REMANENT MAGNETISM AND DUCTILE DEFORMATION IN AN EXPERIMENTALLY DEFORMED MAGNETITE-BEARING LIMESTONE

A limestone with pseudo-single-domain (PSD) pure magnetite is given an isothermal remanent magnetization (IRM). Samples are deformed triaxially at 200 MPa (2 kbar) confining pressure at room temperature and at a strain-rate of 10(-5) s-1. The limestone deforms in a ductile manner by twinning to produce sufficiently homogeneously strained specimens up to about 20% shortening. The uniform, saturation IRM rotates away from the shortening axis, approximately by an amount expected for the homogeneous strain of a non-material line marker. Hydrostatic compaction alone shows that this is time-dependent and compatible with progressive damage of the ferrimagnetic grains. Strain produces an increase in coercivity of the magnetite, an increase in its saturation remanence, and an increase in its anisotropy of magnetization (with respect to IRM). These observations are compatible with reduction in effective grain size and change of shape of magnetite. Experimental deformation reduces the intensity of magnetisation, chiefly by the removal of low-coercivity components of remanence when there is a pure IRM vector.