23. MAGNETIC ANISOTROPY IN SERPENTINIZED PERIDOTITES FROM SITE 920: ITS ORIGIN AND RELATIONSHIP TO DEFORMATION FABRICS1

Anisotropy of magnetic susceptibility (AMS) measurements on serpentinized peridotites from Ocean Drilling Program Site 920 reveal a strong magnetic "fabric, " typically characterized by an oblate susceptibility ellipsoid. Curie temperatures and max-imum unblocking temperatures near 580°C, as well as petrographic observations, suggest that magnetite is the sole magnetic carrier in the serpentinites. Because the magnetic mineralogy is dominated by coarse-grained magnetite, the susceptibility ellip-soid should provide a three-dimensional image of the average elongation of magnetite grains or grain clusters. Petrographic studies of three orthogonal thin sections from a limited number of samples indicate that the preferred shape orientation of mag-netite grain clusters correlates well with the apparent susceptibility maxima and minima in these planes. The magnetite long-axis preferred orientation is typically within-20 ° of the maximum principal axis of the susceptibility ellipsoid. The close corre-spondence between the magnetic foliation and the orientation of magnetite-bearing serpentine veins, together with the petro-graphic evidence for the distribution of magnetite, suggests that the magnetic "fabric " is primarily a reflection of the orientation of these veins. Hence, the AMS ellipsoid may be a more accurate descriptor of the integrated three-dimensional vein orienta-tions than visual orientation measurements made on the split cores