25. MAGNETIC PROPERTIES OF LEG 158 CORES: THE ORIGIN OF REMANENCE AND ITS RELATION TO ALTERATION AND MINERALIZATION OF THE ACTIVE TAG MOUND1

Xixi Zhao,2,3 Bernie Housen,4 Peter Solheid,4 and Weixin Xu5

ABSTRACT

We present detailed rock magnetic results of sulfide and basalt samples recovered at the Trans-Atlantic Geotraverse (TAG) hydrothermal mound on the Mid-Atlantic Ridge near 26N. The study involves a series of rock magnetic measurements and characterization of 46 minicore samples. These measurements were designed to investigate the origin of the magnetic remanence of the samples and allow an unambiguous identification of magnetic minerals and their distribution in the TAG mound. The Leg 158 cores displayed a multicomponent magnetization nature. No reversed magnetization was observed, which was expected as the mound was formed within the last 140 ka. Variations in magnetic properties correlate to changes in lithology that result from differences in the abundance and size of magnetic particles. Almost all sulfide samples from the TAG drilling areas contain small amounts of pyrrhotite and magnetite as shown by the transition around 40 K and by the Verwey transition in the vicinity of 118 K during the low-temperature cycling. The magnetization shows complexity both on a local scale and on a larger scale, with highly variable inclinations over relatively short distances along the core sections. The available evidence suggests that the remanence of sulfide samples is partly or wholly chemical in origin, probably as a result of heating and alteration related to hydrothermal fluid circulation. The combined investigation suggests that the magnetic properties of the basalt samples from the TAG-4 area are controlled mainly by magnetite and minor amounts of maghemite and hematite. The magnetic minerals of the basalts are of variable particle size, but fall within the pseudo-single domain size range (0.2-14 m). The mean inclination of 20.8 obtained from the basalt samples is a significant departure from the expected 55 value and is surprising. One possible paleomagnetic interpretation of the shallow inclination results would be to propose that some tectonic rotation of the TAG mound has occurred since the basaltic lava flows were erupted.

1Herzig, P.M., Humphris, S.E., Miller, D.J., and Zierenberg, R.A. (Eds.), 1998. Proc. ODP, Sci. Results, 158: College Station, TX (Ocean Drilling Program).
2Institute of Tectonics, University of California, Santa Cruz, CA 95064, U.S.A. xzhao@earthsci.ucsc.edu
3Institute of Geophysics, The Chinese Academy of Sciences, Beijing, China.
4Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55455, U.S.A. (Housen present address: Geology Department, Western Washington University, Bellingham, WA 98225, U.S.A.)
5Department of Geology, Arizona State University, Tempe, AZ 85287, U.S.A.