Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp11a0697u&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #GP11A-0697
Physics
1507 Core Processes (1213, 8115), 1560 Time Variations: Secular And Longer, 1595 Planetary Magnetism: All Frequencies And Wavelengths
Scientific paper
The behavior of the early geomagnetic field is key for investigations of Earth's thermal history, evolution of its atmosphere and the development of life. A major obstacle to obtaining primary remanences from the oldest terrestrial rocks is ubiquitous low-grade metamorphism which has resulted in magnetic mineral alteration and magnetic overprints of varying age. Measurements on pristine single silicate crystals provide the oldest unambiguous paleomagnetic evidence of strong thermoremament magnetization from 3.2 Ga rocks (Tarduno et al., 2007). Specifically, these data indicate that Earth's magnetic field at 3.2 Ga was within 50% of the strength of the modern field. Here, we report rock magnetic and paleomagnetic analysis on ~3.45 Ga rocks from the Barberton Greenstone Belt. The data provides the first unambiguous evidence for a geodynamo at ~3.45 Ga. We focus on an exceptionally well-preserved dacitic intrusion of the Hooggenoeg Formation. This lithology is preserved in situ and as clasts comprising stratigraphically overlying conglomerates. Detailed progressive thermal demagnetization of the conglomerate clasts reveals a common overprint component of low unblocking temperature and a high unblocking temperature component which is directionally distinct for each clast. These data comprise a positive conglomerate test, providing paleomagnetic evidence that the high unblocking temperature component was acquired before the formation of the conglomerate. Theory and empirical data indicate that a thermoviscous overprint induced by low-grade metamorphism and carried by multi-domain grains cannot be completely thermally demagnetized, prohibiting the further use of the bulk rock to constrain the paleointensity of the ancient field. The positive conglomerate test also indicates that the underlying dacitic intrusion could hold a primary magnetization. Single-crystal paleointensity experiments on this lithology are underway.
Cottrell Rory D.
Hofmann Andrea
Tarduno John A.
Usui Yoichi
Watkeys Michael K.
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