Field-strength dependence of the viscous remanent magnetization in lunar samples

Measurements of the viscous remanent magnetization (VRM) in lunar soil breccia 14313 have been made. It is shown: (a) that the decay of VRM can be described best by a curve of the form log(t/t+ t 0) where t 0 is the acquisition time, (b) that in fields less than 4 Oe, the slope of the decay curve for t ≪ t 0 is linear, and the intercept of the straight-line portion of the decay curve is equal to the application time, (c) that in fields above 4 Oe, the slope of the decay curve tends to saturate following a Langevin-like function and the intercept time derived from the straight-line portion of the decay curve is greater than the acquisition time. By fitting experimental data for a range of fields including the non-linear portion it is possible to determine values for both the total moment taking part in the viscous process and for the effective volume of iron grains participating in the VRM process.