Geology of the oceanic crust: Magnetic properties of oceanic rocks

A diverse suite of rocks has been sampled from ocean basin escarpments in the North Atlantic and the Caribbean: fresh and weathered basalts, metabasalts (zeolite and greenschist facies), gabbros, metagabbros (greenschist and amphiolite facies), serpentinized peridotites, and actinolite rocks. One hundred and three representative specimens were chosen from this diverse suite of rocks, and the natural remanent magnetization (NRM) intensity and susceptibility for each specimen were measured in the laboratory. Unaltered and unweathered basalts had the highest measured NRM intensities (geometric mean, 10.69 × 10−3 emu/cm3). The weathered basalts, metamorphosed basalts, gabbros, metagabbros, and actinolite rocks all had NRM intensities much lower than those of the fresh basalts (geometric means, 0.11–1.60 × 10−3 emu/cm3). The serpentinized peridotites, however, are strongly magnetic, the geometric mean intensity being 7.86 × 10−3 emu/cm3. The serpentinized peridotites also had high susceptibilities (geometric mean, 3.298 × 10−3 emu/cm3), whereas all the other rock types had low susceptibilities (geometric means, 0.047–0.480 × 10−3 emu/cm3). The measured magnetic parameters for the several rock types are compared with published models of the oceanic crust based on measurements of the magnetic field over the oceans. These geophysical models suggest that the upper part of the oceanic crust is a strongly magnetized layer of basalt and the majority of the crust below is weakly magnetized. The fresh basalts measured in this study have intensities and susceptibilities compatible with those in the geophysical models. The metabasalts, gabbros, metagabbros, and actinolite rocks have weak intensities and therefore could be, on the basis solely of their magnetic character, volumetrically important components of the weakly magnetized part of the oceanic crust. Serpentinized peridotite has intensities and susceptibilities too high to be an important component of the oceanic crust.

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