HR: 1340h
AN: GP53A-0953    [Abstracts]
TI: Estimating Paleointensity Reliability Based on the Physical Mechanism of Natural Remanence
AU: * Smirnov, A V
EM: asmirnov@mtu.edu
AF: Michigan Technological University, Department of Geological and Mining Engineering and Sciences, Houghton, MI 49931, United States
AU: Tarduno, J A
EM: john@earth.rochester.edu
AF: University of Rochester, Department of Earth and Environmental Sciences and Department of Physics and Astronomy, Rochester, NY 14627, United States
AB: Data on the long-term evolution of Earth's magnetic field intensity are crucial for understanding the geodynamo and planetary evolution. However, paleointensity remains one of the most difficult quantities to determine. The conventional Thellier method is based on the assumption that the paleointensity signal is carried by non- interacting single-domain (SD) magnetic grains that hold a thermal remanent magnetization (TRM). Most bulk rock samples, however, deviate from this ideal case. This departure, coupled with the desire to tap the relatively plentiful potential record held by bulk rocks has led to the development of reliability criteria that largely rely on the observed NRM/TRM characteristics (Arai plots). While such methods may identify effects such as non-SD behavior and laboratory alteration, they assume that the paleointensity signal is a TRM. However, many paleointensity estimates in the current database are probably held by thermochemical remanent magnetizations (TCRMs) or crystallization remanent magnetizations (CRMs). Common processes that form such magnetizations include subsolidus reactions in magnetic grains during initial lava cooling (e.g., oxyexsolution), subsequent low- temperature oxidation (e.g., maghemitization), and the formation of secondary magnetic phases (e.g., hydrothermal magnetite). If unrecognized, such magnetizations can lead to large paleointensity underestimates or overestimates. In most cases, these processes cannot be identified using the Arai-based reliability controls. We suggest that additional criteria based on the physical mechanisms of recording and preserving the paleointensity signal should be utilized in order to assess the reliability of data. We introduce criteria based on whether the magnetization represents a TRM, TCRM or/and CRM based on rock magnetic and other analytical techniques. While such a categorization is needed to make further progress in understanding the nominal paleointensity signal of bulk rocks, we also recognize that much of the required supporting data is missing from the current database.
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting