Determination of the paleointensity in the early proterozoic from granitoids of the Shumikhinskii complex of the Siberian craton

Details Determination of the paleointensity in the early proterozoic from granitoids of the Shumikhinskii complex of the Siberian craton Determination of the paleointensity in the early proterozoic from granitoids of the Shumikhinskii complex of the Siberian craton

Jun 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006izpse..42..521s&link_type=abstract

Izvestiya, Physics of the Solid Earth, Volume 42, Issue 6, pp.521-529

Physics

1

91.45.Xz

Scientific paper

The extreme scarcity of data on the behavior of the paleointensity H an in the geological past from rocks older than 400 Ma significantly hinders the development of our ideas of the geomagnetic field evolution and the geological history of the Earth as a planet. This work presents H an determinations for the Early Proterozoic using the Thellier method and meeting modern requirements for their reliability. The data are obtained from 1850-Ma rocks of granite intrusions sampled in the south of the Siberian platform. The rocks are virtually unaltered granites and granitoids. The paleointensity was determined on 15 samples; results from 11 samples were found to be suitable for the calculation of H an, which is good for experiments of this type. The common feature in the behavior of the natural remanent magnetization (NRM) is a very narrow interval of blocking temperatures: destruction of (60 90)% NRM often took place between 500 and 550°C. Because of the large thickness of the sampled magmatic body, the paleointensity estimates were corrected for its slow cooling rate. With regard for this correction, the probable value of the virtual dipole moment (VDM) from the given collection amounts to 5 × 1022 A m2. Analysis of all published data obtained by the Thellier method for the Precambrian and satisfying the well-known minimal criteria of reliability showed that the average VDM value is about 2 × 1022 A m2, which is four times smaller than the VDM value of the last million years. This phenomenon can be interpreted in terms of the hypothesis that the solid inner core formed only in Proterozoic and, in its absence, the generation of the geomagnetic field was relatively weak, which yielded a small intensity value of the geomagnetic field at early stages of the Earth’s evolution.

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