Some Observations of Rotational Remanent Magnetization

Abstract

Wilson & Lomax (1972) have recently described experiments which show that a remanence can be induced in rock specimens when they are rotated in a decreasing alternating field. They called this effect rotational remanent magnetization (RRM), and they suggested that other workers might try to duplicate it. We describe here some similar experiments which confirm the existence of the effect but which show that it can occur with either sign. As Wilson and Lomax point out, RRM, if present, can produce apparently erratic behaviour during alternating field demagnetization with a tumbling mechanism. A number of specimens which appeared to show this effect were selected for more detailed investigation. The specimens represent two rock types. One (E25B) is a basalt from a recent lava flow of the volcano Emurangogalok in the Gregory rift valley, 100 km north-north-east of Lake Baringo, Kenya. The others (K3, K4) are lacustrine tuffaceous silts from the Acheulean archaeological site Kilembe near Rongai 30 km west-north-west of Nakuru, Kenya. In one case the remanence is TRM, and in the other probably DRM, but both have the ability to acquire RRM. The specimens were demagnetized in the Nairobi equipment which consists of a two-axis tumbler with the outer axis rotating at 1.8 rotations per second, and the inner axis at 2.7 rotations per second. If RRM is present it would be expected to appear directed along the inner axis. For each maximum value of the demagnetizing field, the specimen is treated twice, with a selected axis parallel, and then anti-parallel to the inner axis of rotation. By vector subtraction any RRM should appear on the relevant specimen axis, the other two axes having near zero components if no other effects are present. Addition will then eliminate the RRM vector, and yield the demagnetized NRM free of RRM. In this way demagnetization curves for the NRM can be obtained at the same time as curves showing the growth of RRM. This follows standard demagnetization procedures, and will yield results of immediate interest to the practising worker. It should be emphasized however, that this RRM is developed on the inner axis of a tumbling mechanism, and is not directly comparable to the RRM of Wilson & Lomax which is developed along an axis perpendicular to the demagnetizing field. In this note we designate the former as RRM2 and the single axis type as RRM1. Experiments on RRMl are described later. The demagnetization’of NRM and the concurrent growth of RRM2 are shown in Fig. 1 (a)-(c). The curves show a variety of shapes, and the relative importance of NRM and RRM2 at a given field varies widely from specimen to specimen. It is this which governs the actual impact of RRM2 in a given case; contrast K3 and E25B. The sense of the RRM2 vector in the sediment samples K3 and K4 was in the direction of -w in agreement with that observed by Wilson & Lomax for RRMl in all their samples. In contrast the RRM2 vector in E25B was in the direction of +w. This was sufficiently surprising for us to make careful repeat measurements and the effect