Potassium-argon ages and magnetic properties of some dredged submarine basalts and their geophysical implications

Abstract

Magnetic properties and K-Ar ages of five basalts dredged from the western Pacific and two from the East Pacific rise area were studied. The western Pacific samples have an age of a few ten million years, whereas the eastern Pacific sample has a much younger age of about 3 m.y. Most of the samples have intensities of NRM of about 3 × 10−3 emu/cc and susceptibility of about 10−4 emu/cc. Three of the seven dredged basalts studied here showed self-reversal of thermoremanent magnetization upon heating to 300°C in air. Q ratios range from 5 to 57. Judging from the comparison between the thermal decay of NRM and the production of TRM, the NRM of most samples seems to be of TRM origin. These experiments also suggest that the geomagnetic field intensity when these rocks were formed was not much diffrent from the present one. Both Curie temperature Θ and saturation magnetization Is of the submarine basalts increase markedly when they are heated in air or in vacuum. The Is generally increases by a factor of more than 3. Such irreversible increase of Is and Θ is also studied on the ferromagnetic separates from the experimental Mohole basalt. From thermomagnetic, chemical, and X-ray analyses and microscopic observation, it is suggested that these abnormal increases of Θ and Is are caused by rearrangements of ionic configuration in titanomaghemite. On the basis of these magnetic characteristics and K-Ar ages of the submarine basalts, an interpretation of magnetic anomaly lineation pattern of the ocean floor is presented.