THE INFLUENCE OF PHASE INTERFACES IN MAGNETIC MINERALS ON THE NATURE OF MAGNETIC ANOMALIES

Isidore Zietz was a pioneer in applying aeromagnetic surveys to the mapping of regional geology. Today we use magnetics to map anomalies over many length scales, from the planetary to the microscopic. Crustal rocks have magnetic anomalies that reflect the magnetic minerals, which to various degrees respond to the changing planetary magnetic field. Anomalies are influenced by the geometry of geological bodies and the magnetic properties of the constitutive rocks. The shapes of anomalies are also determined by the ratios of remanent versus induced magnetizations (Koenigsberger ratios = Q) and the relative vector orientations of the two. Here we highlight how the nature of phase interfaces in magnetic minerals can affect the nature of magnetic anomalies.

Rocks containing mainly rhombohedral oxides with exsolution lamellae of hematite in ilmenite, or ilmenite in hematite, have high Q values, commonly > 5, and many with values into the hundreds. A large component of their magnetization is carried at the interfaces of the lamellae. These rocks typically have remanent-dominated anomalies. When magnetite is a co-existing oxide, the Q values can be lower, due to the addition of a larger induced component, but commonly Q values are > 2, thus retaining the stronger influence of the remanent component.

Rocks with oxides mainly involving ilmenite and magnetite are characterized by two related exsolution systems. Typically, oxidation-exsolution of ilmenite from multi-domain magnetite produces induced anomalies. By contrast reduction-exsolution of plates of magnetite from ilmenite causes significant magnetic anomalies that are dominated by remanence, even with comparable magnetite domain size. Although the induced component is still quite large, Q values, all > 1, range from 2 to over 10. This strong remanence effect, presumably related to phase interfaces, is yet to be fully understood.