Simulation of long-term changes in the magnetic signal of the oceanic crust

Abstract

We report magnetic properties of submarine basalts 3.5 to 16 Ma in age recovered from depths as great as 530 m in layer 2 near the Mid-Atlantic Ridge at 37° N during Leg 37 of the Deep Sea Drilling Project. The rocks are classified as type-I if they have reversible in-vacuum thermomagnetic curves and as type-Il if they are thermomagnetically irreversible and develop a high-Curie-point phase (believed to be magnetite) when heated. Initial Curie points are low: 140–200 °C in type-I rocks, 250–300 °C in type-II rocks. The phases responsible are thought to be stoichiometric and cation-deficient (oxidized) titanomagnetite, Fe2.4Ti0.6O4, respectively. Only the 3.5 Ma basalts contain any type-I material; the older basalts are completely oxidized.Viscous magnetization is uniformly strong in type-I rocks, weaker and variable in type-II rocks. Hysteresis properties explain this difference. It is not due primarily to the chemical difference between stoichiometric and oxidized titanomagnetites, but to a difference in grain size. Type-I rocks are magnetically very soft: the coercive force (Hc) is 15–90 Oe (1194–7162 A/m), the median demagnetizing field [Formula: see text] of natural remanent magnetization (NRM) is 35–135 Oe (2785–10743 A/m), the ratio between saturation remanence Jrs and saturation induced magnetization Js is generally [Formula: see text] and the ratio of remanent coercive force, HR, to Hc is [Formula: see text]. These results all indicate multidomain grains of titanomagnetite ≥ 40 μm in size. Opaques of this size are seen in polished thin sections. Type-II rocks have Hc > 150 Oe (11937 A/m), [Formula: see text] (27 853 A/m), [Formula: see text] and HR/Hc generally < 2, indicating single-domain or pseudo-single-domain behaviour in micron- or submicron-size grains. The small magnetic grain size in type-II rocks could result from preferential oxidation of fine grains and/or subdivision of larger grains by inhomogeneous oxidation. The pronounced viscous magnetization of type-I rocks is therefore thought to be due to coarse, unoxidized multidomain grains of titanomagnetite.Long-term viscous magnetization is simulated by measuring viscous decay curves at temperatures up to 200 °C. Relaxation times are strongly temperature dependent: relaxation times as long as 106 yr can be activated in laboratory experiments at 75 °C if a low-Curie-point phase like Fe2.4Ti0.6O4 carries the viscous magnetization, or at 200 °C if Fe3O4 is the carrier. Viscous remanent magnetization (VRM) over 106 years seems to be no more than a factor 2 or 3 times the VRM estimated by extrapolating room-temperature data determined over a laboratory time scale. Even in type-I rocks, long term VRM is insufficient to completely erase the NRM.