HR: 1340h
AN: B53B-0988    [Abstracts]
TI: Paleomagnetism of the early Archean Marble Bar Chert Member, Pilbara craton, Western Australia: Implication for Archean geomagnetic reversals and Paleogeography
AU: * Suganuma, Y
EM: suga@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, the university of Tokyo, 1-15-1, Minamidai, Nakano-Ku, Tokyo, 164-8639 Japan
AU: Niitsuma, S
EM: greigite@ganko.tohoku.ac.jp
AF: Tohoku University, 6-3, Aramaki Aza Aoba, Sendai, 980-8578 Japan
AU: Hoashi, M
EM: hoashi@chan.co.jp
AF: Kagoshima University, 1-21-30, Korimoto, Kagoshima, 890-0065 Japan
AU: hamano, Y
EM: hamano@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Niitsuma, N
EM: senniit@ipc.shizuoka.ac.jp
AF: Shizuoka University, 836,Ohtani, Shizuoka, 422-8529 Japan
AU: Hisamitsu, T
EM: hisa@cc.kochi-u.ac.jp
AF: Institute for Research on Earth Evolution, 200, Otsu-Mononobe, Nankoku, 780-8502 Japan
AU: Kodama, K
EM: kdma@cc.kochi-u.ac.jp
AF: Center for Advanced Marine Core Research, Kochi University, 200, Otsu-Mononobe, Nankoku, 780-8502 Japan
AU: Nedachi, M
EM: nedachi@sci.kagoshima-u.ac.jp
AF: Kagoshima University, 1-21-30, Korimoto, Kagoshima, 890-0065 Japan
AB: We conducted Paleomagnetic study on the early Archean rocks in Pilbara Craton, Western Australia. The Pilbara craton is the one of the best exposed and complete record during Archean evolution of the Earth. Archean Biosphere Drilling Project (ABDP) had drilled a continuous 250 m long oriented core from the Towers Formation in north-eastern part of the Pilbara craton. This drilling was essential to obtain the fresh samples from below the influence of intense weathering. Lithofacies of the core are divided into basalt/dolerite and various colored chert named the Marble Bar Chert Member. The Marble Bar Chert Member, dated between 3463 to 3454 Ma (Thorpe et al., 1992), is a world famous early Archean unit with little metamorphism. Initial magnetic measurements of the whole core revealed that the black or red layers in the Marble Bar Chert Member display strong Natural Remanence Magnetization. Petrographic observations revealed that the black and red layers are preserving the primary structure of sedimentation. Hematite, magnetite, and siderite were separately precipitated as primary minerals. Rock magnetic analysis indicates an existence of the large amount of magnetite associated with minor amount of hematite. Through the thermal demagnetization, we had found that the remanent magnetizations define two components: (1) a mid-temperature (<$450\deg$C) component; (2) a high-temperature component (450 to $575\deg$C) with stable decay going to origin. This results indicate that the main magnetic carrier of remanence likely to be magnetite, which is consistent with petrologic and rock magnetic analysis. Characteristics Remanent Magnetization directions of the 2nd component are put on nearly opposite side of the hemisphere. The polarity transition from normal to reverse recorded in continuous stratigraphic section. Petrologic, rock magnetic, and paleomagnetic analysis suggested that the Marble Bar Chert Member is probably preserving the primary magnetization. This result may suggest that the geomagnetic field has reversed at 3.46 Ga, which is the oldest observed geomagnetic reversal. The previous oldest geomagnetic reversal was reported from the Kaap Valley pluton of South Africa at 3.2 Ga (Layer et al., 1996). Our result is at least 260 m.y. older than that reported geomagnetic reversal. We also present a new paleomagnetic pole at 3.5 Ga and argue the APWP and paleogeography before 1.7 Ga. In contrast to the paleomagnetic poles from McElhinny and Senanayake (1980), our paleomagnetic pole is located in southern hemisphere, near the other poles about 2.7 Ga. Therefore, two possibilities should be considered: (1) our paleomagnetic pole is over print, which might be recorded about 2.7 Ga; (2) our paleomagnetic pole is primary, and recording the geomagnetic filed at 3.5 Ga.
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
DE: 1540 Rock and mineral magnetism
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting