CHANGES IN MAGNETIC REMANENCE DURING SIMULATED DEEP SEDIMENTARY BURIAL

Macroscopic hydrostatic compaction of granular rocks causes grain-scale differential stresses as the externally applied load is transmitted through grain contacts. We have compacted rock analogues containing calcite and one of two types of magnetite, bonded with Portland cement. The first type of magnetite is chemically precipitated to give grain sizes in the range 20 nm to 2 mum; these particles were stress-free before compaction. The second type of magnetite was crushed and sieved to a mean grain size of 40 mum; these particles began the experiments in a pre-stressed state. Compaction under confining pressures up to 220 MPa (equivalent to sedimentary compaction at up to 8 km depth) produced strong irreversible changes in the coercivity (and to some extent in other hysteresis parameters) of the samples with initially stress-free magnetite. In contrast, the pre-stressed magnetite exhibited only minimal changes. Composite isothermal remanent magnetisations with orthogonal components in the coercivity ranges 0-30 mT and 30-600 mT were applied prior to compaction. For both sets of samples, the low coercivity component was preferentially progressively demagnetised with increasing compaction stress. This was most efficient for the initially stress-free magnetite. The high coercivity component showed weaker decreases and some spurious increases but there was essentially no change for the samples with pre-stressed magnetite. The changes in magnetic properties of the chemically precipitated magnetite are attributed to the development of dislocation-related impediments to domain wall translation. In contrast, the defect density of the pre-stressed magnetite was acquired under higher differential stresses when it was initially crushed and this was unmodified by the lower experimental stresses. These results may be relevant to the changes expected during rapid sedimentary burial in the absence of pore fluids at low geothermal gradients. One might predict that sedimentary burial could progressively clean out early, soft components of natural remanence, particularly if the magnetite was low in defects (e.g. authigenic).