Ancient secondary magnetizations in rocks

The natural remanent magnetization of many rock formations which show no petrological signs of baking (for example, Bloomsburg formation, Emu Bay shale) includes a secondary component that is not of geologically recent origin. Samples from these formations have been heated for up to 10 hours at constant temperatures (105°C, 200°C, 300°C) in a steady field (1.08 oe) and cooled to room temperature in that field. The remanence acquired is termed ‘viscous partial thermoremanent magnetization’ and may be considered to consist of (a) a PTRM component and (b) a viscous component that increases in intensity with the logarithm of time. Results are broadly consistent with theories of magnetic viscosity. Thermal demagnetization curves show that the rocks have a broad blocking temperature spectrum of magnetic grains. Only the PTRM component is removed by demagnetization (maintaining at maximum temperature in zero field for 1 min) at the temperature of its acquisition; the viscous component may be removed by heating either for a longer time or at a higher temperature. Extrapolation of the experimental data indicates that viscous PTRM acquired in fields of intensity comparable to the earth's field, over periods of the order 103 years and at temperatures not greatly in excess of 100°C, would have comparable intensity and similar demagnetization characteristics to the observed secondary NRM and would be stable for millions of years at normal temperatures. It is suggested that the ancient secondary NRM was acquired under the action of the earth's field for long periods, after which remanence was stabilized owing to increase in effective relaxation time of magnetic grains. Fall in temperature is probably the cause of increase in relaxation time in most of the cases which were studied, but increase in grain size would have a similar effect. Viscous PTRM in rocks with stable thermoremanent NRM is unlikely to reach comparable magnitude to the (supposed primary) thermoremanence.