The Mid-Atlantic Ridge at 45° N. XIV. Oxidation and magnetic properties of basalt; review and discussion

Abstract

The magnetic properties of submarine basalt procured in a traverse across the accreting margin of a lithospheric plate are reviewed. These properties are attributed to two processes. Firstly, the rapid quenching of lava in the axial zone of accretion produces dispersed titanomagnetite of very small grain-size, ranging from about 5 μm down to submicroscopic sizes. During quenching the pillow lava acquires a very intense (0.1 c.g.s. units per cm3) and stable (mean coercivity of remanence 240 Oe) remanent magnetization, with Q of the order 100. The oxidation state is initially low (FeO/Fe2O3 averages about 7) but, presumably because of rapid quenching, it is chemically unstable, and undergoes secondary oxidation, so that at a distance of 10 km in from the accreting margin the FeO/FenO3 ratio has fallen to about unity, the remanence has decreased by a factor 10, and the Curie and blocking temperatures have risen from the range 100–300 °C to the range 300–500 °C. It is suggested that these magnetic changes are caused by very mild hydrothermal activity in the axial zone of accretion. The larger magnitude of the axial geomagnetic anomaly compared with those immediately to each side (a difference of a factor 2 over the Mid-Atlantic Ridge) is attributed to this oxidation process.These magnetic properties are very different from those of basic intrusive and subaerial igneous rocks which characterize the early (that is continental) rifting stages of ocean formation. This is because the grain sizes of their titanomagnetites are larger, commonly ranging beyond 50 μm, which is due to their taking years or decades to cool, rather than the minutes or hours, required by water-quenched basalts. Large grains have a much smaller specific intensity than small grains, so that the remanence of dolerite is generally less than that of submarine lava, and Q is of the order unity. Consequently, in the continental rifting stages, igneous rocks which, by-and-large, are incapable of causing very large anomalies in the geomagnetic field are produced, and this, it is argued, may be the origin of the magnetically smooth zones which border the North Atlantic, which occupy all except the central deep of the Red Sea, and which also may be represented in the inner basins of the Gulf of California.Because of quenching and low initial oxidation state, sulfide minerals are generally present in submarine basalts which therefore may be a potent source of sulfide mineralization.