Авторы: Dunlop D.J.

1983 г.

GJ Int.

    Acquisition and decay of small-field viscous magnetization have been measured as a function of temperature (9—500°C) for four synthetic magnetite dispersions ranging in mean particle size from 0.04 to 0.22 mkm and at room temperature only for an additional six samples (2—100 mkm). Room- temperature viscous magnetization is pronounced in the single-domain range, slight between 0.1 and 5 nm, and again substantial in the intermediate multi- domain range (10-15 mkm). In all experiments, the acquisition rate exceeded the decay rate, resulting in residual undecayed viscous remanent magnetization (VRM) after a zero-field decay time equal to the time of exposure to a field. Viscous magnetization is enhanced at elevated temperatures but not in direct proportion to absolute temperature. Instead, there appears to be a low- temperature (< 20°C) threshold below which viscous effects are negligible and an approach to saturation at high temperature. VRM produced isothermally at a given temperature is significantly more difficult to erase by heating to a higher temperature than most thermal activation theories predict. Furthermore, even in quite brief experiments, the viscosity coefficient S = dJ/d1og t increased with time, the increase being more pronounced at high temperature. The implications of these observations are that viscous overprinting of natural remanent magnetization in magnetite-bearing rocks, particularly during burial or intrusive reheating, may be more extensive and more difficult to erase than previously believed.

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