Theory of the magnetic viscosity of lunar and terrestrial rocks

Abstract

Lunar materials exhibit two distinct types of viscous or time-dependent magnetic behavior. Igneous rocks and largely recrystallized breccias, whose magnetic properties are due to multidomain iron, typically have weak magnetic viscosity, but decay persists for very long times following even a brief exposure to a field. Lunar soils and low metamorphic grade breccias, which contain an impor[ant fraction of metallic iron of singledomain and superparamagnetic size, generally acqtlire an anomalously strong viscous remanence, whose decay time is about equal to the 'time of exposure to the field. Four theories of magnetic viscosity are reviewed in this paper in an attempt to interpret the very different viscous properties of these two types of rocks. The Richter (1937) and N6el (1949) theories are appropriate to soils and low-grade breccias, whereas igneous and recrystal-liZed rocks are better described by the multidomain theories of N6el (1950) and Stacey (1963). Both the Stacey and N•el theories correctly predict logarithmic'magnetic viscosity, in spite of the fact that the central role played by the internal demagnetizing field in multidomain grains is ignored in N6el's formulation. This apparent paradox has been resolved. Analysis of the particularly simple Case of a two-domain particle shows that .the distribution of asymmetrical nonidentical potential barriers required by N•e! is automatically generated from the simpler distribution of symmetrical identical barriers proposed by Stacey through the action of the demagnetizing field. Experimental evidence on many facets of viscous magnetization, from terrestrial as well as lunar materials, is reviewed in detail before a final evaluation of the various theories is made. One interesting conclusion is tha.t the magnetic viscosity of multidomai n particles, although relatively weak, is still too strong to be explained by • displacements of entire domain walls. Either displacements .of small wall segments or rotation of pseudo-single-domain moments could account for the enhancement of magnetic viscosity in these particles.

Viscous magnetization of loess/palaeosol samples from Bulgaria

“…However, the amount of viscous remanence for each sample, that decays in 10 4 sec., is always about 30% lower than the VRM acquired for the same time period. This observation is consistent with the considerations of Dunlop (1973) and Worm et al (1988), where it is shown that for an assembly of SD particles after equally long acquisition and decay times, 37% of the acquired magnetization still survives during VRM decay in zero field. However, this is not observed for sample K1 from the recent soil So, where VRM decay removes almost all of the VRM acquired.…”

The origin and tectonic history of the Southwest Philippine Sea

“…First, the log t relationship is critically dependent on the nature of the grain distribution f(v,Hco) for non-interacting SD grains, where v is the volume and Hco the microscopic coercive force (Dunlop, 1973). Consequently, if a sample contains discontinuities in f(v,H ), the slope S may change co with time.…”

Paleomagnetic Constraints on the Age of Lower Cretaceous Cores from Deep Sea Drilling Project Site 535, Southeastern Gulf of Mexico

“…The possibility that the stable magnetization is a viscous remanent magnetization (VRM) acquired during the current polarity epoch, is highly unlikely because most single domain grains have very much longer relaxation times. Mild heating (~200°C) will erase readily such a VRM (Dunlop, 1973), yet no significant change in magnetization was recognized during thermal demagnetization. Because magnetite is, except under very special conditions, of detrital origin, the stable magnetization was acquired during or soon after deposition.…”