LOW-TEMPERATURE DEMAGNETIZATION AND ICE-PRESSURE DEMAGNETIZATION IN MAGNETITE AND HEMATITE

Low-temperature demagnetization (LTD) is confirmed as an effective method of removing soft (low coercivity) components of remanence in magnetite-bearing specimens by passing the magnetite through its low-temperature transition (almost-equal-to 130K). Haematite has its low-temperature transition at approximately 258K. When some specimens bearing fine-grained haematite pigment pass through this transition, the directional component which is hardest (highest coercivity) may be more reduced than the soft components. In other samples of the same material the opposite may occur, depending on the range of coercivities represented in each directional component. Thus LTD does not consistently isolate hard directional components of remanence in haematite. It is therefore an unsuitable method for routine palaeomagnetic cleaning of natural remanent magnetization carried by haematite. Specimens that contain moisture-for example, from recent drilling with water-cooled equipment-show greater reductions in remanence intensity after LTD. This is attributed to ice-pressure demagnetization due to the effects of stress as pore ice thaws when the specimens warm to room temperature. It is recommended that porous samples are thoroughly dried out before LTD is performed, if consistent results are required.