TEMPERATURE DEPENDENT MAGNETIC PROPERTIES OF MAGNETITE-RICH BANDS OF THE 3.25 GA FIG TREE GROUP FROM BARB4 DRILL CORE

The ~3.25 Ga Fig Tree Group (Barberton Greenstone Belt) in South Africa consists of deep to shallow-water shale, greywacke, jaspilitic BIF, and carbonaceous chert. Fe-bearing rocks are generally considered as good targets for paleomagnetic work (thermal demagnetization). However some sedimentary rocks may undergo irreversible chemical transformations upon thermal treatments (e.g. van Velzen and Zijderveld, 1992). During this study, the magnetite-rich bands of BIF of the Fig Tree Group were subjected to temperature dependent rock magnetic tests in order to constrain any mineral change, and in turn to evaluate the use of concerned rocks in paleomagnetism. Samples were retrieved from the Barb4 drill core (Barberton Drilling Project). XRD analyses achieved in South Africa indicate that our samples are mainly composed of quartz > siderite > magnetite > muscovite. Test of thermal demagnetization of IRM (Isothermal Remanent Magnetization) confirms that magnetite (Curie Temperature (CT) is ~580°c) is the dominant ferromagnetic mineral. Test of temperature dependent MS (magnetic susceptibility) was achieved in normal and argon atmospheres. For the heating cycle, our samples show a flat curve between 0 and 500°c; after 500°c, an increase of MS values occurs, with a peak at 550°c; heating at 580-700°c indicates a sudden loss of susceptibility. The cooling curve is higher than the heating, with two peaks at ~580°c and ~490°c (CT of magnetite and maghemite respectively). This is a clear indication of mineral change. Test of stepwise heating and cooling further demonstrates that primary minerals of our samples are altered to low MS phases at runs 100-500°c, and inverted to strongly magnetic phases over 500°c of treatment; mineral alteration is completed at 700°c. Our results warn of difficulties to obtaining primary magnetic remanence from the studied rocks by thermal demagnetization.

References

Van Velzen, A.J. and Zijderveld, J.D.A. 1992. A method to study alterations of magnetic minerals during thermal demagnetization applied to a fine-grained marine marl (Trubi Formation,. Sicily) Geophys. J. Int. 110, 79-90