Opening Remarks for Symposium on Rock- and Palaeo-Magnetism
(IUGG Kyoto Symposium on Rock- and Palaeo-Magnetism)
By Takesi NAGATA Organizer of the Symposium, GeophysicalInstitute, University of Tokyo
In connexion with the International Conference on Magnetism and Crystallography, which is held on 25th-30th September 1961 at Kyoto under auspices of the International Union of Pare and Applied Physics (IUPAP) and the International Union of Crystal- lography (IUCr), an international symposium on rock-and ppalaeo-magnetism is now held at Kyoto. This symposium is organized according to request by a number of active workers in this branch of science and by many members of the Committee on Secular Variation and Palaeomagnetism of the Association of Geomagnetism and Aeronomy (IAGA) of the International Union of Geodesy and Geophysics (IUGG). This symposium is, therefore, formally sponsored by IUGG, and may be called the "IUGG Kyoto Symposium on Rock - and Palaeo-Magnetism in 1961." We had a scientific session on "Physical Problems of Rock Magnetism" as a part of the Conference on Magnetism and Crystallography. In the session several basic problems of physical characteristics of fine grains of rock-forming metallic oxides are mainly discussed in view of solid state physics. Two main topics in that session were self-reversal of remanent magnetization and superparamagnetism and super- antiferromagnetism of very fine grains of metallic oxides. (The full papers presented to this session will be published as a part of the proceedings of the whole conference, a special volume of the Journal of Physical Society of Japan). In the present rock- and palaeo-magnetism symposium, various important topics in rock-magnetism and palaeomagnetism will be discussed more generally in connexion with geophysics and geology as well as solid state physics of materials. It is our great pleasure that this Symposium can give an opportunity for the first international meeting for joint discussions on palaeogeophysics from both viewpoints of palaeo- magnetism and palaeoclimatology. It seems that general basis of rock-magnetism and consequently that of palaeo- magnetism have been nearly established, though there remain of course a large number of unsolved problems. One of basic problems may be general characteristics of all magnetic minerals in rocks and another may be various origins of remanent magnetism of rocks and its stability. As for rock-forming ferrimagnetic (including parasitically ferromagnetic) minerals, it has been known that minerals having chemical composition of FeO-Fe2O3-TiO2
(63) 64 T. NAGATA ternary system are exclusively predominant in their frequency of occurrence in nature.
As well known, the titaniferous magnetites having composition of xFe2TiO4・(1-x)Fe3O4 are the most common magnetic minerals in rocks. In addition to them, it has been established that the hematite-ilmenite series of yFeTiO3・(1-y)Fe2O3 are also ferrimagnetic when 0.45
An important fact recently found with the FeO-Fe2O3-TiO2 system is that natural occurrence of ferrimagnetic minerals of inverse spinel type crystal structure having chemical composition between the Fe3O4-Fe2TiO4 line and the Fe2O3-FeTiO3 line in the ternary system is very common, and that these minerals which can be produced
by oxidizing the titaniferous magnetites are magnetically unstable owing to the lattice
defect in their crystal structures. Actually, even titaniferous maghemites, which are yFeTiO3・(1-y)Fe2O3 in chemical composition but are of cubic crystal structure, are contained occasionally in geologically oxidized rocks. Every experimental and observed evidence shows that the most unstable remanent magnetization of rocks so far reported can be attributed to predominance of the unstable magnetic minerals in those rocks. Another important progress in rock-magnetism may be concerned with discovery of various different processes of producing remanent magnetization of rocks. It has been fairly well known hitherto that TRM and DRM are predominant origins of stable NRM of rocks, and that IRM must be avoided in palaeomagnetic research. In addition to these three kinds of remanent magnetization, four other processes causing remanent magnetization of rocks have recently been confirmed; they are chem.cal (or crystalline) remanent magnetization (CRM), viscous remanent magnetization (VRM), piezo (or pressure) remanent magnetization (PRM), and anhysteretic remanent magnetization (ARM). CRM seems to be a very important source of NRM of rocks, because CRM is such a phenomenon that fine ferrimagnetic phases exsolved in non-ferrimagnetic minerals by chemical changes in a magnetic field H acquire remanent magnetization parallel to H and proportional to H as far as it is not large, and on the other hand so many rocks in nature show evidence for their chemical change during their geologic history. It seems likely that the basic process of producing of CRM has been roughly under- stood. However, we still know very little of various possible modes of exsolution of ferrimagnetic phases which might take place in rocks extremely slowly during a long period. Studies on CRM, including those on relevant chemical reaction processes, should therefore be highly appreciable in future. The viscous magnetization has been understood to be gradually produced with time t in a ferro- or ferrimagnetic material put in a magnetic field H, it being ex- pressed as S log t, where the viscosity coefficient S is a function of H and temperature Opening Remarks for Symposium on Rock- and Palaeo-Magnetism 65
T as well as magnetic characteristics of the material. After removing the magnetic field, VRM having intensity proportional to S log t remains and it decreases with time in a field-free space with rate of (S/2) log t. In palaeomagnetism where a very large value of t, say for example 106～109 years, is concerned, VRM may not be ignored in some cases. Characteristics of S has been studied, as dependent on T and H, for various ferrimagnetics of different grain sizes. The results have shown that a possible effect of VRM must be taken into account especially in case of DRM of sediments. PRM may also be an important factor of the remanent magnetization of rocks, because the most rocks in situ have been subject more or less to the pressure or stress during their geologic history. Possibility of PRM could be easily expected from existing knowledge of magneto-striction phenomenon of any kind of ferro- and fern-magnetic material. However, only few systematic studies have so far been made on this problem, and their results are not always consistent among each other. Systematic and ex- tensive studies on PRM should therefore be a significant subject in rock-magnetism, though their experimental procedures may contain much difficulty. ARM is an interesting phenomenon as a laboratory process for acquiring remanent magnetization, but it does not seem to be essential in NRM of rocks in situ except several particular cases. Summarizing the above-mentioned results of studies so far made on origin and stability of NRM of rocks, it may be said that the outline of various processes of producing NRM of rocks has been fairly well understood on basis of solid state physics and mineralogy, but it is still far from exact knowledge of details of the phenomena, with which palaeomagnetism can stand upon exact rigid basis. It may be worthwhile to note again in the end of this opening remark that the absolute determination of age of rocks is now extremely important and necessary in the present stage of palaeomagnetism, as already been recommended in the latest assembly of IUGG. In drawing any diagram of palaeomagnetic expression, we now need reliable time scales on the abscissa which can be as accurate as figures of mag- netic data on the ordinate of the same diagram. It will therefore be recommended that the age determination works must be carried out as well as possible together with rock-magnetic ones in palaeomagnetic research hereafter.
DISCUSSION E. Thellier: I don't like so much the name of chemical remanent magnetization. I prefer, with the same letters CRM, crystalline remanent magnetization, because the process may be purely a physical one.
T. Nagata: Somebodies are calling CRM chemical remanent magnetization. So we may call either crystalline (according to the first discoverer, late Prof. Ch. Maurain) R.M. or chemical R. M. It may be better called simply CRM.