HR: 0830h
AN: GP21C-0040    [PDF]
TI: Pyrrhotite in Extraterrestrial Materials: Paleomagnetic Implications of its High-Pressure Transition
AU: Rochette, P
EM: rochette@cerege.fr
AF: CEREGE Unviersity Aix-Marseille 3, BP80 Cedex 4, Aix en Provence, 13545 France
AU: * Gattacceca, J
EM: gattacceca@cerege.fr
AF: CEREGE Unviersity Aix-Marseille 3, BP80 Cedex 4, Aix en Provence, 13545 France
AU: Chevrier, V
EM: chevrier@cerege.fr
AF: CEREGE Unviersity Aix-Marseille 3, BP80 Cedex 4, Aix en Provence, 13545 France
AB: In mineralogical and rock magnetic studies of meteorites, pyrrhotite has been somewhat underscored due to misidentification with troilite and to the poor knowledge of its magnetic properties until the nineties. In the common case of pyrrhotite-magnetite (or metal) mixtures, remanence may be mainly carried by pyrrhotite while thermomagnetic curves points toward metal or magnetite. We have now studied 20 (among a total of 28) independent martian meteorites (SNCs) and found that among the 18 strongly magnetic ones (Mrs more than 10 A/m), 9 have their magnetic remanence carried by pyrrhotite. Among the chondrites, rumurutites (R) contain only pyrrhotite, while a number of carbonaceaous chondrites (including Allende) contain mixture with metal or magnetite. Low unblocking temperatures combined with high AF resistance of NRM may point toward pyrrhotite (or other sulfides) as the main NRM carrier in various published studies. A high-pressure transition to non-magnetic state has been pinpointed to 2.8 GPa (Rochette et al., 2003; GRL doi:10.1029/ 2003GL017359). Remanence measurements on well characterized nearly SD pyrrhotite confirm full demagnetization at this pressure, and 3/4 at 2 GPa. As the majority of meteorites (including all SNCs) has been shocked much above this pressure this implies that the pyrrhotite NRM is post-shock even in meteorites considered to be considered to be "unshocked". Indeed the threshold for petrographic shock evidence is 4-5 GPa. This shock demagnetization may also explain the lack of magnetization at several crater radii from the large impact basins on Mars (Hood et al., 2003; GRL doi10.1029/2002GL016657).
DE: 1519 Magnetic mineralogy and petrology
DE: 1533 Remagnetization
DE: 3924 High-pressure behavior
DE: 5440 Magnetic fields and magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting