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首页> 外文期刊>Surveys in Geophysics: An International Review Journal of Geophysics and Planetary Sciences >Origin of High Electrical Conductivity in the Lower Continental Crust: A Review
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Origin of High Electrical Conductivity in the Lower Continental Crust: A Review

机译：大陆下部地壳高电导率的起源：一个综述

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摘要

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 H_2O (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.
机译：电磁测量表明，下大陆壳在全球范围内具有高电导率和电各向异性的异常电异常（可能有一些局部例外），但是它们的起源是一个长期存在且有争议的问题。下大陆壳的典型电学特性包括：（1）电导率通常为10〜（-4）至10〜（-1）S / m。 （2）在大多数情况下，上覆的浅层地壳和下覆的上地幔导电性较差; （3）据统计，生代时代的电导率比前寒武纪地区的电导率高得多; （4）在许多地区解决了水平各向异性问题; （5）在某些地区，高电导率与其他物理特性（如地震反射）之间似乎存在相关性。基于相互交联的高导电相（例如，晶界区域中的流体，熔体或石墨膜）的导电的解释在解决地球物理解析的电导率以及下地壳的其他化学和物理特性方面存在各种问题。下部大陆壳以镁铁质花岗岩（尤其是在稳定区域以下）为主，名义上无水的斜py，邻苯二酚和斜长石为主要成分，根据异岩体数据，表面热量估算，盛行的温度大多为700-1,000°C。流动和地震成像。辉石在下地壳中的铁含量明显高于上地幔（橄榄岩），斜长石在下地壳中的Na含量高于浅地壳（花岗岩）。下部大陆壳中的矿物质通常含有痕量的水（与H有关的点缺陷），H_2O含量低于100 ppm至1,000 ppm以上（按重量计），浓度通常高于上地幔中的含量。火山相关喷发捕获的异岩砾石的观察表明，下部大陆壳的特征是富辉石层和斜长石层交替发生。对典型的下地壳矿物的实验研究表明，在热力学条件下，对应于下部大陆壳的铁（辉石），钠（斜长石）和水（所有矿物）的含量较高，可以显着提高其电导率。 ，例如，水平可与地球物理实地调查测得的水平相比。含水斜长石的优选取向，例如由于在深地壳中的延性流动以及富含辉石的层和富含斜长石的层的交替矿物织物，可导致电导率的显着各向异性。在下部大陆壳的许多区域，特别是在稳定区域之下的电导率特性，主要是由固态导电引起的，这是主要成分。由于晶界相的原因，其他特殊的，额外的传导机制不是严格必需的。

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