Anorthosites, igneous rocks rich in plagioclase feldspar, are common on the Earth, Moon, and possibly other planets. Though anorthosites are usually not considered to be strongly magnetic, Brown and McEnroe note that their magnetic properties could be useful in investigating mineral deposits in addition to magnetic anomalies on other planets, particularly Mars.
Through investigations of three ancient anorthosite bodies in Norway, the authors find that two anorthosites have large natural remanent magnetization signatures, indicating that they contain strong signatures of the Earth's magnetic field direction that was present when the rocks crystallized roughly 1 billion years ago.
These signatures are comparable in intensity to those found in freshly crystallized basalts. Microscopic observations reveal that, although only one body contains high levels of magnetite, all three anorthosites contain ilmenite and hematite, which are weakly magnetic minerals.
Previous research suggests that submicroscopic plates of ilmenite with hematite intergrowths interact with each other to amplify magnetic anomalies, causing the authors to conclude that anorthosites can be important sources of magnetic anomalies on Earth and perhaps on other planets.
Journal reference: Magnetic properties of anorthosites: A forgotten source for planetary magnetic anomalies? Geophysical Research Letters (GRL) paper 10.1029/2007GL032522, 2008; http://dx.doi.org/10.1029/2007GL032522
Authors: Laurie L. Brown: Department of Geosciences, University of Massachusetts-Amherst, Amherst, Massachusetts, U.S.A.; Suzanne A. McEnroe: Norwegian Geological Society, Trondheim, Norway.
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