The origin of bore-core remanences: mechanical-shock-imposed irreversible magnetizations

Repeated laboratory-induced weak mechanical shocking (c. 0.57 kg m s(-1)) o f marine sandstone samples showing drilling-induced remanence, from commerc ial bore cores from the North Sea and Prudhoe Bay, causes increases in thei r low-field susceptibility (chi) and their ability to acquire an isothermal remanent magnetization (IRM). These enhancements are reduced by some 20 pe r cent by AF demagnetization in 100 mT. Doubling the intensity of the shock doubles the susceptibilities and IRMs acquired. The susceptibility increas e ceases after 300 to 400 shocks for the North Sea samples and 20 to 30 sho cks for those from Prudhoe Bay, while the IRM saturates after 800-1000 and 30-50 shocks respectively. Continental, haematite-bearing sandstones from c ommercial bore cores with no drilling-induced remanence subjected to the sa me shocks do not show these effects. Differences in the magnetic mineralogy of shocked and unshocked marine samples suggest that the magnetic enhancem ent is predominantly due to the creation of pyrrhotite by shock-induced irr eversible crystallographic changes in iron-bearing sulphides. When shocked during commercial drilling, these new ferromagnetic minerals acquire strong chemical (crystalline) remanences, associated with a wide spectrum of grai n sizes, in the magnetic field of the drill string, and these are resistant to both thermal and AF demagnetization. Similar processes are likely in an y situation involving the shock of physically metastable iron-bearing miner als, particularly anoxic sediments. A 5 cm non-magnetic collar between the drill stem and crown should drastically reduce the magnetic intensity of th is effect under commercial conditions, but would not prevent its occurrence .