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Insight Into the Magnetic Mineralogy of Antarctic Rocks

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Insight Into the Magnetic Mineralogy of Antarctic Rocks JOURNALOF GEOMAGNETISMAND GEOELECTRICITY Vol.19,No.3,1967 Insight into the Magnetic Mineralogy of Antarctic Rocks P.J. wasilewski and B.J. Carleton Department of Earth and Planetary Sciences University of Pittsburgh (Received June 15, 1967) Abstract A magnetic powder technique is presented, which is considered to be phase I of a methodology being developed at the University of Pittsburgh, to ultimately relate the mag- netic properties of rocks to the causitive mineralogy. The methodology, termed magnetic petrology, was suggested by an initial study of plutonic rocks from Ellsworth Land, Antarctica. Phase I, the magnetic powder technique utilizes a strained mechanically polished rock surface upon which a magnetite colloid is deposited. After washing the surface with alcohol or acetone, the magnetic minerals or magnetic phases are patterned with magnetite powder ; the non magnetic components remain free of powder. Introduction The magnetization history of rock samples used in paleomagnetic research could be considered the ultimate aim of rock magnetism research. Magnetization history, which is intimately related to the physico-chemical environment of a rock throughout its history begins when the blocking temperature of magnetic minerals is approached and continues until the rock is sampled for paleomagnetic studies. The problem will be somewhat more complicated if solid diffusion (exsolution) is operative at the blocking temperature. A direct correlation is implied between the magnetization history and the geologic history. Rapid progress has been made in rock magnetism research, with respect to instrumen- tation (see, for example, Collinson, et al, ed.,1967) and general understanding of magnetic properties of rocks and minerals, (Nagata, 1965, 1961), and their wide application to earth science (see, for example, Irving,1964). Presently, such measurements as size and shape of magnetic minerals, understanding of intergrowth structures and relationships, and degree of chemical change of the magnetic minerals are becoming increasingly important. Several important studies have been made using reflection-microscopy, (see, for example, Uyeda, 1956; Nagata, et at, 1957; Ozima, et at, 1967; Ozima and Larsen,1967). The present preoccupation of several investigations concerned with the field reversal problem has stim- ulated extensive reflection-microscopy studies (see, for example, Wilson and Haggerty, 1966; Watkins, et at, 1967; Wilson, et at, 1967). Interest in the stability of NRM has further stim- ulated reflection-microscopy (see, for example, Akimoto and Kushiro,1961). * Work supported by U.S. National Science Foundation Grant, NSF GA725, which is gratefully ac- knowledged. (195) 196 R.J. WASILEwsKI AND B.J. CARLETON However, studies of a polished surface using a reflection microscope for these purposes requires extensive experience. Consequently, many geophysicists who need the detailed information about magnetic minerals in rocks have always called upon ore microscopists for assistance. Because of the inherent subjectivity of the reflection technique, even an experienced microscopist can occasionally arrive at wrong conclusions regarding the rela- tionship between mineralogy and magnetic behavior. Basic research objectives become a bit more subtle when Antarctic rock samples are intended for use as paleomagnetic data. The geologic history is mostly unknown, and the structural and petrologic relationships are not vivid. This applies particularly to the Ellsworth Land region (Figure 1). The magnetic crust in Ellsworth Land appears highly variable with magnetic and essentially non magnetic plutons of acid to basic character areally related to acid and intermediate volcanics. There is lack of continuous exposure due to the ice sheet. Preliminary studies of the magnetic properties and representative chemical analyses indicate the following (Wasilewski, 1967): A. A plot of total iron (FeO+Fe2O3) and TItania (TiO2) against silica (SiO2) suggests a simple differentiation trend (Figure 2). B. The oxidation trend is indeterminate in view of limited data. (See figure 2 where Fe2O3/FeO vs. SiO2 is plotted.) C. Modal data (L. Lackey, 1966, written communication) offers no correlation between total iron as in Figure 2 and modal opaque percentage. D. There is no correlation between magnetic susceptibility, remanence, and rock basic- ity. E. Many of the cored specimens do not represent effective dipoles a basic assumption Fig. 1 Outline map of west antarctica locating the research area Insight into the Magnetic Mineralogy of Antarctic Rocks 197 Fig. 2 Lower plo-Filled circles are plot of total iron oxides (Feo+Fe2o3) against silica (SiO2) ; open circles are plot of titania (TiO2) against silica (SiO2) for 15 representative rocks from ellsworth land, antarctica. Upper plot Oxidation index corresponding to rocks in lower plot for paleomagnetic research. Whether the authors like it or not, the vector magnetism in rocks might be used for estima- tion of structural relationship as well as for paleomagnetic studies. This places greater de- mands on the understanding of the continuous physico-chemical environment of the magnetic mineralogy in the rocks. This necessitates a magnetic petrology which would attempt to present a self consistent evaluation of the vector properties of the magnetization within the framework of the Ellsworth Land crustal configuration. This paper is concerned with phase I of this Magnetic Petrology--a magnetic powder technique. The technique is almost identical to one used by Grabovsky, et at, (1960) in which the composition of iron ores was studied. Some of their results were confirmed by the authors' studies in evaluating the reliability of the technique. The Technioue The magnetite powder technique consists of placing a few drops of a magnetite colloid upon the surface of a polished rock specimen. The specimen is then washed with alcohol or acetone and observed. The magnetic grains will exhibit a powder pattern, easily distinguish- ed from the non magnetic grains which are free of powder. The magnetite colloid was prepar- ed according to the Elmore (1938) recipe and resulted in good detail. Grabovsky, et at, (1960) used fine magnetic powders in their studies. Presently, the authors are investigating the use of fine magnetic powders, as well as improved versions of the magnetite colloid (see, for example, Garrood,1962). The specimen usually constitutes the top or bottom of a core approximately one inch in diameter. After smoothing the surface to be polished on 240 and 320 emery paper, the specimen is mounted in quick-set plastic. In 20 to 30 minutes the specimen is ready for sur- face preparation. Initial smoothing is done on 400 and 600 emery paper. It is them high speed 198 R.J. WASILEWSKI AND B.J. CARLETON lapped with 1μgamma alumina. Low speed lapPing with 1μalumina is followed by 0.5μ gamma alumina lapping at high and then at low speed. The time for preparation of most surfaces is about one hour. The prepared surface is washed clean and with water, and then washed with alcohol before deposition of the colloid as a greasy or dirty surface will interfere with the powder pattern. The colloid is placed on the surface to be studied, and then alcohol or acetone is used to remove excess colloid. When the surface is completely dry, it is ready for microscope observations. A photograph is first taken while the surface is patterned with magnetite powder. A second photograph is taken after the surface has been cleaned so that compari- sons can be made. The surface is cleaned without moving the specimen from the stage. To insure that surface defects are not responsible for the observed powder pattern, the surface is then carefully studied. Polaroid pictures were taken at 120x magnification. Reliability of Technioue Precautions were taken to insure that the technique and the information gained were reliable. The strained surface will not yield any inf romation about intrinsic magnetic struc- ture. However, this intrinsic detail is not necessary, and effort was concentrated on deter- mination of reliability and effectiveness of the technique. The following points might be considered as indications of the reality of the powder patterns. A. Grinding is done in a random manner with consequent random strain. Low power magnification of the patterned surface reveals no maze pattern, which is particu- larly evident on the surface of exsolved grains where a uniform powder pattern is observed. B. A large magnetostatic energy is associated with the strained surface and hence, holes or inclusions are distinctly outlined by the powder. Nell spikes do not form, a hole is filled with powder and appears black, while an inclusion, if magnetic, is patterned and, if non magnetic, remains clear. C. Successive applications of the colloid with the surface cleaned between successive applications results in a reproduction of the pattern. D. Where boundaries of magnetic grains are sharp, the powder pattern is sharp, and where they are diffuse, a diffuse powder pattern results. E. In a pyrrhotite ore a banded pattern was observed. Each grain exhibited continuous banding which stopped at the grain boundary. Detailed study of the powder free surface proved convincingly that this banded pattern was due to magnetization contrasts within the grain and not to surface defects due to polishing. This pat- tern was reproduced after successive application of the colloidal magnetite. F. Small surface scratches may or may not interfere with the patterned surface.
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