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 maximum 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 ellipsoid 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 magnetite 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 correspondence between the magnetic foliation and the orientation of magnetite-bearing serpentine veins, together with the petrographic 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 orientations than visual orientation measurements made on the split cores.

Date of initial receipt: 17 August 1995
Date of acceptance: 22 March 1996

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