caa a22 4500 445814292 CHVBK 20180317145221.0 cr unu---uuuuu 170323e20111101xx s 000 0 eng 10.1007/s10712-011-9145-z doi (NATIONALLICENCE)springer-10.1007/s10712-011-9145-z Yang Xiaozhi Bayerisches Geoinstitut, Universität Bayreuth, 95440, Bayreuth, Germany aut Origin of High Electrical Conductivity in the Lower Continental Crust: A Review [Elektronische Daten] [Xiaozhi Yang] 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 usually10−4 to 10−1S/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 1,000ppm 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. Springer Science+Business Media B.V., 2011 Electrical conductivity nationallicence Electrical anisotropy nationallicence Lower continental crust nationallicence Conduction mechanism nationallicence Granulite-facies assemblages nationallicence Grain boundary secondary phases nationallicence Solid-state conduction nationallicence Surveys in Geophysics Springer Netherlands 32/6(2011-11-01), 875-903 0169-3298 32:6<875 2011 32 10712 https://doi.org/10.1007/s10712-011-9145-z text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s10712-011-9145-z text/html Onlinezugriff via DOI NATIONALLICENCE 100 1- Yang Xiaozhi Bayerisches Geoinstitut, Universität Bayreuth, 95440, Bayreuth, Germany aut NATIONALLICENCE 773 0- Surveys in Geophysics Springer Netherlands 32/6(2011-11-01), 875-903 0169-3298 32:6<875 2011 32 10712 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer