Partial anhysteretic remanence and its anisotropy: Applications and grainsize-dependence

The distribution of remanence coercivities in a rock sample can be obtained by differentiating alternating field (AF) demagnetization curves of anhysteretic remanent magnetization (ARM); however, significant errors can be introduced by grain interactions and by the noise inherent in differentiation. We have found that 1) this error can be significantly reduced by measuring partial ARM's (pARM's) and 2) pARM patterns show a strong empirical correlation with grain-size. pARM's are imparted by exposing a sample to a small direct magnetic field in the presence of a large, decreasing AF; the direct field is switched on over a specified range of AF's, thus magnetizing a fraction of the magnetite grains in the corresponding coercivity range. We have further found in both synthetic and natural sediments that measurement of the anisotropy of pARM enables us to detect the preferred orientations of distinct subpopulations of magnetite particles, related to the size and shape of the grains.