Seminar Thomas Ferrand

Abstract

Electrical conductivity measurements on well-characterized materials in the laboratory allow accurate interpretations of high-conductivity anomalies within the lithosphere and asthenosphere, both affected by substantial deformation over geological times. However, only a few experiments so far have measured rock conductivity during controlled deformation at high pressures (≥1 GPa) and temperatures (500–1,000°C). Here, we report the first successful deformation experiments performed in a new-generation Griggs-type apparatus adapted for electrical measurements. As a proof of concept, one successful experiment was conducted on Carrara marble at a confining pressure of 1 GPa and temperatures of 500, 650, and 800°C. A conductivity peak reveals percolation of supercritical CO2 at the onset of mechanical relaxation. Three other experiments were performed at the same pressure to explore the electrical conductivity of Åheim dunites at 500, 650, and 800°C, respectively. Our results show very different electrical responses in the elastic and plastic regimes. Stress and strain can significantly impact the electrical conductivity of peridotites by changing the thickness, number, or geometry of grain boundaries. At fixed P-T conditions, the electrical conductivity varies within an order of magnitude during elastic loading and unloading, which motivates reappraisal of interpretations of electrical anomalies in mantle rocks, at least in tectonically active regions. Upon additional development to achieve deformation up to 4 GPa (≃120 km depth), the design presented here opens a fully new research field, which will help to more deeply understand electrically conductive anomalies in rocks under stress at depth, notably within the lower crust, upper mantle and subducting slabs.