A survey of the natural remanent magnetization and magnetic susceptibility of Apollo whole rocks

Magnetic properties of lunar rocks may help understand the evolution of the past magnetic field of the Moon. However, these properties are often measured on small subsamples, which may not be representative of the bulk lunar rocks. In this study, a rough magnetic characterization of a large fraction of the Apollo collection was performed using non-destructive methods, with a focus on large samples (mean mass 500 g). The natural remanent magnetization of 123 Apollo samples and the magnetic susceptibility of 154 Apollo samples were measured in the Apollo storage vault at the Lunar Sample Laboratory Facility in Houston. Magnetic susceptibility is strongly controlled by lithology and is clearly enhanced by impact processing and associated incorporation of meteoritic metal, as well as formation of nanometric iron particles by space weathering. The natural remanent magnetization is lower by a factor of similar to 3 than that measured on small samples (similar to 100 mg), suggesting magnetic contamination during small-sample preparation or handling, or directional heterogeneity in remanence within large samples. The ratio of natural remanence magnetization to magnetic susceptibility may be used as selection tool for further paleomagnetic study. It is a crude paleointensity proxy and its overall temporal evolution is coherent with the existence of a high magnetic field from similar to 4 Ga to 3.5 Ga and a subsequent weaker field epoch as previously defined in the literature. Our data also raise the possibility that the intensity of the paleomagnetic field during the high field epoch may have been more variable and lower on average than previously suggested.