HR: 10:50h
AN: GP22A-03    [Abstracts]
TI: Thermoviscous Magnetization in Planetary Lithospheres
AU: * Dunlop, D J
EM: dunlop@physics.utoronto.ca
AF: Physics Dep't, Univ. of Toronto - Mississauga, Mississauga Rd. N., Mississauga, ON L5L 1C6 Canada
AB: The nature of magnetic anomaly sources in the deep crust or uppermost mantle remains poorly constrained. I investigate the rock magnetic character of potential source rocks and the relative importance of induced, thermoviscous, and remanent magnetization, starting from first principles. Neel single-domain theory gives a useful picture of thermoviscous effects. It can be adapted to describe multidomain grains provided self-demagnetization is taken into account. Lower crustal rocks from deep drilling or uplifted crustal blocks have multidomain magnetite as their principal magnetic phase. Measured at room temperature, both remanent and induced magnetizations are in the range 0.1-0.5 A/m, but at high T in the lower crust, induced magnetization remains constant or increases while remanence decreases by a factor 2-4. Thermoviscous magnetization produced over the Brunhes chron likely causes a 30-60% increase compared to short-term induced magnetization produced by the present Earth's field. Data on thermoviscous effects at high T are mainly for synthetic magnetites but one set of data for serpentinized peridotites implies that these rocks in situ in the uppermost oceanic mantle could be sources of both broad-scale induced magnetic anomalies and of lineated remanent anomalies. Mars is a special setting because although most of its surface is magnetically barren, the remaining areas have magnetic anomalies more intense than any on Earth. Mars lacks a present-day field so that remanent sources magnetized in an ancient field before 4 Ga and occupying a substantial fraction of the crust must be responsible for the anomalies. Many minerals are possible, including magnetite, hematite, titanohematite and pyrrhotite. The brief (100-200 Ma) existence of a dynamo field combined with slow cooling prior to 4 Ga result in a narrow T window for TRM acquisition in the juvenile crust. Only limited ranges of crustal depths likely acquired a stable TRM. On the other hand, subsequent cooling of only 20-25 C stabilized this TRM for the rest of Mars' history.
DE: 1517 Magnetic anomaly modeling
DE: 1519 Magnetic mineralogy and petrology
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting