Nature of magnetic grains in basalts and implications for palaeomagnetism

Abstract

Investigations involving temperature dependence of low-field susceptibility and of low- and high-field hysteresis have been carried out on more than one thousand basalt samples of ages ranging from less than 3 million years to more than 1 billion years. Combined application of these measurements makes it possible to distinguish rapidly the effective particle sizes and oxidation states of the magnetic minerals in the rock.

One interesting finding is that, in basalts of widely different ages, a strongly cation-deficient phase of magnetite having distinct magnetic properties predominates over other types of magnetic grains inferred to be present. Properties attributable to multidomain magnetite were found often in basalts older than Cretaceous, but seem to be present only occasionally among the younger basalts. Despite the fact of a frequent association of magnetite with titanium in basalts, results of our tests carried out on such basalts do not in general show a magnetic behaviour consistent with the magnetic material being a solid-solution titanomagnetite. Some implications of these results for palaeomagnetism are discussed.