Some new insights about electrical discontinuities in the cratonic lithosphere

Abstract

Craton is an important geological unit of the continental lithosphere, which can provide significant information about the Earth's geological history, especially, composition, structure and evolution of continental lithosphere. Magnetotellurics (MT) is a useful tool to image the composition and structure of continental lithosphere because electrical conductivity is sensitive to temperature, bulk composition, volatile (H2O or CO2) and/or melt/fluid content, and the presence of highly conductive minerals. In this paper, we reviewed the conductivity features and electrical discontinuities revealed by MT surveys in the worldwide cratonic lithospheres. By integrating various geophysical and petrological observations including surface heat flow, bulk composition, thermal conductivity, electrical conductivity, and xenolith data, the thermal structure and conductivity-depth profiles were constructed in the continental lithosphere. The plausibility of water in nominally anhydrous minerals (NAMs), silicate melt, and carbonate melt to account for the high conductivity anomalies observed in the cratonic lithosphere was evaluated based on the geothermal structure and composition within the cratonic lithosphere, and laboratory measurements of mineral physics. Our modelled results clearly indicate that the thermal structure of the cratonic lithosphere acts as a primary control on its internal melting state and electrical conductivity and thereby its dynamic evolution. When the craton has anomalous high surface heat flow (for example the North China Craton), the origin of the enhanced conductivity may be explained by the presence of silicate melt or carbonate melt, rather than water in NAMs. However, because of low temperature, silicate melt can be ruled out as a candidate for cratons with relatively low surface heat flow. Instead, either a small amount of carbonate melt or water in NAMs may explain the high conductivity anomalies observed in the deep part of the cratonic lithosphere. Although metasomatism products (i.e., phlogopite or amphiboles) or sulfides may produce conductivity anomalies as well at the shallow lithosphere, they require the formation of interconnected pathways over large scales, which rarely occurs in the deep cratonic lithosphere.