Is there a significant long-term evolution in the dipole field intensity since the Precambrian ?

Details Is there a significant long-term evolution in the dipole field intensity since the Precambrian ? Is there a significant long-term evolution in the dipole field intensity since the Precambrian ?

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....6005m&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #6005

Physics

Scientific paper

New determinations of absolute paleointensity studies published during the past few years have significantly improved the database. Of particular interest are the results obtained for the period between 1 and 3 Ga which give us better clue about the initial stages of the geodynamo and could also be potential indicators about the inner core formation. We present a summary of measurements involving 7 mafic dyke swarms from the Superior and Slave Provinces in Canada which were radiometrically dated with ages distributed between 1 Ga and 2.4 Ga. Because small grains of almost pure magnetite dominate the remanent magnetization characterized by a narrow range of very high unblocking temperatures, these dykes carried a very stable original magnetization and were exempt of any alteration. The determinations of paleointensity were successful for one hundred samples using a Thellier-type method. The virtual dipole moments average around a value of 3x1022 A.m^2 . They are found in good agreement with previous and recent results for the same periods. Thus, it seems that the geomagnetic field was significantly lower during these remote periods, i.e. between 3 and 1 Ga, than during the following period. Unfortunately the next 600 Ma remains too poorly documented in the database to observe the pattern and the timing of evolution towards a stronger field. However the number of field determinations within certain time intervals is large enough to plot the evolution of the dipole moment with time. The robustness of the evolution was tested using different age intervals. Twenty sequences of 40 samples within every time interval were also randomly drawn and provided a further conclusive test. In light of these results we will discuss if there are significant long-term trends in the dipole field intensity, which would reflect changes in field generation that in turn could be associated with major phases in the evolution of the planet (formation of the inner core, changes at the core-mantle boundary...).

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