Magnetic viscosity, diffusion after‐effect, and disaccommodation in natural and synthetic samples

Summary. The magnetic viscosity of some oceanic basalts from IPOD site 417D is investigated. The samples are fine‐grained pillow basalts and coarse‐grained massive flows. Experiments on the viscous behaviour of these samples include (1) acquisition and decay of weak‐field viscous remanent magnetization (VRM) and (2) the influence on VRM produced by the zero field storage time following demagnetization by an alternating magnetic field. In all cases the aquisition of VRM is found to be more rapid than its subsequent decay. The time dependence of the intensity of VRM is observed to be significantly non‐logarithmic during acquisition, but less so during decay. The length of time a sample remains in zero field following demagnetization produces a decrease in the aquisition and decay coefficients of VRM. These results cannot be reconciled entirely with predictions based on existing models of magnetic viscosity due to thermal fluctuations. Available evidence also suggest that the effect on VRM produced by (1) zero field storage following demagnetization or produced by (2) different initial states of magnetization, is attributed to susceptibility disaccommodation. Unfortunately, the available data are insufficient fully to ascertain the importance of diffusion after‐effect and disaccommodation to long‐term viscous behaviour. It is suggested, however, that diffusional processes may contribute to VRM in young pillow basalts (< 0.1 Ma) near mid‐ocean ridges where the oxidation of titanomagnetite produces an increase in the number of octahedral lattice site vacancies and the elevated temperature of the crust enhances the diffusive reordering of these vacancies and ferrous ions. In order to gain some additional insights into VRM and attempt to explain some of these observations, theoretical and experimental results pertaining to diffusion after‐effect and disaccommodation in magnetic materials are reviewed.