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 cr ustal magnetic anomalies due to remanent magnetization, while on the Earth both remanence and induction can contribute to the magnetic anomaly, becaus e 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 o ther magnetic minerals. We investigated the thermoremanent magnetization (TRM) acquisition of hemat ite to determine if the remanent and induced magnetization of hematite coul d compete with magnetite in weak magnetic fields up to 1 mT. TRM acquisitio n curves of magnetite and hematite show that multidomain hematite approache s TRM saturation (0.3-0.4 A m(2)/kg) in fields as low as 0.1 mT, However, m ultidomain 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 pr oviding requisite magnetization contrast, Consequently, we should reevaluat e where multidomain hematite exists in significant concentration, allowing a better insight into the role of remanent magnetization in the interpretat ion of the magnetic anomalies. (C) 2000 Elsevier Science B.V. All rights re served.