HR: 1340h
AN: GP13A-0032    [Abstracts]
TI: Defining the Magnetic Field of the Early Earth Through Rock Magnetic and Paleomagnetic Analyses of Single Silicate Crystals
AU: * Bauch, D G
EM: bauch@earth.rochester.edu
AF: Dept. of Earth & Environmental Sciences, University of Rochester 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Tarduno, J A
EM: john@earth.rochester.edu
AF: Dept. of Earth & Environmental Sciences, University of Rochester 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Cottrell, R D
EM: rory@earth.rochester.edu
AF: Dept. of Earth & Environmental Sciences, University of Rochester 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Watkeys, M K
EM: watkeys@ukzn.ac.za
AF: School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041 South Africa
AB: The current uncertainty on the age of the inner core, and its role in the geodynamo, highlights the need for improved paleomagnetic constraints based on Proterozoic to Archean-age rocks. However, most of the rocks available for sampling have seen low-grade metamorphic conditions; extreme care is needed in selecting suitable samples, conducting rock magnetic and paleomagnetic analyses, and interpreting the results. David Dunlop's many contributions in rock magnetism, from efforts to understand the time-temperature characteristics crucial for the preservation of magnetizations, to more recent work defining the domain state and recording characteristics of mafic minerals separated from dikes, have greatly assisted our efforts to learn more about the early magnetic field. Here we present new rock magnetic, paleomagnetic and paleointensity data from single silicate crystals separated from plutonic rocks of the Kaapvaal Craton of southern Africa. Magnetic hysteresis data demonstrates that different silicate minerals from these rocks have magnetic inclusions with vastly different magnetic domain states, suggesting that their potential to preserve primary magnetizations should vary considerably. In particular hornblende carries multidomain inclusions, whereas quartz and microcline have single to pseudo-single domain inclusions. Warming of an SIRM acquired at low temperatures (data acquired using the MPMS at the IRM) shows the Verwey transition for quartz and microcline crystals, indicating the presence of magnetite. We also will present joint paleomagnetic and paleointensity data derived from oriented crystals obtained using a stepwise CO₂ laser heating approach, and field tests of the age of magnetization. These analyses will be used to discuss the strength of the mid-Archean field (3.0-3.6 Ga), its geometry and variation, and the implications for magnetic shielding in the early Earth.
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005