Hematite vs. magnetite as the signature for planetary magnetic anomalies?

Crustal magnetic anomalies are the result of adjacent geologic units having contrasting magnetization. This magnetization arises from induction and/or remanence. In a planetary context we now know that Mars has significant crustal magnetic anomalies due to remanent magnetization, while on the Earth both remanence and induction can contribute to the magnetic anomaly, because of the presence of the Earth's magnetic field. If there is a significant induced magnetization (IM) then magnetite is commonly assumed as the source, since it has a much greater magnetic susceptibility, when compared with other magnetic minerals. We investigated the thermoremanent magnetization (TRM) acquisition of hematite to determine if the remanent and induced magnetization of hematite could compete with magnetite in weak magnetic fields up to 1 mT. TRM acquisition curves of magnetite and hematite show that multidomain hematite approaches TRM saturation (0.3-0.4 A m(2)/kg) in fields as low as 0.1 mT, However, multidomain magnetite reaches only a few percent of its TRM saturation in a field of 0.1 mT (0.02-0.06 A m(2)/kg). These results suggest that a mineral such as multidomain hematite and, perhaps, other minerals with significant remanence and minor induced magnetization may play an important role in providing requisite magnetization contrast, Consequently, we should reevaluate where multidomain hematite exists in significant concentration, allowing a better insight into the role of remanent magnetization in the interpretation of the magnetic anomalies. (C) 2000 Elsevier Science B.V. All rights reserved.