Magnetic properties of anorthosites: A forgotten source for planetary magnetic anomalies?

Abstract

Anorthosites, igneous rocks very rich in plagioclase, rarely considered to be strongly magnetic, are common on Earth, and the Moon, and inferred to be on other planets. Magnetic properties of anorthosites could be important in investigating associated mineral deposits and in studying magnetic anomalies, especially on Mars. Here we investigate three late Proterozoic anorthosites in Rogaland, Norway, for magnetic and petrographic properties. Two of the anorthosites have large natural remanent magnetization (NRM), with intensities comparable to Tertiary basalts. Susceptibility, NRM and hysteresis properties provide information about the magnetic minerals present and their response to inducing fields. Microscopic observations show ubiquitous hemo‐ilmenite in the anorthosites, whereas magnetite is common in the Håland‐Helleren, but rare in the Åna‐Sira and Egersund‐Ogna bodies. This study illustrates that anorthosites can be important sources of magnetic anomalies, and can retain a remanent field over geologic time. It also supports the recently described property of ‘lamellar magnetization’.