Magnetization changes caused by burial and uplift

Abstract

Theoretical blocking curves for hematite and magnetite are given over temperature ranges from room temperatures to Curie temperature and over time scales from 1 second to 10 9 years. In principle these curves predict how remanent magnetization in rocks is gradually lost during the heating accompanying burial and metamorphism, and how it is replaced by a new remanence during uplift and cooling. A series of experiments are described which give the decay of remanent magnetization and the build up of viscous magnetization during long-term heating at temperatures up to 400°C. Tests showed that in the selected samples minimal chemical changes occured. The treatment is considered to be analogous to the burial of rocks to 5–10 km for several millions of years. It is shown that burial magnetizations acquired in this way should be removed by standard magnetic cleaning procedures. A second series of experiments at somewhat higher temperatures (400–530°C) were carried out in order to observe the unblocking of magnetite and hematite in long-term heating experiments, and to compare the with the theoretically derived curves. In the second experiments there is a discrepancy with theory, and a compromise is proposed which indicates that very little, if any remanence can survive high greenschist facies metamorphism, and that the age of the remanence in high-grade metamorphic rocks is between the Rb—Sr whole-rock isochron and K—Ar mica ages. It is also argued that burial magnetizations are being generated in the present geomagnetic field in rocks at depths of the order of 10 km and are a potent source of magnetic anomalies over the continental crust.