Russian Journal of Earth Sciences
Vol. 3, No. 2, June 2001
Petromagnetism of the continental lithosphere and the origin of regional
magnetic anomalies: A review
D. M. Pechersky and Yu. S. Genshaft
Abstract
The paper discusses the current ideas of regional magnetic anomaly sources and
experimental evidence for the formation and behavior of various magnetic
minerals within a wide range of pressure, temperature and oxygen fugacity. Four
thermodynamic zones of formation conditions of magnetic minerals are shown to
exist; these are the hematite, magnetite, silicate and metal-Fe zones
successively changing, as the oxygen fugacity decreases (from strongly oxidizing
to strongly reducing conditions). The effects of pressure and diffusion
processes on titanomagnetite alterations, as well as the oxygen fugacity and
fluid composition implications for the composition and concentration of magnetic
minerals, are considered. Experimental studies show that ferromagnetic minerals
do not form from silicates under "dry'' conditions or in the presence of water
vapor. The paper presents results derived from our studies of petromagnetic
characteristics of rocks that formed under near-surface conditions (basalts and
gabbroids) and in the lowermost continental crust(xenoliths in igneous rocks of
Afar, Mongolia, the Lesser Caucasus, Kurile Islands, and Yakutia), as well as
rocks from Archean-Proterozoic metamorphic sequences (Aldan and Anabar shields
and Voronezh crystalline massif). The implications of secondary processes, such
as chloritization and amphibolization, for the alterations in the ferromagnetic
fraction and magnetic properties of these rocks are considered. Our results and
literature data reviewed indicate that, since the Archean, igneous rocks formed
in extension zones under surface and near-surface conditions have made a major
contribution to the crust magnetism and regional magnetic anomalies. This
situation is presently preserved in spite of metamorphism and substantial
recrystallization at various depths.