HR: 0830h
AN: GP31C-0763    [PDF]
TI: Back to Basics: Allard Lake to Mars
AU: * McEnroe, S A
EM: suzanne.mcenroe@ngu.no
AF: Geol. Survey of Norway, N-7491, Trondheim, 7491 Norway
AU: Langenhorst, F
EM: Falko.Langenhorst@uni-bayreuth.de
AF: Bayerisches Geoinstitute,, Univ. Bayreuth, Bayreuth, D-95440 Germany
AU: Robinson, P
EM: peter.robinson@ngu.no
AF: Geol. Survey of Norway, N-7491, Trondheim, 7491 Norway
AU: Clark, D
EM: David.Clark@csiro.au
AF: Advanced Magnetics Group,, CSIRO, North Ryde, NSW 1670 Australia
AB: Rob Hargraves worked on hemo-ilmenites from the Allard Lake area, Quebec for his Ph.D. thesis in 1959 and returned with great interest to these samples in relation to the concept of lamellar magnetism. Because the hemo-ilmenite ores produce large negative magnetic anomalies that can only be modeled with a significant remanent component, they are good analogs for the newly discovered Martian magnetic anomalies. The Allard Lake samples typically have high MDFs, above 100mT, however the hysteresis properties vary significantly with the presence or absence of magnetite. Ms values range from 620 to 23500 A/m, Hc from 3.7 to 169 mT, and Hcr values from 4 to 247 mT. The ratios of Mrs/Ms (0.062 to 0.842) and Hcr/Hc (1.1 to 11.5) reflect the mineralogy, and domain sizes of the exsolution lamellae. Some authors favored a magnetization carried by large multidomain lamellae of hematite, but failed to take the Ilm-Hem phase diagram into account properly. The first coarse generation of titanohematite lamellae, if not further exsolved, would be paramagnetic until appx. 300C. However with further exsolution of ilmenite lamellae, and a Ti-depleted hematite host, magnetic interactions develop at lamellar interfaces between CAF hematite and PM ilmenite, resulting in a chemical remanent magnetization (CRM) that we call lamellar magnetism. Thermoremanent magnetization (TRM) experiments made by Rob years earlier that showed the TRM to be far lower than the NRM, supporting a CRM mechanism, such as lamellar formation, for the NRM. Recent experiments with low-temperature demagnetization of the NRM and SIRM by cooling to 10 K or liquid nitrogen temperatures, and rewarming to room temperature in zero field, indicate a self-reversed magnetization at low temperatures. TEM images show exsolution lamellae down to the nanometer scale. Most small lamellae are fully coherent, which could enhance coercivity greatly. Rob's recent work with the concept of lamellar magnetism appears to provide the most accurate interpretation of the magnetization of the Allard Lake samples and also provides a candidate mineral for Martian magnetic anomalies.
DE: 1517 Magnetic anomaly modeling
DE: 1540 Rock and mineral magnetism
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 5410 Composition
DE: 5440 Magnetic fields and magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting