HR: 10:30h
AN: GP12A-01 INVITED     [Abstracts]
TI: On the Conditions for Self-reversal of Remanent Magnetization in Titanomaghemite: Constraints From DSDP Site 307 Basalts
AU: * Doubrovine, P V
EM: pavel@earth.rochester.edu
AF: University of Rochester, Dept. of Earth and Environmental Sciences, Hutchison Hall 227, University of Rochester, Rochester, NY 14627 United States
AU: Tarduno, J A
EM: john@earth.rochester.edu
AF: University of Rochester, Dept. of Earth and Environmental Sciences, Hutchison Hall 227, University of Rochester, Rochester, NY 14627 United States
AB: The possibility of self-reversed chemical remanent magnetization (CRM) carried by titanomaghemite in altered submarine basalts has been discussed in the literature since the late 1950's (Verhoogen, 1956), but no unequivocal example was found in natural oceanic rocks until recently, when we reported a prominent case of partial self-reversal in Late Cretaceous pillow lavas from Detroit Seamount in the northwestern Pacific Ocean (Doubrovine and Tarduno, 2004). Here we present new paleomagnetic, rock-magnetic and compositional data from severely oxidized mid-ocean ridge tholeiitic basalts of Late Jurassic age collected at Mesozoic magnetic anomaly M21 of the Hawaiian magnetic lineations (DSDP Site 307). Through a series of rock-magnetic, X-ray diffraction and energy-dispersive X-ray spectrometry analyses, we show that the magnetic carriers in Site 307 basalts are highly oxidized, pseudo-single domain titanomaghemites. In thermal demagnetization data for the majority of samples, we observed a small, reversed polarity magnetization component unblocking between ~200-275°C, antiparallel to the magnetizations of normal polarity isolated at higher and lower temperatures. However, alternating field demagnetization of thermally untreated sister samples showed only a single, normal polarity magnetization component, suggesting that the apparent component of reversed polarity is an artifact of the inversion of titanomaghemite, rather than the unblocking of a true self-reversed CRM. If this interpretation is correct, the compositions of titanomaghemites from Site 307 basalts constrain the self-reversal field of titanomaghemite to extremely high oxidation states, requiring almost complete maghemitization (z≥0.95) to produce a self-reversed CRM. Further rock-magnetic experiments performed to test this hypothesis will be discussed.
DE: 1519 Magnetic mineralogy and petrology
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly