Palaeomagnetic cleaning strategies

P. W. Schmidt*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    63 Citations (Scopus)

    Abstract

    Cleaning is a term used in palaeomagnetism to refer to laboratory methods designed to demagnetize preferentially the less stable components of magnetization. Although the diversity of methods suggests that there is a wide choice available, this is illusory. Except in simple cases, the wrong cleaning strategy may yield misleading results. Cleaning methods include thermal, magnetic (alternating field or a.f.), low-temperature, chemical, shock and microwave cleaning. A cleaning strategy involves one or more cleaning methods applied in a specific sequence. The design of a strategy is made easier with the a priori knowledge of magnetic mineralogy and granulometry, or in other words, rock magnetic properties. Although not a substitute for other rock magnetic experiments, the variation of low-field susceptibility with temperature (k-T) not only provides information on magnetic mineralogy and granulometry, but it also draws attention to chemical alteration that may occur during thermal cleaning. Apart from the rapid, though reversible, decrease of k associated with Curie temperatures, other diagnostic k-T features include the low-temperature (about -140°C) peak typical of multidomain (MD) magnetite, the high-temperature (Hopkinson) peak typical of fine-grained magnetite (and other magnetic minerals) and the classic lepidocrocite-maghemite-haematite profile, which is irreversible. Examples are given where the remanence caused by lightning can be effectively eliminated only by applying a.f. or low temperature (LT) pre-treatment. Such remanence may be an isothermal remanent magnetization (IRM), or a combination of IRM and anhysteretic remanent magnetization (ARM). As the unblocking temperature of relatively low-coercivity MD grains may extend to high-temperature, it is often desirable to suppress MD remanence. An example is given of palaeointensity determination, with the without LT pre-treatment. The nature of the LT transition is briefly addressed in the light of a rare double low-temperature peak, which may reflect both the isotropic point and the Verwey transition.

    Original languageEnglish
    Pages (from-to)169-178
    Number of pages10
    JournalPhysics of the Earth and Planetary Interiors
    Volume76
    Issue number1-2
    DOIs
    Publication statusPublished - 1993

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