On the alternating field demagnetization characteristics of multidomain thermoremanent magnetization in magnetite

It is demonstrated that the stability against af demagnetization of TRM carried by multidomain magnetite increases with increasing strength of the inducing field. This multidomain field-dependence trend of stability against af demagnetization is opposite to that of single-domain particles. A simple test to distinguish multidomain and single-domain carriers of remanence is designed that makes use of the distinctive af demagnetization characteristics. The test was applied to a variety of igneous rocks. None of these showed truly multidomain characteristics. Thus, although some TRM is undoubtedly multidomain in these rocks, the TRM is dominated by single-domain carriers. These results are consistent with the growing evidence of the importance of very fine grained magnetite as a carrier of NRM in many igneous rocks.