Geophysical Journal | 2000 volume 22 2

Electrical properties of the Ukrainian Shield gneisses in high thermobaric conditions

© Lebedev N.S., Shepel S.J.

Thermobaric changes of the electrical characteristics of collections of various gneisses of the Central Ukrainian Precambrian Shield are studied. Experiments were made on dry air - dried and fluid - saturated samples at different (including high ones) temperature () and pressure (P). An analysis of mean values of electrical parameters shows that least resistant are pyroxene - biotite gneisses. At the same time, they show the highest dielectric constant (e) values. The discovered regularity is due to the fact that the pyroxene - biotite gneisses are intensively effected by secondary transformations. As a result, we observe serpentinization of plagioclase, development of chlorites and amphiboles among dark-colored minerals. Such processes cause the appearance of additional defective domains in crystalline lattices of mineral matter. It decreases the activation energy of the current - carriers (E) and the specific electrical resistance (r). We see r decrease for gneisses when proceding from dry to water - saturated state the former being more intensive for highly resistant varieties. With mineralization growth from 0,1 to 100 g/l, r of gneisses decreases on average by ca. 1,5 orders. In the same range of concentrations of salts saturating the solutions, e grows more intensively in the lower frequency range. Rather clear relation between the porosity and electrical parameters of gneisses has been discovered. It is shown that r of water - saturated gneisses decreases with increasing porosity. At the same time, their e increases. Besides, the differences in absolute r and e values of gneisses of potentially gold - productive and unproductive masses of this region are preserved even at high temperature. The studies of the character of the chang of electrical resistance of dry, air-dried and water-saturated gneisses at a simultaneous effect of high quasi - hydrostatic pressure and temperature have revealed its notable differences for rocks with different moisture content. The quasi-hydrostatic pressure increase notably lowers E and temperature of the transition from the extrinsic and in trinsic conductivity. On the basis of the system analysis of the experimental data obtained and the information of P and distribution in the Earth's crust of the Central Ukrainian Shield and possible depth of the occurrence of gneisses, prognostic values of their specific electrical resistance have been determined to 15 km depth.

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