高温高压下碳酸盐熔体对地幔岩电导率的影响

为了观测含碳酸盐地幔岩部分熔融过程中电导率的变化,厘清碳酸盐熔体在金伯利岩岩浆形成过程中所起的作用,并探讨Slave克拉通中部Lac de Gras地区约80～120km深处的高导成因,我们利用DS 3600t六面顶压机和Solartron 1260阻抗/增益-相位分析仪在1.0～3.0GPa、673～1873K温压条件下分别测量了含碳酸钠(Na_2CO_3)、碳酸钙(CaCO_3)和大洋中脊玄武岩(MORB)的地幔岩样品的电导率.实验结果表明,地幔岩样品的电导率主要受到温度和组分的影响,而压力对其影响较小.在温度低于1023K时,含Na_2CO_3地幔岩样品的电导率明显高于含同比重CaCO_3和MORB的;温度达到1023K时,含Na_2CO_3地幔岩样品开始熔融;但在之后的200K温度区间内,该部分熔融样品的电导率随温度的增加几乎不发生变化.这一现象或许揭示:地幔深部的碳酸质岩浆在快速上升过程中会同化吸收岩石圈地幔中的斜方辉石(Opx),进而形成金伯利岩岩浆,期间岩浆的电导率几乎不发生变化.含CaCO_3和MORB的地幔岩样品分别在1723K和1423K开始熔融,其部分熔融样品的电导率随温度的增加而快速增加.依据前人的研究结果和我们的实验结果,我们认为可以用含碳酸盐的部分熔融样品来解释Slave克拉通中部Lac de Gras地区约80～120km深处的异常高导现象,并推测熔体中碳酸盐的熔体比例小于2wt.%.     

基于电性结构模型的青藏高原东缘上地幔热结构研究

研究青藏高原东缘地区的深部物质结构对于理解青藏高原的隆升及扩张机制具有重要的科学意义.本文将青藏高原东缘实测大地电磁测深剖面反演所得的岩石圈电性结构模型与高温高压岩石物理实验测得的上地幔矿物和熔融体导电性定量关系相结合,通过Hashin-Shtrikman(HS)边界条件建立上地幔电导率与温度、熔融百分比等参数的定量关系,在此基础上计算得到了青藏高原东缘上地幔热结构及熔融百分比分布模型.研究结果表明在青藏高原东缘地区通过大地电磁测深方法所探测到的上地幔低阻体可以解释为由高温作用所产生的局部熔融区域.其中,松潘—甘孜地块上地幔高导体对应的温度介于1300～1500℃之间,熔融百分比可高达10%,支持前人将松潘—甘孜地块内部的低阻体解释为局部熔融的观点.龙门山断裂带以东、四川盆地西缘的上地幔高导体温度介于1200～1400℃之间,熔融百分比介于1%～5%左右,表明扬子克拉通的西缘可能正在经历一定程度的活化作用.龙门山断裂带下方的上地幔高阻体温度介于1100℃附近,基本没有发生局部熔融,具有较冷的刚性块体特征,与该区域频发的地震活动相吻合.四川盆地东部的扬子上地幔温度介于800～900℃之间,没有发生局部熔融,符合古老稳定的克拉通块体的基本特征.     

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

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.     

Analyzing electromagnetic induction data: Suggestions from laboratory measurements

Recent inversions of electrical profiles of the upper mantle beneath the oceans permit a variety of conductivity-depth profiles ranging from models with monotonically increasing conductivity to layered models having decreases of conductivity with depth. Laboratory data on possible mantle materials can physically explain high mantle conductivities in terms of a fluid phase (partial melt, hydrous fluid) or a good solid conductor (amorphous or graphitic carbon) and favor a profile having a high conductivity layer (HCL) underlain by a more resistive layer.     

Electrical conductivity of a spinel lherzolite and a garnet peridotite to 1550°C: relevance to the effects of partial melting

Electrical conductivity σ of two ultramafic rocks (a spinel lherzolite and a garnet peridotite) has been investigated to melting temperature at 1 bar under known oxygen fugacity environment. The electrical conductivity of the two rocks is found to increase with degree of partial melting and an ～ 15% melt fraction is necessary for the electrical conductivity to increase by ～ 1 order of magnitude. For a given melt fraction electrical conductivity of a spinel lherzolite is lower than that of a garnet peridotite and may be attributed to the differences in the composition of the melts formed.     

碳酸盐化橄榄岩的电性研究

为进一步探讨上地幔的高导层成因,了解碳酸盐在上地幔电性方面的作用并估算上地幔高导层的碳酸盐含量,本文对不同碳酸盐含量的橄榄岩及玄武岩样品在2~3 GPa、300~1300℃的条件下进行了电性实验研究.研究初步发现:碳酸盐熔体显著增强橄榄岩、玄武岩样品的导电能力;单纯用含硅酸盐熔体的橄榄岩或单纯用含水橄榄岩可能难以解释上地幔某些区域的异常高导现象;同样,单纯用碳酸盐化的橄榄岩可能也难以解释上地幔某些区域的高导现象;上地幔的高导区很可能是碳酸盐熔体、硅酸盐熔体及水的共存区域.     

Structure-sensitive physical properties of partially melted rocks

Structure-sensitive physical properties (or generalized conductivity) such as electrical conductivity, heat conduction, viscosity and elastic properties significantly change on partial melting. Though the connectivity of a melts is of decisive importance, these properties are also considerably affected by the size, surface properties, shape and orientation of inclusions. Lateral discretness of the asthenosphere can be explained as being a result of inversion in melt distribution pattern, a transition from isolated inclusions and films to interconnected films, which causes drastic changes of the generalized conductivity.     

计算部分熔融岩石电导率方法的综述

部分熔融岩石的电导率是由固体岩石的低电导率和硅酸盐熔体的高电导率组成，因此熔体的几何形态和空间分布对部分熔融岩石的电导率将产生重大影响.本文介绍了熔体在岩石中的分布特征和它们的几何分布形态，同时，评述了各种数学模型在模拟计算部分熔融岩石电导率中的优缺点.     

A modified Archie’s law for two conducting phases

Many types of mixing model are used widely within the earth sciences to determine the electrical properties of porous media consisting of solid and fluid phases of known conductivities, volume fractions and distributions (i.e. phase connectivities). Most models are valid for two or more conducting phases. However, of the simple models only Archie’s law includes a variable term, the Archie cementation exponent m, that describes the connectivity of the phases. Unfortunately, Archie’s law is only valid for one conducting phase distributed within a non-conducting phase, which makes it inapplicable in instances where the rock matrix has a significant conductivity such as for clay-rich rocks and in calculations involving partial melting. More complex models exist which account for multiple conducting phases and control over phase conductivity. We have adapted the conventional Archie’s law to produce a simple modified Archie’s law that can be used with two conducting phases of any conductivity and any volume fraction, while retaining the ability to model variable connectivities within those phases that result from variations in their distribution. The modified model has two exponents (m and p) that describe the connectivity of each of the two phases. The exponents are related through an equation that depends also on the volume fractions of the two phases. The modified and the conventional versions of Archie’s law have been tested on a granular analogue porous medium with a conducting matrix and a pore space saturated with a range of saline fluids with different salinities and conductivities. The new model describes the experimentally determined electrical behaviour of the system extremely well, improving greatly on the conventional Archie’s law.     

Relations between electrical conductivity and petrological parameters of the crust and upper mantle

Laboratory data of the electrical conductivity of rocks and minerals pertinent to the deeper crust and upper mantle and summarized. They are discussed in the context of a theory to calculate effective conductivities of materials in the state of partial melt. Most published data have been obtained by too rapid measurements, i.e. without reaching an equilibrium state of the sample. Conductivity measurements on a material similar to the composition of pyrolite are not het known, their importance is outlined. A global conductivity distribution obtained by electromagnetic induction studies is represented by a few results covering oceanic and continental areas. Till today it seems to be a doubtful venture to deduce the temperature of the upper mantle or even the existence of a partial molten asthenosphere from a global conductivity distribution. On a more local scale the correlation of electrical conductivity with temperature and state of the material seems to be more realistic. This is tentatively shown by two petrological models of the Afar depression in Ethiopia and of the midoceanic rift.     

Correlations between electrical and elastic properties of solid–liquid composites with interfacial energy-controlled equilibrium microstructures

Electrical conductivity and seismic velocity are studied for plausible pore geometries in the Earth's interior for reliable quantitative analysis of experimental data such as seismic tomography and magnetotelluric explorations. Electrical conductivity of a two-phase system with equilibrium, interfacial energy-controlled phase geometry is calculated for the dihedral angles θ = 40°–100° that are typical for rock–aqueous fluid and θ = 20°–60° for rock–melt systems of lower crust and upper mantle for the case of tetrakaidecahedral grains. Electrical conductivity vs. seismic velocity correlations are acquired by combining of the simulated electrical conductivities with the seismic velocity calculated with the help of equilibrium geometry model Takei [Takei, Y., Effect of pore geometry on V_P/V_S: From equilibrium geometry to crack. J. Geophys. Res. 107 (2002): 10.1029/2001JB000522.] for the same pore geometries. The results show that electrical conductivity gradually decreases reaching zero when seismic velocities reach seismic velocities of intact rock for rock–melt systems, while for rock–aqueous fluid systems with θ ≥ 60° conductivity drops to zero at velocities up to 10% smaller. This can explain the seeming discrepancy of the low seismic velocity region, attributed to the high fluid fraction, and the low electrical conductivity of the same region, which is sometimes faced at collocated electromagnetic and seismic experiments.     

Partial melt in the asthenosphere: Evidence from electrical conductivity data

Modelled conductivity variations with depth in the upper mantle obtained from geomagnetic induction and magnetotelluric results are compared to predicted conductivity variations obtained from laboratory measurements on dry material assumed to occur in the mantle. Higher than predicted conductivities at the base of the oceanic lithosphere suggest the presence of highly conductive partial melt. Using a known relationship between observed conductivity and the conductivity and volume fraction of the fluid, estimates of the melt volume fraction have been made assuming the melt to be in continuously connected network. In the sub-oceanic asthenosphere these values range from approximately 0.45 to 9%, whereas in sub-continental asthenosphere the partial melt volume fraction appears to be too low to increase the bulk conductivity. The partial melt content of the sub-continental asthenosphere may reach a few percent if the melt exists in isolated pockets. The apparent difference in melt content in sub-oceanic and sub-continental asthenosphere is discussed in terms of the different velocities of Pacific-type plates (which carry no continental block) and Atlantic-type plates (which carry a continental block).     

氧化镍、二氧化钛高压相变及电导率压力效应

采用金刚石对顶砧压腔静高压技术，对氧化镍、二氧化钛在０-３２ＧＰａ压力范围内的电导率进行了测量．结果表明，电导率－压力关系曲线上的一些特征点与同一物质的高压相交点具有一致性．     

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 Fe²⁺ to Fe³⁺. 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 highΣM²/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 ΣM²⁺/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 ΣM²⁺/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.     

Effective conductivity of periodic media with cuboid inclusions

This paper presents a numerical solution for the effective conductivity of a periodic binary medium with cuboid inclusions located on an octahedral lattice. The problem is defined by five dimensionless geometric parameters and one dimensionless conductivity contrast parameter. The effective conductivity is determined by considering the flow through the “elementary flow domain” (EFD), which is an octant of the unitary domain of the periodic media. We derive practical bounds of interest for the six-dimensional parameter space of the EFD and numerically compute solutions at regular intervals throughout the entire bounded parameter space. A continuous solution of the effective conductivity within the limits of the simulated parameter space is then obtained via interpolation of the numerical results. Comparison to effective conductivities derived for random heterogeneous media demonstrate similarities and differences in the behavior of the effective conductivity in regular periodic (low entropy) vs. random (high entropy) media. The results define the low entropy bounds of effective conductivity in natural media, which is neither completely random nor completely periodic, over a large range of structural geometries. For aniso-probable inclusion spacing, the absolute bounds of K_eff for isotropic inclusions are the Wiener bounds, not the Hashin-Shtrikman bounds. For isotropic inclusion and isoprobable conditions well below the percolation threshold, the results are in agreement with the self-consistent approach. For anisotropic cuboid inclusions, or at relatively close spacing in at least one direction (p > 0.2) (aniso-probable conditions), the effective conductivity of the periodic media is significantly different from that found in anisotropic random binary or Gaussian media.     

Electrical conductivities of two granite samples in southern Tibet and their geophysical implications

There are clear differences in the electrical conductivities of the crustal granites of the Qinghai-Tibet Plateau.Because these granites are among the major rock types on the Qinghai-Tibet Plateau, it is very important to detect the electrical conductivity of granites under high temperatures and pressures to study the electrical conductivity structure of this area. Using impedance spectroscopy at a frequency range of 10.1–106 Hz, the electrical conductivity of the muscovite-granite collected from Yadong was investigated at a confining pressure of 1.0 GPa and temperatures ranging from 577 to 996 K, while the electrical conductivity of the biotite-granite collected from Lhasa was investigated at a pressure of 1.0 GPa and temperatures ranging from587 to 1382 K. The calculated activation enthalpies of the Yadong muscovite-granite sample is 0.92 eV in the low-temperature range(577–919 K) and 2.16 eV in the high-temperature range(919–996 K). The activation enthalpies of the Lhasa biotite-granite sample is 0.48 eV in the low-temperature range(587–990 K) and 2.06 eV in the high-temperature range(990–1382 K). The change in the activation enthalpies of the granites at different temperature ranges may be associated with the dehydration of the two samples. The electrical conductivities of the granite samples obtained in the laboratory using impedance spectroscopy correspond well with field observations conducted near the sampling points, both in terms of the actual conductivity values and the observed variations between the low-temperature and high-temperature regimes. This correlation of laboratory and field conductivities indicates that the conductivities of the crustal rocks in the two regions closely correspond to granite conductivities.We calculated the electrical conductivities of muscovite-granite and biotite-granite samples using the effective medium and HS boundary models. When applied to the crustal rocks of southern Tibet, the results of the geophysical conductivity profiles lie within the range of laboratory data. Thus, the electrical characteristics of the crustal rocks underlying the southern Qinghai-Tibet Plateau can largely be attributed to granites, with the large changes to high conductivities at increasing depths resulting from the dehydration of crustal rocks with granitic compositions.     

Low-degree partial melting trends recorded in upper mantle minerals

The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Minerals in a spinel lherzolite nodule from Grande Comore island contain glass inclusions which, after homogenization by heating, exhibit a continuous suite of chemical compositions clearly distinct from that of the host basanitic lava. The compositions range from silicic, with nepheline–olivine normative, 64 wt% SiO₂ and 11 wt% alkali oxides, to almost basaltic, with quartz normative, 50 wt% SiO₂ and 1–2 wt% alkali oxides. Within a single mineral phase, olivine, the inferred primary melt composition varies from 54 to 64 wt% SiO₂ for MgO content ranging from 8 to 0.8 wt%. An experimental study of the glass and fluid inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at moderate temperature and pressure (T≈1230°C and P≈1.0 Gpa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr₂O₃ and poorer in SiO₂, K₂O, Na₂O, Al₂O₃ and Cl with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts, and indicate that primary melts with SiO₂ >60 wt%, alkali oxides >11%, FeO <1 wt% and MgO <1 wt% are generated during incipient melting of spinel peridotite. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO₂ and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO₂ and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated.     

Experimental studies of electrical conductivities and P-wave velocities of gabbro at high pressures and high temperatures

The studies on the physical properties of minerals and rocks in combination with the work in petrology, mineralogy and geochemistry are not only a useful mean to look into the composition and structure of the earth抯 interior, but also can provide extreme…     

Effect of high-pressures on the electrical resistivity of natural zeolites from Deccan Trap, Maharashtra, India

We report here the electrical resistivity measurements on two natural zeolites–natrolite and scolecite (from the Killari borehole, Maharashtra, India) as a function of pressure up to 8 GPa at room temperature. High-pressure electrical resistivity studies on hydrous alumino-silicate minerals are very helpful in understanding the role of water in deep crustal conductivities obtained from geophysical models. The results obtained by magneto-telluric (MT) soundings and direct current resistivity surveys, along with the laboratory data on the electrical resistivity of minerals and rocks at high-pressure–temperature are used to determine the electrical conductivity distribution in continental lithosphere. The electrical resistivity of natural natrolite decreases continuously from 2.9 × 10⁹ Ω cm at ambient condition to 7.64 × 10² Ω cm at 8 GPa, at room temperature. There is no pressure-induced first order structural phase transitions in natrolite, when it is compressed in non-penetrating pressure transmitting medium up to 8 GPa. On the other hand scolecite exhibits a pressure-induced transition, with a discontinuous decrease of the electrical resistivity from 2.6 × 10⁶ to 4.79 × 10⁵ Ω cm at 4.2 to 4.3 GPa. The observed phase transition in scolecite is found to be irreversible. Vibrational spectroscopic and X-ray diffraction studies confirm the amorphous nature of the high-pressure phase. The results of the present high-pressure studies on scolecite are in good agreement with the high-pressure Raman spectroscopic data on scolecite. The thermo gravimetric studies on the pressure-quenched samples show that the samples underwent a pressure-induced partial dehydration. Such a pressure-induced partial dehydration, which has been observed in natural scolecite could explain the presence of high conductive layers in the earth's deep-crust.     

Numerical models on the influence of partial melt on elastic,anelastic and electric properties of rocks. Part I: elasticity and anelasticity

With the aim of a simultaneous interpretation of elastic, anelastic and electric in situ data from the asthenosphere a comprehensive set of numerical models is developed for partial melt in different geometrical configurations. For the elastic and anelastic modulus use is made throughout of the melt squirt mechanism. Frequency dependence is not treated in detail but estimated from the limiting cases of the relaxed and unrelaxed modulus. This has the advantage that quantitative values of viscocity and flow path dimensions are not required. In the models melt can be assumed to occur in the form of tubes, films, and triaxial ellipsoidal inclusions of arbitrary aspect ratio. The conditions in which the solutions for triaxial ellipsoidal inclusions can be approximated by simpler ones for spheroidal inclusions are discussed. It is then shown up to which aspect ratio a published model on melt films is applicable. The problem of interconnection of inclusions is treated with a statistical numerical approach. It is found that a reduced degree of interconnection may have a significant influence on anelastic relaxation at melt fractions corresponding to a moderate modulus decrease. A useful representation of the anelastic melt models is introduced by plotting the relaxation strength against the effective modulus, both of which depend on the state of melting. Such diagrams allow a clear distinction between the different melt geometries and may be used for the interpretation of observed data. Finally, different melt geometries are superimposed and it is found that under certain conditions bulk dissipation may reach the order of that for shear.