Observed effects of mechanical grain-size reduction on the domain structure of pyrrhotite

Abstract

The magnetic mineral grains that carry natural remanent magnetization (NRM) in rocks can undergo changes in volume, chemical composition, or both. Because domain structure is a function of particle size, composition, and the dominant anisotropy, a change in these properties could trigger changes in the number of domains, drive domain walls to new positions, and thus reset natural remanence. The manner in which active grain-size reduction alone affects domain state has not been studied, however. To address this problem, we have observed Bitter patterns on two mutually perpendicular surfaces of natural pyrrhotite grains, whose sizes were reduced mechanically in the laboratory. Both one-dimensional and two-dimensional thinning experiments were performed. Each grain was thinned until at least one dimension was approximately 20–50% of its original size. Domain widths, the overall positions of surviving walls, and even many small-scale details in the shapes of curved walls were remarkably insensitive to thinning. These results lead to three conclusions. First, local energy minimum (LEM) domain states in pyrrhotite could be stable across a wide range of grain sizes and shapes. Second, defects that pin domain walls could inhibit LEM–LEM transitions by impeding the wall motions required for such transitions. Third, in pyrrhotite the defects which prevent walls from adjusting to thinning-induced changes in demagnetizing field appear to be densely and rather homogeneously distributed across a wall’s surface, rather than being volumetrically rare. When viewed together, these results strongly suggest that an NRM direction, originally carried by two-domain grains, would survive grain-size attrition as the two-domain parents were reduced to stable single-domain daughters.