Induced polarization of volcanic rocks. Part 7. Kimberlites

Abstract

SUMMARY In the field, kimberlites are characterized by high electrical conductivities (about 0.1 S m−1) compared to most igneous rocks. The reason for these high conductivities has not been fully elucidated to date. We investigate here the spectral induced polarization of seven core samples of kimberlites in the frequency range 1.43 mHz–20 kHz. The measurements are made at pore water conductivities ranging from 0.07 to 2.4 S m−1 (NaCl, 25 °C). We also measured the cation exchange capacity (CEC), the specific surface area (SSA) and the magnetic susceptibility of the core samples. We characterized the samples by optical microscopy as well as the X-ray diffraction and thermogravimetric analyses. Based on the electrical measurements, we obtained values of the surface conductivity produced by the double electrical layer coating the solid particles, and the normalized chargeability values characterizing the polarization magnitude of these materials. Mineralogical analyses show significant amount of magnetite (from 2 to 9 wt. per cent, approximately 1 to 4 per cent in vol. content) and smectite (from 1 to 44 wt. per cent) in the core samples. The main contributor of the CEC is smectite because of its very high CEC. The quadrature conductivity, the normalized chargeability, and the surface conductivity are controlled by the CEC normalized by the tortuosity of the pore space (product of the formation factor by the porosity). Our data demonstrate that the conduction and polarization of kimberlites are both controlled by the presence of smectite rather than associated with magnetite. Comparing the new data set and data recently obtained with volcanic rocks from both shield and strato-volcanoes in the previous papers of this series, we show that the model of polarization of the dynamic Stern layer correctly describes the complex electrical conductivity of kimberlites as well. Our results also explain the cause of electrical conductivity anomalies detected at kimberlite pipes and offer new perspectives in using induced polarization method for the exploration of kimberlites around the world.