Magnetic properties of dredged oceanic gabbros and the source of marine magnetic anomalies

Abstract

Summary. Magnetic property studies (natural remanent magnetization, initial susceptibility, progressive alternating field demagnetization and magnetic mineralogy of selected samples) were completed on 45 samples of gabbro and metagabbro recovered from 14 North Atlantic ocean-floor localities. The samples are medium to coarse-grained gabbro and metagabbro which exhibit subophitic intergranular to hypidiomorphic granular igneous textures. The igneous mineralogy is characterized by abundant plagioclase, varying amounts of clinopyroxene and hornblende, and lesser amounts of magnetite, ilmenite and sphene. Metamorphic minerals (actinolite, chlorite, epidote and fine-grained alteration products) occur in varying amounts as replacement products or vein material. The opaque mineralogy is dominated by magnetite and ilmenite. The magnetite typically exhibits a trellis of exsolution-oxidation ilmenite lamellae that appears to have formed during deuteric alteration. The NRM intensities of the gabbros range over three orders of magnitude and give a geometric mean of 2.8 x 10-4gau~~ and an arithmetic mean of 8.8 x 10-4gauss. The Konigsberger ratio, a measure of the relative importance of remanent to induce magnetization, is greater than unity for the majority of the samples and indicates that remanent magnetization on average dominates the total magnetization of oceanic gabbros in the Earth's magnetic field. The magnetic properties of fresh and metamorphosed gabbros appeared to be similar. The majority of gabbros studied were characterized by median destructive fields greater than 200 Oe. The high stability is attributed largely to the effective subdivision of the magnetite grains by the