Magnetic properties of oceanic ridge basalts from the Ocean Drilling Program, Leg 187

*Yen-Hong Shau[1] ,Masayuki Torii [2],Chorng-Shern Horng [3]
Shu-Fang Ou [1],Wen-Tzong Liang [3]
National Sun Yat-Sen University[1]
Okayama University of Science[2]
Institute of Earth Science, Academia Sinica[3]

Magnetic properties of ocean-ridge pillow basalts that were obtained from the Indian- and Pacific-type oceanic crusts in the Southeast Indian Ridge area have shown that the magnetic carrier in these rocks is titanomaghemite, which has formed by maghemitization of primary titanomagnetite. On the contrary, the intercalated massive flows are dominated by less oxidized titanomagnetite as the main magnetic mineral, indicating permeability as a main controlling factor to the maghemitization. The NRM values of the samples from the different drilled sites show a general trend of decreasing NRM with increasing crustal age.

We have studied rock magnetic properties of core samples of oceanic ridge basalts (including Indian- and Pacific-type), which were recovered from 23 drilled holes at 13 sites in the Southeast Indian Ridge (SEIR) area near the Australian Antarctic Discordance (AAD) during the Ocean Drilling Program (ODP) Leg 187. On the basis of the results of thermomagnetic curves, Curie temperatures, and low-T experiments, the dominated magnetic carrier in most of the basalt samples is titanomaghemite, which has presumably formed by maghemitization of primary titanomagnetite. Few samples from nearly unaltered massive flows contain less oxidized titanomagnetite as the main magnetic mineral, which is characterized by irreversible thermomagnetic curves with ~200-250 C Curie points and a transition at ~55 K in the low-T IRM measurements. The titanomaghemite-bearing samples generally show wasp-waisted hysteresis loops and have hysteresis parameters, Mrs/Mr = 0.40 - 0.70 and Hcr/Hc = 1.2 - 2.0, indicating a mixture of single domain (SD) and multidomain (MD) grains. The unaltered massive flows show much lower Mrs/Mr ratios (0.05 - 0.1) and higher Hcr/Hc ratios (2.3 - 2.6), indicating a MD feature. The values of natural remanent magnetization (NRM) for the samples in this SEIR area are in the same order as those for the other oceanic ridge basalts. They show a general trend of decreasing NRM with increasing crustal age. However, the NRM and other magnetic properties do not seem to correlate with the mantle provinces (Indian- versus Pacific-type).