Abstract
The deepest electric conductivity increase determined by electromagnetic induction methods can be attributed to the conductivity increase observed in the laboratory during rock phase transition. It can be called the Ultimate Conducting Increase (or UCL), since no further increase of conductivity has yet been detected at greater depths. A positive dP/dT gradient is characteristic of rock phase transition, where P is the pressure in BARS, T the temperature in °C.
The aim of the research reported here was to look for an indication of a positive dP/dT gradient in the data set available, i. e. whether greater depths of the UCL (or greater pressures) correspond to greater temperatures at the depth of the phase transition. The surface heat flow (q in HFU), the characteristic part of which is coming from the upper mantle, can be used as an indicator of the temperature. Since in the upper mantle of the territories with lowest heat flow (q<1HFU) there is no partial melting, it can be assumed that the depth of 250-300km of the conductivity increase determined on platforms and crystalline shields corresponds to the depth of the rock phase transition. Using these data, which are very critical for our interpretation, an empirical formula was calculated by least squares fit between the depth of the conductivity increase HUCL (km) and the surface heat flow q (HFU): HUCL=16.3+292.5q. This formula indicates a positive dP/dT gradient. On the basis of the data set, the average HUCL is 420km and the average heat flow 1.38HFU. From this formula, taking dP/dT=30BARS/°C, it can be concluded that the temperature at a depth of 300-400km below the platform areas is about 1, 000°C less than the average.
