Multiphase magnetizations: Problems and implications |
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| Authors: | JL Roy PL Lapointe |
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| Institution: | Geomagnetic Laboratory, Earth Physics Branch, Ottawa, Ont. K1A 0Y3, Canada |
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| Abstract: | Experiments combining different cleaning and analytical techniques indicate that multiphase magnetizations may be quite common. However, these may not be recognized because of limited experimental work. Alternating field (AF) cleaning is often the only treatment applied to igneous and metamorphic rocks; thermal and/or AF cleanings are usually the only treatments applied to sedimentary rocks. In many instances, AF and thermal treatments are limited to 100 mT and 550°C respectively. Investigations based on such limited cleaning often fail to detect some of the phases of magnetization contained in the rock. Failure to detect one or more phases means that valuable data are not recovered and the whole magnetic history of the rock has not been unfolded. Most importantly, the undetected phase might be the initial so that a secondary magnetization can easily be mistaken for the initial with an erroneous interpretation as a result. It is therefore imperative to recognize all phases of magnetizations and it possible to separate them.Procedures that have been used to recognize and unravel multiphase magnetizations are described. These procedures make use of chemical, thermal and AF cleaning treatments, two-stage demagnetization, vector analysis, slicing of specimens and vector diagrams. The combination used depends on the rock studied. For example, it is found that AF followed by thermal treatment can be very useful for the study of igneous rocks; chemical leaching is by far the most effective cleaning technique for the study of red beds. A three-phase model describing the magnetizations process of certain red beds is proposed. The slicing of specimens is used to explain intermediate directions with respect to field reversals. It is shown that graphical representation by vector diagrams can greatly facilitate the interpretation of the results. The examples show that, although a statistically well defined magnetization may be obtained after limited cleaning, it cannot be assumed to be the initial. One must ascertain that another magnetization has not remained undetected. This necessitates detailed and extensive experimental work using and devising new analytical procedures in an attempt to unfold the whole magnetic history of the rock. It is noted that tentative polar paths constructed from results obtained after inadequate experimental work cannot be up-graded by adding more data points of doubtful or unproven quality. The evolutionary process of polar paths is dependent upon increasing the reliability of palaeomagnetic results. |
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