HR: 14:45h
AN: GP33A-03 INVITED     [Abstracts]
TI: Titanomagnetite oxyexsolution and thermochemical remanent magnetization: A warning sign on the road to paleointensity determination?
AU: * Smirnov, A V
EM: aleksey.smirnov@yale.edu
AF: Yale University, 210 Whitney Avenue, New Haven, CT 06511, United States
AB: Data on the long-term evolution of the Earth's magnetic field strength are crucial for understanding mechanisms of geodynamo and constraining models of planetary evolution. Correct interpretation of paleointensity data, however, is ultimately based on our understanding the processes responsible for the formation and preservation of paleointensity signal in rocks. The Thellier method, most commonly used method of paleointensity determination, requires that the paleointensity signal is carried by thermoremanent magnetization (TRM). However, during initial cooling of some rocks, titanomagnetite undergoes the oxyexsolution into fine-scale lamellar intergrowths of low-Ti and high-Ti phases (with magnetite and ilmenite as the end-members). Several lines of evidence indicate that this process may continue at temperatures as low as 500 °C, in which case the resulting remanence is (at least, in part) a thermochemical remanent magnetization (TCRM). The presence of TCRM may seriously afflict paleointensity determinations. For example, it is likely to result in a significant low-field bias in mafic dikes and plutonic rocks. This bias may be responsible for the dominance of low paleointensity values in the Precambrian database, which includes a large portion of data from slowly cooled intrusive rocks. Interestingly, TCRM has been recently suggested as a source of paleointensity overestimates in modern lavas. Understanding how TCRM is acquired in rocks and records the field is a major challenge for the use of many rocks in defining the strength of the geomagnetic field (especially, in the Precambrian). Ongoing and future studies will be discussed, including theoretical estimates of the TCRM/TRM ratio as well as numerical and laboratory modeling of the oxyexsolution process.
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly