A method for the removal of rotational remanent magnetization acquired during alternating field demagnetization

Experimental evidence has confirmed the existence of rotational remanent magnetization (RRM), a spurious remanence imparted to rock specimens that have been rotated in a decreasing alternating field.Sediments with low magnetic stability systematically acquired RRM while being demagnetized in fields of 50-200 Oe.Our results were obtained from demagnetization equipment utilizing a four-axis specimen holder.When a demagnetization run was repeated in the same peak field, but with the specimen holder's direction of rotation reversed, RRM was created that was equal and opposite to the RRM of the previous run.The spurious magnetization can be removed by averaging the paleomagnetic vectors obtained from two demagnetization runs carried out with forward and backward rotations of the specimen.Besides removing RRM, doubledemagnetization is desirable because it gives an estimate of the directional dispersion due to random noise in the applied field, and thus provides a check on the reproducibility of the AF treatment.Alternating field demagnetization, when applied to a rotating rock specimen, can systematically add magnetic remanence that masks the specimen's true paleomagnetic direction.This paper presents a method for the removal of spurious remanence created by demagnetization equipment that employs a rotating sample holder.Wilson & Lomax (1972) demonstrated that this spurious component is not ordinary anhysteretic remanence (ARM) due to the superposition of a weak DC field on the alternating field.Instead, the remanence, termed rotational remanent magnetization (RRM), is an intrinsic property of magnetic specimens that have been rotated in an alternating field.Although RRh4 is usually insignificant in fine-grained lavas, the effect can mask the true paleomagnetic directions of sediments and coarse-grained igneous rocks.Fortunately, RRM can be removed mathematically from these specimens because the remanence consistently occurs along one axis, and the intensity of RRM is highly reproducible over a series of demagnetization runs carried out in identical peak fields.From experiments with a single-axis device Wilson & Lomax (1972) found that RRM is always