Origin and stability of drilling induced remanence

Abstract

To provide reliable palaeomagnetic data, rock specimens must be cleaned of less stable, usually secondary magnetizations. Yet the very process of preparing standard cylindrical specimens with diamond drills and saws can impart to the specimens of some coarse magnetite-bearing rocks a strong magnetization whose stability to alternating field demagnetization may exceed the cleaning field selected using minimum dispersion, stability indices or other criteria. This added magnetization, here referred to as drilling induced remanence (DIR), was studied using both natural (NRM) and artificial thermal (ATRM) remanent magnetization of two granitic plutonic rocks. Although the DIR is less stable than the NRM or ATRM, its coercivity spectrum extends at least to 925 Oe. It originates as a stress-aided viscous magnetization acquired in the direction of the ambient field during cutting of the larger magnetite grains. Because the grains are exposed, they, and the DIR, can be removed by etching with concentrated HCl. This solution, however, may have serious drawbacks if fine-grained magnetite carrying the stable remanence can also be attacked. The best approach is to demonstrate that the particular rocks being studied are immune to DIR acquisition or, if not, to avoid imparting a DIR by preparing specimens in low magnetic fields.