On the stability of the viscous remanent magnetization in sedimentary rocks when submitted to AF demagnetization: New experimental results

Abstract

We present new results concerning some properties (especially the intensity and the hardness with respect to AF treatment) of laboratory-acquired viscous remanent magnetization (VRM) in sedimentary rocks. We compare these properties with the characteristics of the natural secondary magnetization, with the object of verifying its assumed viscous nature. This study is made on samples coming from three French sedimentary formations which were previously investigated for palaeomagnetism: two hematite-bearing red formations and a clayous titanomagnetite-bearing volcano-detrital formation. The experimental VRMs were obtained either in the Earth's field (in the case of the clayous formation) or in a 10 Oe field (in the case of the red rocks). In order to compare the secondary magnetization with the experimental VRM, the samples are firstly divided into groups according to the preponderance in the natural remanent magnetization (NMR), either of the primary or of the secondary magnetization after storage in field free space. In the red formations, the samples in which the secondary magnetization is predominant are further subdivided according to the degree of stability of their NRM direction when they are subjected to AF treatment. The points of this study to be borne in mind are the following: 1. (1)|In both the red rocks and the volcano-detrital clay, the intensity of the experimental VRM acquired in the Earth's magnetic field is never negligible compared to the NRM intensity. The mean viscosity index, estimated by comparing the VRMs, acquired during several weeks or several months, to the NRMs measured after a long storage in field free space, varies, according to the group, between 10 and 95% in the red formations; it is approximately equal to 100% in the volcano-detrital clay. The VRM acquired in situ is therefore in all cases a large component of the NRM. 2. (2)|The resistance of the VRM of AF treatment is most often very high in the red rocks, and weaker but never very low in the volcano-detrital clay. In the red rocks, the median destructive field of a VRM acquired during several days or several months is most often of about several hundreds Oe r.m.s.; in the volcano-detrital clay it is in general about 50 Oe r.m.s. The inefficiency of the AF cleaning, established for the red rocks, undoubtedly originates from the extremely high resistance of the VRMs to this treatment. 3. (3)|It is fairly often possible to display a rough parallel between the properties of the experimental VRM and the characteristics, deduced from the study of the total natural remanence, of the secondary magnetization. Nevertheless, some of the results seem inconsistent with the assumption likening secondary magnetization and VRM. This leads us to conclude that the secondary magnetization aligned with the present Earth's field is not, in some of the cases, exclusively viscous.