UNIQUE THERMOREMANENT MAGNETIZATION OF MULTIDOMAIN SIZED HEMATITE: IMPLICATIONS FOR MAGNETIC ANOMALIES

Abstract

Intense magnetic remanence (100-1000 A/m) associated with MD hematite and/or titanohematite and associated with high Koenigsberger ratios (40-1000) indicate that magnetic remanence may dominate the total magnetization if these minerals are volumetrically significant. Titanohematite behaves similarly to hematite and, thus, the grain size dependence of TRM acquisition in hematite is considered as a generalization. The transition between truly MD behavior and tendency towards SD behavior in hematite has been established to be between grain sizes of 0.1 and 0.05 mm. In contrast to magnetite and titanomagnetite, hematite exhibits inverse grain size dependence, with MD hematite acquiring a relatively intense TRM in the geomagnetic field, comparable to sub-micrometer sized magnetite and only an order of magnitude less than SD magnetite. Consequently MD hematite (and by analogy titanohematite) remanence may be of significance as a source of magnetic anomalies at all scales. MD hematite exhibits TRM weak field acquisition behavior that is different from all other magnetic minerals, being the only magnetic mineral having an REM (TRM/SIRM) value #0.1 for TRM acquisition in the geomagnetic field. The very different TRM behavior of MD hematite in contrast to magnetite is due to two factors. The first is the lesser influence of demagnetizing energy with respect to wall pinning energy, at temperatures almost up to the Curie temperature for hematite. The second is the greater importance of the magnetostatic energy in the applied field, which for hematite dominates the total energy at high temperatures.