Magnetism of basalt cores from the Nazca Plate and implications for magnetic anomaly interpretation

Abstract

Measurements of magnetic and thermomagnetic properties of basalt from leg 34 of the Deep-Sea Drilling Project indicate that our 46 samples contained stable single-domain (SD) or pseudo-SD magnetite or titanomagnetite. The mean inclination after af and thermal demagnetization is Ī= −15°, close to the present dipole field, at site 321 but is anomalous (Ī = +53°) at site 319. In 31 samples of massive basalt from both sites the stable remanence at 20°C was masked by a low-coercivity component. High-field hysteresis loops are narrow at 20°C and broad below the −155°C magnetite transition. These samples, which we denote type 1, exhibited wide or constricted Rayleigh loops in 10 Oe. Although the magnetite in type 1 samples tends to be coarse (>100 μm), our findings demonstrate that the effective source of type 1 magnetite properties is not multidomain structures but fine predominantly superparamagnetic interacting particles that become stable SD at low temperatures. The other 15 samples (type 2), containing finer-grained magnetite or titanomagnetite, were stable at 20°C, did not show Rayleigh loops, and produced highly irreversible thermomagnetic curves consistent with SD behavior and oxidation upon heating. Between 20° and 105°C all type 1 k-T curves rose spectacularly towards a Hopkinson-type peak, while the natural remanence and the Koenigsberger ratios (∼1–10 at 20°) decreased sharply. Therefore in areas underlain by such rocks, magnetic anomalies must be interpreted with care.