Compaction-induced inclination shallowing of the post-depositional remanent magnetization in a synthetic sediment

Abstract

Summary. A synthetic sediment comprised of kaolinite, distilled water and either equidimensional or acicular magnetite was given a post-depositional remanent magnetization (PDRM) by being stirred in a magnetic field. This sediment was compacted under pressures which varied continuously from 0 to 0.14 MPa in a water-tank consolidometer and to higher pressure steps (~2.53 MPa) in a standard soil consolidometer. Compaction took place in the same magnetic field in which the sample was given its PDRM. The compaction caused shallowing of the sample's magnetic inclination. This shallowing was found to be a function of the sample's initial magnetic inclination and the degree of sample compaction; tan (I,) = (1 - aAv) tan (lo), where IR is the remanent inclination after compaction, AV is the volume change, I. is the initial magnetic inclination, and a is an empirically derived constant. The data show a maximum inclination shallowing of 12 for an initial inclination of 54, in good agreement with the maximum inclination shallowing predicted by the above equation. We propose an electrostatic mechanism to be the cause of the inclination shallowing. In this model positively charged magnetite grains adhere to the surface of negatively charged clay grains with their long dimension parallel to the clay grain's surface. As the clay grains become reorientated due to compaction the easy axes of magnetization are rotated away from the axis of compression. Alternating field demagnetization data reveal that our samples have shallower characteristic magnetizations than their post-compaction NRMs, implying that the smaller, higher coercivity grains are most affected by the compaction process. These data support our model since a surface charge mechanism for inclination shallowing would predict that the smallest magnetic grains would be preferentially affected.