Induced Polarization and Magnetic Responses of Serpentinized Ultramafic Rocks From Mid‐Ocean Ridges

Abstract

AbstractThe outcrops of ultramafic rock along mid‐ocean ridges experience variable extents of serpentinization related to tectonic and hydrothermal processes. It involves the water‐assisted alteration of primary ferromagnesian minerals (e.g., olivine and pyroxenes) to produce secondary minerals including serpentine, magnetite and brucite. This hydration process induces profound changes in both the rheological and petrophysical properties of the ocean lithosphere. That said, the evolution of the low‐frequency electrical polarization properties of these rocks along the serpentinization path remains poorly explored. A total of 47 core samples were collected at the seafloor along the Southwest Indian Ridge (SWIR) in order to investigate the process of serpentinization in terms of combined magnetization and polarization signatures. We found a linear relationship connecting chargeability and magnetic susceptibility for both fresh basalts and ultramafic rocks. A unique trend between these two properties is associated with the production of magnetite during serpentinization of peridotites. Magnetite acts indeed as a semi‐conductor responsible for a high chargeability in a low‐frequency (<10 kHertz) electrical field and because of its magnetic properties, it increases the magnetic susceptibility of the ultramafic rocks during serpentinization. In addition, both the chargeability and the magnetic susceptibility change with the grain density of the core samples, which depends on the degree of serpentinization.