Mixed domain states of magnetic grains in basalts and implications for palaeomagnetism

Abstract

Abstract The physical size of the magnetic mineral grains in basalts can vary from 100 μm. Such a possible distribution in the size of the grains should, in itself, have prompted rock magnetists to treat the magnetic properties of basalts as if they arose from mixed domain states, unless they were shown to be otherwise. Moreover, submicroscopic structures or changes in large magnetic grains might impart single-domain behaviour to some basalts. However, such considerations were neglected until recently, and even now these aspects are not usually discussed to the extent required. The large amount of data now available on the intrinsic magnetic properties of basalts, both continental and marine, is made use of to obtain an insight into the domain state aspects of these rocks. Also, the possible implications of mixed domain states of magnetic grains in basalts for the palaeomagnetism are discussed using some case histories.