Origin of High Electrical Conductivity in the Lower Continental Crust: A Review

Electromagnetic measurements have demonstrated that the lower continental crust has remarkable electrical anomalies of high conductivity and electrical anisotropy on a global scale (probably with some local exceptions), but their origin is a long-standing and controversial problem. Typical electrical properties of the lower continental crust include: (1) the electrical conductivity is usually 10⁻⁴ to 10⁻¹ S/m; (2) the overlying shallow crust and underlying upper mantle are in most cases less conductive; (3) the electrical conductivity is statistically much higher in Phanerozoic than in Precambrian areas; (4) horizontal anisotropy has been resolved in many areas; and (5) in some regions there appear to be correlations between high electrical conductivity and other physical properties such as seismic reflections. The explanation based on conduction by interconnected, highly conductive phases such as fluids, melts, or graphite films in grain boundary zones has various problems in accounting for geophysically resolved electrical conductivity and other chemical and physical properties of the lower crust. The lower continental crust is dominated by mafic granulites (in particular beneath stable regions), with nominally anhydrous clinopyroxene, orthopyroxene, and plagioclase as the main assemblages, and the prevailing temperatures are mostly 700–1,000°C as estimated from xenolith data, surface heat flow, and seismic imaging. Pyroxenes have significantly higher Fe content in the lower crust than in the upper mantle (peridotites), and plagioclase has higher Na content in the lower crust than in the shallow crust (granites). Minerals in the lower continental crust generally contain trace amounts of water as H-related point defects, from less than 100 to more than 1,000 ppm H₂O (by weight), with concentrations usually higher than those in the upper mantle. Observations of xenolith granulites captured by volcano-related eruptions indicate that the lower continental crust is characterized by alternating pyroxene-rich and plagioclase-rich layers. Experimental studies on typical lower crustal minerals have shown that their electrical conductivity can be significantly enhanced by the higher contents of Fe (for pyroxenes), Na (for plagioclase), and water (for all minerals) at thermodynamic conditions corresponding to the lower continental crust, e.g., to levels comparable to those measured by geophysical field surveys. Preferred orientation of hydrous plagioclase, e.g., due to ductile flow in the deep crust, and alternating mineral fabrics of pyroxene-rich and plagioclase-rich layers can lead to substantial anisotropy of electrical conductivity. Electrical conductivity properties in many regions of the lower continental crust, especially beneath stable areas, can mostly be accounted for by solid-state conduction due to the major constituents; other special, additional conduction mechanisms due to grain boundary phases are not strictly necessary.     

河北廊坊-天津大港剖面地壳上地幔电性结构特征

在河北廊坊-天津大港一线部署了110 km的MT勘探剖面,通过资料远参考与Robust估算处理及反演解释,对剖面的构造维数及构造方向做了分析;揭示了冀中坳陷、沧县隆起、黄骅坳陷三个不同构造单元及边界接触关系;对地壳、上地幔电性构造层进行了划分和分析。MT成果显示岩石圈体现为纵向分层、横向分块的特征;沧东及大城断裂是区内重要的深大断裂,控制了隆起两侧坳陷的沉积和形成;断裂深部对应上地幔高导层的局部隆起,两侧存在中下地壳高导层的错动。     

MoisturEC: A New R Program for Moisture Content Estimation from Electrical Conductivity Data

Noninvasive geophysical estimation of soil moisture has potential to improve understanding of flow in the unsaturated zone for problems involving agricultural management, aquifer recharge, and optimization of landfill design and operations. In principle, several geophysical techniques (e.g., electrical resistivity, electromagnetic induction, and nuclear magnetic resonance) offer insight into soil moisture, but data‐analysis tools are needed to “translate” geophysical results into estimates of soil moisture, consistent with (1) the uncertainty of this translation and (2) direct measurements of moisture. Although geostatistical frameworks exist for this purpose, straightforward and user‐friendly tools are required to fully capitalize on the potential of geophysical information for soil‐moisture estimation. Here, we present MoisturEC, a simple R program with a graphical user interface to convert measurements or images of electrical conductivity (EC) to soil moisture. Input includes EC values, point moisture estimates, and definition of either Archie parameters (based on experimental or literature values) or empirical data of moisture vs. EC. The program produces two‐ and three‐dimensional images of moisture based on available EC and direct measurements of moisture, interpolating between measurement locations using a Tikhonov regularization approach.     

Electrical Struture of the Crust in Manas Earthquake Area,Xinjiang,China

Magnetotelluric sounding data obtalned recently in Manas earthquake area were processed. Inthe result, curves of apparent resistivity, impedance Phase, skewness and optimum rotationangle versus period and the real magnetic induction vectors were obtained. Then the data ofall measuring points were interpreted by 2D automatic inversion. The result indicates thatalong the sounding profile the shallow crust can be divided into 5 segments and the deep crustcan be divided into 3 segments, with faults or deep-seated fault zones as the contactboundaries between them. The sedimentary cover along the profile extents down to depthabout 12 km in maximum and a low-resistivity body exists in the crust in southern section ofthe profile. The interpretation results are well consistent with geological and othergeophysical data. The Manas M7. 7 earthquake occurred near a contact zone where theelectrical structure of the crust sharply changes.     

On the Structures Observed in Thin Rotating Layers of a Conductive Fluid and the Anomalies of the Geomagnetic Field

The results of the laboratory and numerical experiments in circular rotating trays with thin layers of a conductive fluid under the MHD generation of small-scale velocity fields are presented. The configurations of constant magnets for MHD generation were determined based on the numerical calculations with shallow water equations. Both the laboratory and numerical experiments with rotating trays demonstrate the emergence of nonaxisymmetric structures and large-scale near-circular vortices caused by the energy transfer from the system of the externally generated small-scale vortices to the large-scale velocity fields under the action of the Coriolis force. The near-circular vortex has areas with differential rotation when the angular velocity of rotation decreases with the radius. The single large-scale vortices and wide jet flows arise in the regimes of subrotation and superrotation relative to the external rotation depending on its angular velocity. The emergence of the flow structures with the azimuthal wave number m = 2 is demonstrated, and their probable relation to the anomalies of the geomagnetic field observed on the Earth’s surface is considered.     

Generation of EMIC Waves in the Magnetosphere and Precipitation of Energetic Protons: Comparison of the Data from THEMIS High Earth Orbiting Satellites and POES Low Earth Orbiting Satellites

Regions of the detection of electromagnetic ion-cyclotron (EMIC) waves on the THEMIS satellites near the equatorial plane and the precipitation of energetic protons on POES low Earth orbiting satellites are compared with the magnetospheric magnetic field model. It is confirmed that low Earth orbiting satellites detect the precipitation of energetic protons in the regon associated with observations of EMIC waves in the magnetosphere. This is consistent with the idea that protons are scattered in the loss cone as a result of ioncyclotron interaction. Thus, observations of fluxes of energetic protons in low Earth orbits can be used to monitor ion-cyclotron instability regions in the magnetosphere. Simultaneous observations at high and low Earth orbits contribute to the construction of a spatiotemporal pattern of the interaction region of EMIC waves and energetic protons. In addition, it is shown that proton precipitation associated with EMIC waves can cause errors in determining the latitude of the isotropic boundary (the equatorial boundary of isotropic fluxes of energetic protons), which is an indicator of the configuration of the magnetic field in the magnetosphere.     

Deep Electrical Conductivity in the Vicinity of the Orsha Depression: 2D REBOCC Inversion of Synthetic and Observed Magnetotelluric Data

Izvestiya, Physics of the Solid Earth - Abstract—The geoelectric structure of the junction region between three largest segments of the East European Craton (EEC)–Volga–Uralia,...     

CHe in volatile fluxes from the solid Earth: implications for carbon geodynamics

In order to picture C geodynamics past and present, theC³He ratios of the relevant reservoirs are considered. Evaluation of publishedC³He ratio in conjunction with new results for MORB glasses worldwide, suggests that this ratio is unfractionated during magma outgassing, a best estimate being 2 × 10⁹.C³He ratios from other volcanic emissions (hot spots and arcs) do not appear significantly different when the subducted component is omitted.This result permits scaling of the CO₂ degassing flux to that of³He and yields a value of 2 × 10¹² mol/yr which corresponds to a model degassing duration of 3.9 Gyr when recycling to the mantle is disregarded.A bulk Earth chondritic ratio of about 2 × 10⁹ is calculated, very close to the MORB value. On the other hand the reconstructed exospheric (“Rubey inventory”) value of4 ± 1 × 10⁷ is very different from both basaltic and chondritic values.Among the possible interpretations of these results the following two are retained: (1) CO₂ was not released in the early age of the Earth because of the reducing conditions prevailing at that time in the mantle. Formation of the core changed this picture and permitted subsequent degassing of CO₂. (2) Carbonates need a continental crust of significant size to become stabilized in the exosphere. Therefore accumulation in the exosphere was delayed until crustal formation.Alternatively, a similar degassing behaviour for both He and CO₂ requires a massive recycling of carbonates throughout time. This possibility is in contradiction with the present-day maximum recycling rate and the severe imbalance with the observed outgassing flux on one hand and with the small fraction of carbon now present in the exosphere on the other.We conclude that carbon has never been severely degassed. The mantle acts as a buffer for C and most carbon is still retained there, possibly as graphite (or diamond?) or dissolved in minerals.     

An Integrated Study on Physical Properties of a KTB Gneiss Sample and Marble from Portugal: Pressure Dependence of the Permeability and Frequency Dependence of the Complex Electrical Impedance

— Pressure-induced variations in pore geometry were studied on dry- and fluid- saturated samples by means of electrical impedance spectroscopy and permeability measurements. Hydrostatic pressures (up to 120 MPa) and uniaxial pressures (up to failure) were applied. Hydrostatic pressures reduce the aspect ratio of cracks and thus cause a decrease of permeability and electrical bulk conductivity. The opposite was observed in uniaxial pressure experiments where new cracks were formed and consequently permeability and electrical conductivity were increased. More specific informations of these generated observations were derived from the interpretation of the frequency dispersion of the complex electrical conductivity. This least-squares-refinement considers electrochemical interactions between the fluid pore electrolyte and the inner surface of the sample, thus providing informations on the pore geometry and pressure-induced variations. Consequently changes in aspect ratio, size and geometry of the pore system can be detected by means of impedance spectroscopy.     

Deep Electrical Conductivity of the Archaean Blocks of Kola Peninsula in the Light of the Results of Murman-2018 Experiment: A Review

Izvestiya, Physics of the Solid Earth - Abstract—In this paper, we review the results of the deep electromagnetic soundings carried out on the Archaean blocks of the Kola Peninsula over the...     

Crust and upper mantle heterogeneities in the southwest Pacific from surface wave phase velocity analysis

Direct earthquake-to-station Rayleigh and Love wave data observed on high gain broadband records are analyzed in order to improve the lateral resolution of the uppermost mantle in the southwest Pacific region. We used data of nine permanent Geoscope and Iris stations located in the southern hemisphere and nine other stations as part of two temporary networks, the first one installed in New Caledonia and Vanuatu (hereafter named Cavascope network) by ORSTOM and the EOST from Louis Pasteur University in Strasbourg (France) and the second one installed in the Fiji, Tonga and Niue islands (hereafter named Spase network) by Washington University in St. Louis (USA). In order to collect more significant details on the surficial structures, we included the analysis of short period waves down to 8 s. A multiple frequency filtering technique has been used to recover phase velocities of Rayleigh and Love waves for selected earthquakes with magnitude greater than 5.5 and with known centroid moment tensor (CMT). About 1100 well-distributed seismograms have been processed in the period range 8–100 s and corrections for topography and water depth have been applied to the observed phase velocities. The geographical distribution of phase velocity anomalies have then been computed using the tomographic method developed by Montagner [Montagner, J.P., 1986a. Regional three-dimensional structures using long-period surface waves. Ann. Geophys. 4 (B3), 283–294]. Due to a poor knowledge of dense, well-distributed, crustal thickness values and corresponding velocity models, we did not perform or speculate on the construction of an S-wave 3D velocity model; therefore, we limited this study to the interpretation of the phase velocity distribution. The location of phase velocity anomalies are well determined and the deviations are discussed within the framework of the geological context and compared with other tomographic models. At long periods, from 40 s to 100 s, our results agree well with most of previous studies: the tomographic imaging shows a large contrast between low and high phase velocities along the Solomon, New Hebrides and Fiji–Tonga trenches. The lowest phase velocity anomalies are distributed beneath northern and southern Fiji basins and the Lau basin (corresponding to the volume situated just above the dipping slabs), whereas the highest values are displayed beneath the Pacific plate and the eastern part of Indian plate downgoing under the North Fiji basin. At shorter periods, our results show that the phase velocity distributions are well correlated with the large structural crustal domains. The use of local temporary broadband stations in the central part of the studied area gives us the opportunity to observe surface waves showing well-dispersed trains, allowing extended velocity measurements down to 8 s although aliasing due to multipaths become important. The continental regions (Eastern Australia, New Guinea, Fiji islands and New Zealand) show low velocities which are likely due to thick continental crust, whereas the Tasmanian, D'Entrecasteaux, and the Northern and Southern Fiji basins are characterized by higher velocities suggesting thinner oceanic crust. Additional analysis including the anisotropic case and S-wave velocity inversion with depth is in progress.     

中国华北地区壳内低速高导层（体）成因模式的探讨

依据高温高压下华北地区地壳主要岩石的物理性质－波速，电性测定的结果，提出了华北地区低速高导层可能的成因模式以及不同模式的适应范围。认为；碳酸盐岩在深部一定温度，压力和氧逸度条件下碳的析出会导致高导层体的出现；深部韧性剪切带组成矿物的定向排列，可使岩石的波速，电生产生各向异性行为，导致低速高导层的产生；绿片岩相和角闪岩相石中含水矿物的脱水作用会导致上，中地壳岩石物理力学性质的突变，这可能是该地区低速     

Scales of Heterogeneities in the Continental Crust and Upper Mantle

—A seismological characterization of crust and upper mantle can refer to large-scale averages of seismic velocities or to fluctuations of elastic parameters. Large is understood here relative to the wavelength used to probe the earth.¶In this paper we try to characterize crust and upper mantle by the fluctuations in media properties rather than by their average velocities. As such it becomes evident that different scales of heterogeneities prevail in different layers of crust and mantle. Although we cannot provide final models and an explanation of why these different scales exist, we believe that scales of inhomogeneities carry significant information regarding the tectonic processes that have affected the lower crust, the lithospheric and the sublithospheric upper mantle.¶We focus on four different types of small-scale inhomogeneities (1) the characteristics of the lower crust, (2) velocity fluctuations in the uppermost mantle, (3) scattering in the lowermost lithosphere and on (4) heterogeneities in the mantle transition zone.     

Permeability and pore-connectivity variation of pumices from a single pyroclastic flow eruption: Implications for partial fragmentation

The relationship between permeability and vesicularity in volcanic rocks has been used to infer the degassing behavior of hydrous magma. Recent data on natural samples from various eruptions show a wide variation, fitting a power–law relationship of the percolation models with low (< 30%) critical vesicularity (Ф_C). In this study, we present data on permeability and pore-connectivity of juvenile rhyolitic pumice clasts in a pyroclastic flow around Onikobe volcano, NE Japan, and investigate their relationship with vesicularity developed in a single eruption event. The permeability of the pumices having a relatively low abundance of microlites and microphenocrysts shows a trend increasing by 4 orders of magnitude (from 10^− 13.8 to 10^− 10.1 m²) in a high and narrow vesicularity range (from 72 to 80%). This trend intersects at a high angle with the fit to the permeability–vesicularity data in the previous studies that has a low Ф_C, and is located on the extension of the trend for the products of isotropic decompression experiments. The two-dimensional (2D) connectivities of pores for the pumices were also measured from thin sections. From the point of view of percolation theory, connectivity provides information about the probability of percolation. They showed a steep increase from ca. 0 to 0.7 in an almost similar vesicularity range, as compared to their permeabilities. We attribute the increase in 2D connectivity to the increasing amount of ruptured bubble walls, which might have provided less-tortuous paths through larger apertures for gas flow. This, in turn, would cause an effective increase in the permeability. Aggregates of bubble-wall-shaped glass shards were found in the pumices, and their amount and degree of welding are higher in the pumices that have a higher abundance of microlites and microphenocrysts. These pumices have relatively high permeability and 2D connectivity at low vesicularity, which is accounted for by the existence of large irregularly shaped pores. These textural characteristics suggest that a series of partial fragmentation processes, including local rupturing of bubble walls and subsequent foam-collapse with permeable gas flow, might have occurred before the ultimate bulk fragmentation, thus resulting in the increase in permeability. We suggest that the 2D connectivity of pores is a useful parameter to quantify the degree of fragmentation of bubble walls and has the potential for use to assess their permeability.     

地震共轭破裂及极限主应力随地壳深度的变化——以1975年海城7.3级地震为例

许多震例表明，地震可以形成共轭破裂，地震震源处，其温度和压力与地壳浅处及地表不同，温，压越高，共轭破裂角越大。随着深度的变浅（温，压减小），地震共轭破裂角减小，地震烈度共轭角也减小，本文用数学力学方法和岩石力学实验结果，探讨了海城地震共轭破裂及极限主应力随地壳深度的变化。     

Temperature and Pressure Dependencies of Thermal Transport Properties of Rocks: Implications for Uncertainties in Thermal Lithosphere Models and new Laboratory Measurements of High-Grade Rocks in the Central Fennoscandian Shield

Measurements on thermal conductivity and diffusivity as functions of temperature (up to 1150 K) and pressure (up to 1000 MPa) are presented for Archaean and Proterozoic mafic high-grade rocks metamorphosed in middle and lower crustal pressures, and situated in eastern Finland, central Fennoscandian Shield. Decrease of 12–20% in conductivity and 40–55% in diffusivity was recorded between room temperature and 1150 K, which can be considered as typical of phonon conductivity. Radiative heat transfer effects were not detected in these samples. Pressure dependencies of the samples are weak if compared to crystalline rocks in general, but relatively typical for mafic rocks.The temperature and pressure dependencies of thermal transport properties (data from literature and the present study) were applied in an uncertainty analysis of lithospheric conductive thermal modellings with random (Monte Carlo) simulations using a 4-layer model representative of shield lithosphere. Model parameters were varied according to predetermined probability functions and standard deviations were calculated for lithospheric temperature and heat flow density after 1500 independent simulations. The results suggest that the variations (uncertainties) in calculated temperature and heat flow density values due to variations in the temperature and pressure dependencies of conductivity are minor in comparison to the effects produced by typical variations in the room temperature value of conductivity, heat production rate or lower boundary condition values.     

第四届海峡两岸地球科学夏令营纪要

经国务院台湾事务办公室和中国科学技术协会批准,由中国地震学会和台湾中央大学地球科学系共同组织的“第四届海峡两岸地球科学夏令营”（以下简称“夏令营”）,于2005年7月25日至8月21日分别在大陆和台湾地区举办。本届“夏令营”从时间上分两期举行。     

A Dislocation Model of Seismic Wave Attenuation and Micro-creep in the Earth: Harold Jeffreys and the Rheology of the Solid Earth

—A microphysical model of seismic wave attenuation is developed to provide a physical basis to interpret temperature and frequency dependence of seismic wave attenuation. The model is based on the dynamics of dislocation motion in minerals with a high Peierls stress. It is proposed that most of seismic wave attenuation occurs through the migration of geometrical kinks (micro-glide) and/or nucleation/migration of an isolated pair of kinks (Bordoni peak), whereas the long-term plastic deformation involves the continuing nucleation and migration of kinks (macro-glide). Kink migration is much easier than kink nucleation, and this provides a natural explanation for the vast difference in dislocation mobility between seismic and geological time scales. The frequency and temperature dependences of attenuation depend on the geometry and dynamics of dislocation motion both of which affect the distribution of relaxation times. The distribution of relaxation times is largely controlled by the distribution in distance between pinning points of dislocations, L, and the observed frequency dependence of Q, Q, Q∝ω^α is shown to require a distribution function of P(L)∝L ^-m with m=4-2α The activation energy of Q ^?1 in minerals with a high Peierls stress corresponds to that for kink nucleation and is similar to that of long-term creep. The observed large lateral variation in Q ^?1 strongly suggests that the Q ^?1 in the mantle is frequency dependent. Micro-deformation with high dislocation mobility will (temporarily) cease when all the geometrical kinks are exhausted. For a typical dislocation density of ～ 10⁸ m^?2, transient creep with small viscosity related to seismic wave attenuation will persist up to the strain of ～ 10^?6, thus even a small strain (～ 10^?6?10^?4) process such as post-glacial rebound is only marginally affected by this type of anelastic relaxation. At longer time scales continuing nucleation of kinks becomes important and enables indefinitely large strain, steady-state creep, causing viscous behavior.     

Seismic Velocity and Q Structure of the Middle Eastern Crust and Upper Mantle from Surface-wave Dispersion and Attenuation

—Observed velocities and attenuation of fundamental-mode Rayleigh waves in the period range 7–82 sec were inverted for shear-wave velocity and shear-wave Q structure in the Middle East using a two-station method. Additional information on Q structure variation within each region was obtained by studying amplitude spectra of fundamental-mode and higher-mode Rayleigh waves. We obtained models for the Turkish and Iranian Plateaus (Region 1), areas surrounding and including the Black and Caspian Seas (Region 2), and the Arabian Peninsula (Region 3). The effect of continent-ocean boundaries and mixed paths in Region 2 may lead to unrealistic features in the models obtained there. At lower crustal and upper-mantle depths, shear velocities are similar in all three regions. Shear velocities vary significantly in the uppermost 10 km of the crust, being 3.21, 2.85, and 3.39 km/s for Regions 1, 2, and 3, respectively. Q models obtained from an inversion of interstation attenuation data show that crustal shear-wave Q is highest in Region 3 and lowest in Region 1. Q’s for the upper 10 km of the crust are 63, 71, and 201 for Regions 1, 2, and 3, respectively. Crustal Q’s at 30 km depth for the three regions are about 51, 71, and 134. The lower crustal Q values contrast sharply with results from stable continental regions where shear-wave Q may reach one thousand or more. These low values may indicate that fluids reside in faults, cracks, and permeable rock at lower crustal, as well as upper crustal depths due to convergence and intense deformation at all depths in the Middle Eastern crust.     

Nanoscience and technology: the next revolution in the Earth sciences

Nanoscience and technology are not exactly new, but nevertheless rapidly expanding fields that are providing revolutions in all sciences on the scale of what genomics and proteomics have done in recent years for the biological sciences. Nanoscience is based on the fact that properties of materials change as a function of the physical dimension of that material, and nanotechnology takes advantage of this by applying selected property modifications of this nature to some beneficial endeavor. The prefix ‘nano’ is used because the property dependence on physical size is generally observed close to the nanoscale, somewhere around 10⁻⁹ m. The dimensions at which changes are observed depend on the specific material and the property in question, as well as which of the three dimensions are restricted in real space (e.g. small particles vs. thin films vs. ‘one-dimensional’ phases). Properties change in these confined spaces because the electronic structure (i.e. the distribution of electron energies) of the material is modified here in the gray area between the bulk and atomistic/molecular realms, or equivalently between the continuum and strictly quantum domains. Earth materials with at least one dimension in the nanorange are essentially ubiquitous. Many have been known for several decades and more are being discovered all the time. But the scientific emphasis has now shifted to that of measuring, understanding and ultimately predicting the property changes from the bulk to nanodomains, and to the understanding of the significant ways that Earth processes are affected by these changes. In addition, where possible, Earth scientists are using nanoscience to develop nanotechnology that should play important roles in Earth sustainability issues of the future.