AF demagnetization characteristics of NRM, compared with anhysteretic and saturation isothermal remanence: an aid in the interpretation of NRM

Abstract

AF demagnetization characteristics and isothermal remanent magnetization (IRM) acquisition of a variety of rocks are analysed in the form of Cisowski plots and plots of progressive demagnetization of natural remanent magnetization (NRM) and anhysteretic remanent magnetization (ARM) against IRMs. The igneous rocks studied carrying a primary NRM acquired during initial cooling have a ratio of NRM:IRMs of parts in 100, whereas altered igneous rocks have much smaller ratios of parts in 1000 or 10,000. Conversely, the detrital sedimentary rocks carrying a primary NRM have a ratio of parts in 1000, but altered sedimentary rocks carrying secondary magnetization have ratios of parts in 100. Although not as discriminatory as the ratios of NRM:IRMs, altered igneous and sedimentary rocks frequently have strongly concave up curves, reflecting the common admixture of a small amount of hard material in samples carrying secondary magnetization, The ratio of ARM to IRMs serves as a proxy for weak field magnetization and is an indicator of grain size.