Laboratory Simulation of Magnetization Changes Caused by Burial Metamorphism

Abstract

<p> Magnetization associated with the emplacement or a rock body may be thermo-remanent magnetization (TRM) in an igneous body or detrital-remanent magnetization (ORM) in a sedimentary deposit. At the time or Formation the acquired remanence will tend to lie in the ambient field direction. This primary remanence may not remain unchanged through geologic time. Viscous demagnetization may progressively destroy the remanence in the primary direction or it may be reset as VRM in a later different ambient field direction. The amount or acquired VRM will depend on the temperature the rock is heated to and the length or time the heating lasts as well as the magnetic properties or the remanence carriers.</p> <p> The remanence may also be changed by chemical reactions taking place in the magnetic minerals. The remanence acquired during these chemical changes (CRM) is round, by this work, to be a determining factor in the stability or and initial NRM during thermal remagnetization. </p> <p> Synthetic samples were stored for up to 32 days at 400 C it is possible to access geologic time. To simulate viscous changes over geologic time elevated temperatures applied for laboratory times are related to longer times at lower temperatures using the thermal activation curves. Changes in remanence during the storage were observed at various times throughout the experiment. The remanence in samples with an initial NRM was diminished, remanece in samples with a weak initial NRM increased in intensity but the remanence direction lay in the Field direction after as little as l/2 a day storage time. </p> <p> Thermal demagnetization allows separation of the magnetization on the basis of blocking temperature spectra. During the storage the initial magnetite was oxidized to cation deficent magnetite with significantly higher blocking temperatures than those found in the magnetite. Hysteresis measurements, and thermomagnetic mesurements indicate that this change results primarily From the shirt in Tc due to cation deficency. Apparently the change in T did not significantly affect the mechanisms responsible for blocking remanence but merely shifted the blocking temperatures by a similar amount. In samples with a weak initial NRM the chemical change completely reset the magnetization and in samples stored for times as short as only 8 days the initial remanence direction could not be recovered. </p>