Chemical Demagnetization of the Natural Remanent Magnetization of Rocks

In 1963, Kawai1) proposed a technique to clarify mineralogical control of the natural remanent magnetization (nrm) of the thin sections of the rock which contained the ferromagnetic minerals such as titanomagnetite-series and also ilmenite-series. He found that first the pure magnetite easely dissolves in concentrated hydrochloric acid and then follows titanomagnetites. The dissolving rate of these ferromagnetic minerals becomes slow as the titanium content increases in titano-magnetite towards ulvospinel. The present author has tried to apply this technique to bulk basalt samples taken from the so-called 'intermixing zone' of both normal and reversed nrm at Kawajiri-Misaki, Yamaguchi Prefecture, West Japan. The contained ferromagnetic minerals of these rocks are mainly magnetites and titanomagnetites2)3). Eight child-specimens were cut4)5) from one fist sized rock sample come from this zone and two in eight have shown the normal nrm and the remainders the reversed nrm. The specimens submitted to the present study were picked up one of each of both the normally and reversely magnetized child-specimens. Moreover the test specimens used were prepared such a way that three thin plates, of the order of 10×20×0.2 mm in size, were cut perpendicularly each other from one fist sized rock sample. Before cutting thus child-specimen was measured of its direction and intensity of nrm by means of an astatic magnetometer. And each thin test plate was also undertaken to check what changes may occur in direction and intensity of the remanent magnetization before and after the etching by the *) This article is a part of the paper which was presented to the 3rd. Seminar on Palaeo-magnetism of U. S.-Japan Scientific Cooperation held at Kyoto on October 27-29, 1966.