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.