The magnetic properties of Deep Sea Drilling Project basalts from the Atlantic Ocean

Abstract

Thermomagnetic, paleomagnetic and opaque petrology studies were carried out on 53 specimens from 16 samples of oceanic basalt obtained from Deep Sea Drilling Project holes in the Atlantic Ocean. The specimens exhibited irreversible thermomagnetic curves and contained homogeneous titanomagnetites that were slightly maghemitized. They were categorized as deuteric oxidation class I basalts that had undergone slight to moderate regional hydrothermal alteration. The natural remanent magnetization intensities (geometric mean value 1.0 × 10 −3 Gauss) and the initial susceptibilities (geometric mean value 1.7 × 10 −4 G/Oe) were considerably lower than those reported for oceanic basalts obtained from dredge hauls, but the ratios of the two parameters (geometric mean value 6) were comparable to values reported for other oceanic basalts. The remanent magnetizations were very stable against alternating field demagnetization with median destructive fields around 300 Oe. The site paleolatitudes deduced from the remanent inclination agreed well with a model reconstruction of the opening of the Atlantic Ocean, except at two sites where the basalts may have been drilled from sills. The weaker magnetizations and susceptibilities of these specimens were attributed to a lower titanomagnetite concentration than found in dredge-hauled specimens. The amplitudes of model profiles computed with the observed NRM intensities agreed well with the amplitudes of marine magnetic anomalies measured near two of the DSDP holes. The model required an effective thickness of 2.5 km for the magnetized layer.