Conductivity Of Olivine Research Articles

The role of hydrogen in the electrical conductivity of the upper mantle

INTERPRETATIONS of the observed electrical conductivity of the upper mantle have relied on laboratory studies of dry olivine and basaltic melts1,2. These studies indicate that the conductivity of the asthenosphere is higher than that of olivine, suggesting that partial melting or other high-conductivity mechanisms such as grain-boundary phases are required2,3. Recent experimental studies4–6, however, have demonstrated that various kinetic processes in silicate minerals are greatly enhanced by the dissolution of hydrogen. As hydrogen is probably present in the upper mantle, studies on dry olivine may not therefore be relevant to the real Earth. Here I use experimental data on the solubility and diff usivity of hydrogen in olivine5,7 to estimate its effect on the electrical conductivity. I conclude that the observed high conductivity in the asthenosphere can be attributed to solid-state conductivity in olivine if a small amount of hydrogen is present. Partial melting of the asthenosphere on a global scale is therefore not required.

Electrical conductivity of olivine, a dunite, and the mantle

Laboratory studies of the electrical conductivity of rocks and minerals are vital to the interpretation of electromagnetic soundings of the Earth's mantle. To date, the most reliable data have been collected from single crystals. We have extended these studies with electrical conductivity measurements on a dunite from North Carolina, in the temperature range of 600°–1200°C and under controlled oxygen fugacity. Observations of conductivity as a function of oxygen fugacity and temperature demonstrate that conduction in the dunite is indistinguishable from conduction in single olivine crystals. Thus the common practice of exaggerating the single‐crystal conductivities to account for conduction by grain boundary phases in the mantle is unnecessary. Because the dunite conductivity is consistent with that published for single crystals under similar conditions, we have made a combined analysis of these data. Conductivity as a function of temperature between 600° and 1450°C displays three conduction mechanisms whose activation energies may be recovered by nonlinear least squares fitting, yielding activation energies of 0.21±2.56×10−19 J (0.13±1.60 eV) below 720°C, 2.56±0.02×10−19 J (1.60±0.01 eV) between 720°C and 1500°C and 11.46±0.90×10−19 J (7.16±0.56 eV) above 1500°C. The behavior of conductivity as a function of oxygen fugacity is well explained by a model in which an ƒo2‐independent population of charge carriers is supplemented at high oxygen fugacities with a population that is proportional to ƒo20.3. This parametrization produces a clear correlation of the ƒo2 dependent term with iron content, which is otherwise obscured by variations in conductivity among olivines.

Occurrence of small changes in electrical conduction of olivine arising from high‐temperature creep

New measurements of the electrical conductance of single crystals of olivine during creep in the temperature range 1325–1725 K, under controlled oxygen fugacity and under uniaxial stresses of 60–180 MPa, are discussed. These measurements indicate a small but definite increase in the conductivity of olivine as a result of creep. This increase is weakly correlated with strain and stress. The change in conductivity of olivine that results from deformation is not diminished by annealing at high temperature. Similar measurements on corundum crystals lend support to the measured increase in olivine conductivity. These results suggest that dislocations do not directly act as channels for enhanced conduction in olivine; however, they may indirectly influence conduction through their interaction with other defects. Possible mechanisms to account for this influence include interactions between dislocations and point defect complexes and destruction of the preexisting cellular arrangement of dislocations with redistribution of impurities. The present study suggests that deformation is not a viable mechanism in producing high‐conductivity layers in the upper mantle.

Experimental study of pressure dependence of electrical conductivity of olivine at high temperatures

The electrical conductivity of sintered polycrystalline olivines (Mg 1 − x Fe x ) 2SiO 4 ( x = 10, 28, 52, 76 and 100%) has been measured at high pressures (2.9–7.0 GPa) and high temperatures (900–1900 K). Measurements at constant temperature and various pressures show that electrical conductivity increases with pressure in the case of high Fe content, whereas it decreases in the case of low Fe content. The results are interpreted as being due to two competing conduction mechanisms: ionic and electronic, which have different pressure dependences.

Electrical conduction in olivine

This paper reports detailed measurements of electrical conductivity σ and thermoelectric effect S in the mineral olivine and in synthetic forsterite as functions of temperature in the range from 1000° to 1500°C and oxygen partial pressure in the range from 10−10 to 104 Pa. The two most striking observations are strong conductivity anisotropy in forsterite and a sign change in S in olivine at 1390°C. These results are interpreted to show that both materials have mixed ionic and extrinsic electronic conduction under these conditions. On the basis of these interpretations, we infer that forsterite conductivity is dominated by electronic conduction in the a and b directions and probably by movement involving magnesium vacancies in the c direction, where far higher, PO2‐independent conductivity is observed. Olivine appears to show mixed conduction under all the circumstances observed; at low temperatures, electron holes dominate but are superseded by magnesium vacancies at high temperatures.

Electrical conductivity of olivine during high‐temperature creep

The electrical conductance of single crystals of olivine has been measured during creep in the temperature range 1610–1725 K, under controlled oxygen fugacity and under uniaxial stresses of 60–140 MPa. During deformation at a constant uniaxial load, the measured conductance increases continuously with time. During high‐temperature annealing in the absence of load, the conductance of deformed crystals remains constant with time. The total change of conductance resulting from deformation is found to be strongly dependent on the total strain. Comparison between this conductance change and that expected from measured geometrical shape changes of the specimens indicates that the latter is the dominant contributor to the total change in conductance. The results presented indicate that the generation and movement of dislocations and other defects associated with plastic deformation do not significantly contribute to the electrical conductivity of olivine. In addition, the results suggest that the electrical conductivity of the upper mantle will not be affected by the lateral variations in tectonic stresses.

High temperature a.c. electrical properties of olivine single crystal with varying oxygen partial pressure: implications for the point defect chemistry

The electrical conductivity and dielectric constant of a natural olivine single crystal were measured as a function of frequency over 0.1–1000 Hz. The measurements were made at temperatures of 1000–1400°C under a controlled oxygen partial pressure that was varied within the range of the olivine stability field. The observed dielectric constant of olivine increased with increasing oxygen pressure, and the change was greater at higher temperature and lower frequency. When the oxygen pressure Po 2 was relatively high, the dielectric constants measured at 0.1 Hz were proportional to P o 2 1 6 . This observation can be interpreted as a polarization effect of magnesium vacancies that are partially blocked at the specimen electrodes. The conductivity value and the frequency dependence of the dielectric constant indicate that holes as well as magnesium vacancies contribute to electrical properties of olivine. At lower oxygen pressure, the conductivity and dielectric constant are less dependent on oxygen pressure, suggesting that magnesium interstitial ions and/or electrons also contribute to the electrical conduction in olivine.

Measurements of iron(III)-rich fayalites

Recent measurements are reported here of the Mossbauer spectra of selected fayalites (members of the olivine group) from various localities, which indicate large amounts of iron(III) in several of the samples. These features in the spectra are not attributable to magnetite or haematite contamination, but are due to Fe3+ ions contained in the fayalite crystal structure. Such data are contrary to the generally held belief that fayalite is incapable of accommodating much iron(III) in its crystal structure. This discovery may have severe implications for the thermodynamics of the FeO–SiO2 system, as well as for the thermal conductivity of olivine.

The interpretation of upper-mantle electrical conductivities

Abstract Since most past attempts to interprete the upper-mantle electrical conductivities derived from induction theory appear content to support temperature distributions calculated with out-dated static earth models, this paper explores the possibility of finding solutions that are compatible with current notions of a dynamic earth. This has necessarily meant abandoning olivine conductivity functions as a basis for the interpretation. The key notion of the heat transport process as a self-regulating mechanism for viscosity suggests that viscosity rather than temperature be used as the principle variable controlling conductivity. Using a fundamental relation, expressed as an inequality, between measured values of conductivity and viscosity, it is shown that consistent interpretations are possible involving average temperatures many hundreds of degrees below the usual estimates. Local conductivity anomalies are attributed to deformational heating which has a pronounced tendency to become spatially concentrated in zones. The conductivity of oceanic mantle as well as its low Q at depths ~100–400 km is thought to be due to an aqueous phase.

Interpretation of the low velocity zone in terms of the presence of thermally activated point defects

Experimental and theoretical work indicates that point defects can significantly affect sound velocities. Since point defects may exist in thermal equilibrium, their effect on sound velocities may be calculated as an activation phenomenon. Alternatively, activation parameters may be calculated from sound velocity data. Because of the shape of the goetherm it is possible to treat the low velocity zone using the theory of activated processes. The activation parameters calculated from Vp and Vs are in reasonable agreement with one another and with values determined from laboratory experiments on electrical conductivity of olivine and orthopyroxene.

Electrical conductivity of magmatic liquids: effects of temperature, oxygen fugacity and composition

The effects of temperature, f O2 and composition on the electrical conductivity of silicate liquids have been experimentally determined from 1200 to 1550°C under a range of f O2 conditions sufficient to change the oxidation state of Fe from predominantly Fe 2+ to Fe 3+. Oxidation of ferrous to ferric iron in the melt has no measurable effect on the conductivity of melts with relatively low ratios of divalent to univalent cations. Under strongly oxidizing conditions a minor decrease of conductivity is detected inth high ΣM 2/ΣM + ratios. It is concluded that for purposes of estimating the conductivity of magmatic liquids, f O2 may be ignored to a first approximation. Both univalent and divalent cation transport is involved in electrical conduction. Melts relying heavily on divalent cations for conduction, i.e. melts with relatively large ΣM 2+/ΣM + ratios, show strong departures from Arrheenius temperature dependence with the apparent activation energies decreasing steadily as the temperature increases. Conductivities dominated by the univalent cations, in melts with relatively small ΣM 2+/ΣM + ratios, show classical Arrhenius temperature dependence. These observations are discussed in terms of the general characteristics of the melt structure. Compositional variations within the magmatic range account for much less than an order of magnitude variation in electrical conductivity at a fixed temperature. This observation, combined with previous measurements of the conductivity of olivine (A. Duba, H.C. Heard and R. Schock, 1974) make it possible to state with reasonable confidence that melts occurring within the mantle will be more conductive by 3–4 orders of magnitude than their refractory residues. Potential applications to geothermometry are discussed.

Lunar temperature and global heat flux from laboratory electrical conductivity and lunar magnetometer data

Three-layer monotonic electrical conductivity models for the lunar interior to a depth of 600 km are used in conjunction with laboratory measurements of the electrical conductivity of olivine and pyroxene to estimate a temperature–depth profile. The temperatures calculated for depths of 400–600 km are consistent with attenuation of the seismic shear wave. The temperature calculated at a depth of 100–250 km yields a heat flow that is in good agreement with the directly measured lunar heat flow. The temperature, however, is sufficiently close to melting that mascon anisostasy would not be maintained. Thus a better conductor is required at this depth.

Electrical conductivity of olivine at high pressure and under controlled oxygen fugacity

Electrical conductivity σ in the [100] direction has been determined for the Red Sea olivine (Fo 91) to 1440°C and 8 kbar in argon. No systematic variation of σ with pressure was observed. The effect of an 8-kbar variation in pressure over the 1270°–1440°C range is equivalent to a temperature uncertainty of ±5°C. We have also determined σ on the same sample up to 1660°C with controlled oxygen fugacity ƒo2 at 1 bar of total pressure. By using published σ-depth profiles and assuming olivine as the major phase in the earth's upper mantle with ƒ o 2 = 10 −6 -10 −3 bar, temperatures of the upper mantle are calculated as a function of depth. The temperature uncertainty due to possible pressure effects is 2–5 times smaller than that resulting from the ambiguity in published σ-depth profiles.

The effect of ferric iron on the electrical conductivity of olivine

Wet chemistry and Mössbauer techniques indicate that the major chemical difference between olivines whose electrical conductivities differ by several orders of magnitude is the oxidation state of iron in the olivine. Below 1100°C, activation energies less than 1 eV are associated with the presence of Fe3+, while energies greater than 1 eV occur for olivines with no detectable Fe3+.

Temperatures in the lunar interior and some implications

Data on the electrical conductivity of olivine and pyroxene obtained under redox conditions similar to those that exist in the moon indicate that the moon is at temperatures near the melting point at depths of 600–900 km. This temperature profile, combined with information on the distribution of radioactive elements and evidence of extensive differentiation of the moon, lead to the conclusion that the moon accreted at temperatures between 600–1000°C. This high accretion temperature can be reconciled with the presence of FeS and the probable FeO/MgO ratio in the lunar interior if the moon accreted from material which was depleted in H 2 relative to the solar nebula.

The influence of oxidation state on the electrical conductivity of olivine

The electrical conductivity of a single crystal of San Carlos olivine (Fo 92, 0.16 wt.% Fe 2O 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 earth's 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, internal temperatures and thermal evolution of the moon

The electrical conductivity of olivine and pyroxene is a strong function of the fugacity of oxygen in the atmosphere with which the mineral is in equilibrium. Lunar temperature profiles calculated from data on the electrical conductivity of these two minerals at oxygen fugacities similar to those which exist in the Moon indicate considerably higher temperatures for the lunar interior than obtained from conductivity data collected under normal atmospheric conditions. These high interior temperatures, the extensive differentiation associated with the formation of the lunar maria, and the radioactive element content of the Moon indicate that the Moon accreted at temperatures between 600 and 1000°C. Gravitational heating during accretion would lead to melting of at least the outer 200 km of the Moon and would produce conditions favourable to separation of a metal-sulfide melt sufficient to form a core of 200–300 km radius. Such a core would reach the center of the Moon within a few million years after accretion. This core could produce the remanent magnetization observed in the surface rocks. Dynamo action would cease with the cessation of convective motion within the core as the temperature of the surrounding mantle increased due to radioactive heating. With the radioactivity assumed in the present model and the high accretion temperature, this event would require less than 2 b.y., but more than 1.6 b.y.

Electrical conductivity of olivine

The electrical conductivity σ of single crystals of olivine of 0, 7.7, 8.2, 9.4, and 26.4 mole % fayalite has been measured up to 1200°C and 7.5 kb. Samples from different localities with approximately the same fayalite content and impurity levels have electrical conductivities that differ by 2–3 orders of magnitude. It is proposed that the oxidation state of the iron in the natural olivines determines σ below 1100°C. No reversible change in activation energy was noted for any sample below 1100°C. In this temperature range activation energies for different samples vary from 0.7 to 2.0 ev. At 1150°C there is a reversible increase in σ, with a poorly determined activation energy of 7–8 ev. Mantle temperatures calculated from the present data on olivines with almost the same fayalite content differ by more than 700°C at any depth. The temperature difference becomes even larger with depth if the pressure effects noted in this study are used.

Electrical conductivity of olivine and the lunar temperature profile

The measured electrical conductivity of olivine has been shown to depend strongly on thePo2 in which the sample has been equilibrated. At the present time, no definite limits can be placed on temperatures in the lunar interior assuming a model composition of olivine. It is certain that such limits will be at least several hundred degrees above the original estimates.

III. The electrical conductivity of the Moon - The electrical conductivity of the Moon’s interior

Analysis of the geomagnetic variations shows that in the Earth’s mantle at a depth of 700 km there is a rapid rise of electrical conductivity. Other considerations show that this rise flattens off in the lower mantle. The phenomenon is well explained by the semi-conduction of the silicate minerals. Experimental results on the electrical conductivity of olivine with tempera­ture and pressures up to those of the Moon’s centre will be described. Different models of the temperature distribution yield widely varying conductivity distributions.