In this manuscript, we present rock magnetic results of samples recovered during Leg 183. The Leg 183 cores were recovered from six drill sites and display variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Cretaceous subaerial basalt samples with titanomagnetite exhibit a strong Verwey transition in the vicinity of 110 K and have frequency-dependent susceptibility curves that resemble those of synthetic (titano) magnetites. These results are in good agreement with the thermomagnetic characteristics where titanomagnetites with Curie temperatures of ~580ºC were identified. The hysteresis ratios suggest that the bulk magnetic grain size is in the psuedo-single-domain boundary. These subaerial basalts experienced high-temperature oxidation and maintained reliable paleomagnetic records. In contrast, the 34-Ma submarine pillow basalts do not show the Verwey transition during the low-temperature experiments. Thermomagnetic analysis shows that the remanent magnetization in this group is mainly carried by a thermally unstable mineral titanomaghemite. The frequency-dependent relationships are opposite of those from the first group and show little sign of titanomagnetite characteristics. Rocks from the third group are oxidized titanomagnetites and have multiple magnetic phases. They have irreversible thermaomagnetic curves and hysteresis ratios clustering toward the multidomain region (with higher Hcr/Hc ratios).
The combined investigation suggests that variations in magnetic properties correlate with changes in lithology, which results in differences in the abundance and size of magnetic minerals. The rock magnetic data on Leg 183 samples clearly indicate that titanomagnetite is the dominant mineral and the primary remanence carrier in subaerial basalt. The generally good magnetic stability and other properties exhibited by titanomagnetite-bearing rocks support the inference that the ChRM isolated from the Cretaceous sites were acquired during the Cretaceous Normal Superchron. The stable inclinations identified from these samples are therefore useful for future tectonic studies.
1Zhao, X., Antretter, M., Solheid, P., and Inokuchi, H., 2002. Identifying magnetic carriers from rock magnetic characterization of Leg 183 basement cores. In Frey, F.A., Coffin, M.F., Wallace, P.J., and Quilty, P.G. (Eds.), Proc. ODP, Sci. Results, 183 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/183_SR/005/005.htm>. [Cited YYYY-MM-DD]
2Center for the Study of Imaging and Dynamics of the Earth, Institute of Geophysics and Planetary Geophysics, University of California, Santa Cruz CA 95064, USA. xhao@es.ucs.edu
3Institut fur Allgemeine und Angewandte Geophysik, Ludwig-Maximilians-Universitat Munchen, Theresienstrasse 41, 80333 Munchen, Germany.
4Institute for Rock Magnetism, 291 Shepherd Laboratories, University of Minnesota, Minneapolis MN 55455, USA.
5School of Humanity, Environment Policy, and Technology Himeji Institute of Technology Shinzaikehonmachi 1-1-12 Himeji, Hyogo 670-0092, Japan.
Initial receipt: 24 June
2001
Acceptance: 14 May 2002
Web publication:
4 December 2002
Ms 183SR-005