Experimental deformation of synthetic magnetite‐bearing calcite sandstones: Effects on remanence, bulk magnetic properties, and magnetic anisotropy

Abstract

We have quantified effects of experimental deformation on the magnetic properties of a set of synthetic “calcite sandstone” samples containing magnetite. The deformation was carried out in a microcomputer‐controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10 −5 s −1 . Most samples were deformed under dry conditions, but a few were deformed with a pore fluid present; the samples deformed under dry conditions required substantially higher differential stresses. Macroscopically ductile shortening strains of up to 25% produced the following irreversible changes in magnetic properties: (1) increased bulk coercivity, remanence coercivity, and mean anhysteretic remanence susceptibility; (2) decreased mean low‐field susceptibility; (3) decreases in the component of remanence parallel to shortening; (4) smaller decreases for most samples in the component normal to shortening, resulting in a net “rotation” of the remanence away from the shortening axis; (5) larger decreases in the normal component in a few samples, resulting in a net “rotation” of the remanence towards the shortening axis; (6) increased magnetic anisotropy; and (7) increased “deformation” of initial magnetic ellipsoids. A comparison of data for samples deformed under dry and wet conditions (higher and lower differential stresses, respectively) indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain (i.e., rotation and displacement of particles), whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress.