Paleomagnetism of the Astrobiology Drilling Project 8 drill core, Pilbara, Western Australia: implications for the early geodynamo and Archean tectonics

Details Paleomagnetism of the Astrobiology Drilling Project 8 drill core, Pilbara, Western Australia: implications for the early geodynamo and Archean tectonics Paleomagnetism of the Astrobiology Drilling Project 8 drill core, Pilbara, Western Australia: implications for the early geodynamo and Archean tectonics

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v12b..04b&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #V12B-04

Biology

1525 Paleomagnetism Applied To Tectonics: Regional, Global, 1535 Reversals: Process, Timescale, Magnetostratigraphy, 8125 Evolution Of The Earth (0325), 9330 Australia, 9623 Archean

Scientific paper

Paleomagnetic measurements from the Archean Pilbara craton have recently been used to argue for the presence of a substantial magnetic field at 3.2 Ga (Tarduno et al., 2007), as well as for extremely fast plate motions or true polar wander (Strik et al., 2003, Suganuma et al., 2006). Paleomagnetic records in the Archean are fundamentally limited by the scarcity of well-preserved, low metamorphic grade Archean rocks. Where such rocks are exposed, paleomagnetic sampling is often difficult or impossible due to pervasive lightning remagnetization and deep weathering of the cratonic surface. More pristine samples can potentially be obtained from shallow drill cores like those obtained by the Astrobiology Drilling Project (ABDP). We present a paleomagnetic analysis of the ~350 m deep ABDP-8 drill core, which was drilled in the East Strelley greenstone belt and which penetrated the Double Bar Formation of the Warrawoona Group, as well as the unconformably overlying Euro Basalt and Strelley Pool Chert units of the Kelly Group. Full sample orientation (declination and inclination) was achieved through the use of a Ballmark orientation system. A strong drilling overprint was removed for most samples by alternating field demagnetization to 20 mT. Subsequent thermal demagnetization revealed single-polarity magnetic directions within the Euro Basalt and Double Bar Formation carried by magnetite. The directions from these two Formations are statistically different to >95% confidence, which constitutes a positive unconformity test and indicates that the Euro Basalt direction is primary. Upon tilt correction, the ~3.34-3.37 Ga Euro Basalt direction is indistinguishable from the tilt-corrected direction found previously in the ~3.46 Ga Duffer Formation of the Warrawoona Group (McElhinny and Senanayake, 1980). The Euro Basalt direction, if taken at face value, implies small relative motion of the Pilbara Craton from ~3.46 Ga to ~3.34 Ga. This is inconsistent with the apparent polar wander path presented for the ~3.46 Ga Marble Bar Chert Member of the Towers Formation (Suganuma et al., 2006). The lack of reversals in the sequence is consistent with a low reversal frequency in early Earth history, as has been suggested by dynamo models for the Earth with a small inner core (Coe and Glatzmaier, 2006).

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