dissociation constant of electrolyte can be derived from conductivity data. To develop generally applicable quantitative models of electrical conductivity of melt/fluid addressing the dependences on temperature, pressure, and composition, it requires more electrical conductivity measurements of representative systems to be implemented in an extensive P - T range using up-to-date methods.

A New Cell for Electrical Conductivity Measurement on Saturated Samples at Upper Crust Conditions

Electrical resistivity soundings are used by geophysicists to determine the structure and composition of the Earth’s crust and mantle and to explore natural resources (ore, oil, gas, water). Their interpretations in terms of composition and in-situ physical conditions depend mainly on laboratory measurements of electrical conductivity of rocks at simulated crustal conditions of temperature, pressure, saturation and pore pressures. These measurements present a numbers of limitations, in particular, in the case where conductive pore fluids are present, as in the case of deep reservoir conditions, where temperature exceeds 250 °C. Here, we present a new cell capable of measuring electrical conductivity of large saturated samples at confining pressure up to 200 MPa, pore pressure up to 50 MPa, and temperature up to 500 °C. The measurement cell has been developed in a commercial, internally heated, gas pressure apparatus (Paterson press). It is based on the concept of “guard ring” electrode, which is adapted to samples that are jacketed by a very conductive, metallic material. Numerical modeling of the current flow in the electrical cell allowed defining the optimal cell geometry. Calibration tests have been performed on Fontainebleau sandstones saturated with electrolytes of different conductivities, up to 350 °C. The resulting electrical formation factor and temperature dependence of electrical conductivity are in very good agreement with previous studies. This new cell will improve the exploration and exploitation of deep fluid reservoirs, as in unconventional, high enthalpy geothermal fields. In particular, the investigations address possible effects of fluid-rock interactions on electrical resistivity of a reservoir host rock.

In-situ control of oxygen fugacity for laboratory measurements of electrical conductivity of minerals and rocks in a multi-anvil press

In this paper, a new of oxygen fugacity controltechnique that can be widely applied to in-situ measurement of the grain interior electrical conductivities of minerals and rocks is presented for high temperature and high pressure. Inside the sample assembly, a metal and corresponding metal oxide form a solid oxygen buffer. The principle of this technique is to randomly monitor and adjust oxygen fugacity in the large-volume multi-anvil press by changing the types of solid oxygen buffer, metal shielding case and electrodes. At a pressure of up to 5.0 GPa and a temperature of up to 1423 K, the electrical conductivities of the dry peridotite are tested under the conditions of different oxygen fugacities. By virtue of this new technique, more and more reasonable and accurate laboratory electrical property data will be successfully obtained under controlled thermodynamic conditions.

Laboratory measurements of electrical conductivity of hydrous and dry silicic melts under pressure

An interpretation of the electrical signature of molten rocks within the Earth’s interior in terms of nature and temperature conditions of magma requires additional laboratory data on the electrical conductivity of silicate melts. This paper describes an experimental setup and presents measurements of the electrical impedance of dry and hydrous (1–3 wt% H 2O) metaluminous obsidians determined in an internally heated pressure vessel, in the range of 50–400 MPa and 350–1325°C. It is shown that the temperature and pressure dependences of electrical conductivity of both hydrous and dry obsidian can be fitted by an Arrhenius law applying in the melt and the glass regions. This suggests that a similar transport mechanism operates in both melt and glass. The determined activation energies are 70 kJ/mol for the dry obsidian and 65 and 61 kJ/mol for the 1 and 3 wt% H 2O melts, respectively. The activation volume is 20 cm 3/mol. Combination of tracer diffusion and electrical conductivity reveals that sodium is the dominant charge carrier in the hydrous and dry obsidians. The temperature and pressure effects on conductivity are therefore interpreted in terms of activation energy and activation volume for Na mobility in dry and hydrous rhyolites. An increase in conductivity associated with addition of water was observed and is shown to reflect the effect of water incorporation in melts on the mobility of sodium. As a prospective, it is anticipated that both the mobility and content of sodium could control the electrical conductivity of most terrestrial silicate melts. Magma under differentiation becomes more conductive due to sodium and water enrichment associated with fractional crystallization; therefore, its electrical signature must reveal its nature and maturity.

Vegetation indicators of salinity in northern Queensland

Abstract The possibility that plants could serve as indicators of underlying soil salinity in northern Queensland, Australia, was investigated using 2197 site observations recorded during a land resources inventory of the Dalrymple Shire. The area surveyed intersects three biogeographical regions: the desert uplands, the Einasleigh uplands and the northern brigalow belt. The three dominant plant species in the lower, middle, and upper vegetation strata were recorded, along with laboratory measurements of electrical conductivity down each soil profile. Correspondence analysis, generalized additive models and clustering were used to investigate the relationship between plant occurrence, climatic and edaphic factors. The results of these statistical analyses strongly suggest a relationship between salinity and brigalow plant communities. A generalized additive model to predict soil salinity from environmental variables including vegetation data is presented.

Laboratory‐based electrical conductivity in the Earth's mantle

Recent laboratory measurements of electrical conductivity of mantle minerals are used in forward calculations for mantle conditions of temperature and pressure. The electrical conductivity of the Earth's mantle is influenced by many factors, which include temperature, pressure, the coexistence of multiple mineral phases, and oxygen fugacity. In order to treat these factors and to estimate the resulting uncertainties, we have used a variety of spatial averaging schemes for mixtures of the mantle minerals and have incorporated effects of oxygen fugacity. In addition, to better calculate lower mantle conductivities, we report new measurements for electrical conductivity of magnesiowüstite (Mg0.89Fe0.11)O. Because the effective medium theory averages lie between the Hashin‐Shtrikman bounds for the whole mantle, a laboratory‐based conductivity‐depth profile was constructed using this averaging scheme. Comparison of apparent resistivities calculated from the laboratory‐based conductivity profile with those from field geophysical models shows that the two approaches agree well.

The electrical conductivity and thermal profile of the Earth's mid‐mantle

Electrical conductivity in the Earth's mantle is sensitive to temperature and chemical environment. Recent laboratory measurements of electrical conductivity are combined with candidate mantle geotherms to produce synthetic electrical conductivity profiles. These profiles are used to forward model the Earth's geomagnetic response function C, results of which are compared with the observed globally averaged response function at periods of 3.5 days to 4 months. Candidate lower mantle geotherms, representing whole‐mantle and layered convection end‐members, are compared using published electrical conductivity measurements on alumina‐bearing and alumina‐free perovskite in the conductivity models. Comparison of the predicted response functions with the observed geomagnetic response of the Earth shows that a) if lower mantle alumina is incorporated into perovskite, then the lower mantle must be cool, and b) if the alumina is not incorporated in perovskite then the results are only consistent with a hot lower mantle. In addition, the maximum alumina content of lower mantle MgSiO3 perovskite is constrained at 4%.

The electrical conductivity of lherzolite

Electrical conductivity as a function of oxygen fugacity (ƒO2) between 10−5 Pa and 1 Pa, temperature between 700°C and 1200°C, and time over a period of 1700 hours are reported for a lherzolite nodule from Mount Porndon, Australia. Analysis of complex impedance collected at 100 Hz, 1 kHz, and 10 kHz indicates that there is significant frequency dispersion in this rock at temperatures below 900°C–1000°C. By choosing the resistance of an equivalent parallel RC network at the frequency having minimum phase, the effect of dispersion is largely avoided. Conductivity as a function of time, collected following changes in ƒO2 at 1010 and 1200°C, has been analyzed to determine the diffusivity of defects relating ƒO2 to the electrical conduction mechanism in olivine. The diffusivities so obtained are in remarkably good agreement with those determined from strain measurements during creep tests, which implies that magnesium vacancies are the rate‐limiting step for conductivity reequilibration after ƒO2 changes. A longer‐term process is observed in this rock in which the conductivity drifts upward or downward with a time constant of hundreds of hours after initial reequilibration to gas mix ( ƒO2) changes. We speculate that this long‐term drift could be related to equilibration of the iron distribution between coexisting olivine and pyroxene as a function of ƒO2. The sense is an increase in conductivity (olivine gains iron) at low ƒO2 and vice versa. The lherzolite conductivity data are not significantly different from measurements made on olivine single crystals and polycrystals, even though the rock contains about 34 modal % pyroxene. This consistency of laboratory measurements of electrical conductivity of olivines from many localities and geological settings supports the use of recent models relating mantle temperature with electrical conductivity in the interpretation of mantle geo/electromagnetic soundings.

Laboratory measurements of electrical conductivity of biotites

For the sake of helping to clarify the anisotropic behaviour of KTB borehole rock samples, AC (103 Hz) and DC electrical conductivities of powdered biotites and flogopites (sheet silicates) were investigated in various atmospheres (air, argon) at temperatures of 20–1200°C.

A review of laboratory measurements of the electrical conductivity of rocks and minerals

The use of laboratory measurements of electrical conductivity for rocks and minerals at high temperatures and pressures helps us to interpret magnetotelluric measurements and allows us to obtain further information about various physical and mineralogical properties of the Earth's interior. A survey of recent laboratory measurements of electrical conductivity of rocks and minerals at high temperatures and pressures is outlined. The method and results of estimating the probable depth distribution of electrical conductivity based on the temperature dependence of conductivity and the depth distribution of temperature are briefly presented. Results of some comparisons of magnetotelluric measurements with laboratory results are also presented. As we usually make measurements on dry samples, laboratory results are in good agreement with magnetotelluric soundings for depths below ∼ 30 km, but apparently differ above ∼ 20 km. Some of our results concerning the effect of various factors on measured values of conductivity are discussed. The influence of pressure, atmosphere used during measurements, duration of measurements, chemical composition, and difference between d.c. and a.c. conductivity is demonstrated by consideration of the temperature dependence of the electrical conductivity of the hydrous mineral tourmaline.

Temperature dependence of electrical conductivity and lunar temperatures

Numerous investigations of the electrical conductivity of lunar and terrestrial materials as a function of temperature have been performed to date in an attempt to provide data on which to base lunar interior temperatures from magnetometer-derived lunar conductivity profiles (Schwereret al., 1971, 1972, 1973; Dubaet al., 1972 and others). There are several pitfalls inherent in the extrapolation of lunar temperatures from laboratory measurements of electrical conductivity. These include the choice of representative material for the lunar interior, appropriate environmental conditions (pressure, fugacity, etc.) and the various measurement difficulties.