Abstract
Drilling-induced remanent magnetization(DIRM) in drill cores can limit their use for magnetosratigraphic studies and preclude the use of secondary viscous remanence for their azimuthal orientation. DIRM was studied in Ocean Drilling Program(ODP) Leg 118 drill cores. A total of 500.7 m of continuous, vertical, oceanic gabbroic section was recovered during this leg. The gabbros obtained exhibited various degrees of alteration and deformation, which gave us a good opportunity to study the magnetic properties of oceanic gabbros.
DIRM was observed in many of Fe-Ti oxide gabbros and the magnetite-and ilmenite-rich gabbros whose remanence is controlled by grains ranging from multidomain(MD) to single domain(SD). DIRM in ODP Leg 118 cores has the following properties: (1) it is characterized by high intensity and low stability; (2) it is directed up; and (3) it is relatively more dominant and more intense in magnetically less stable MD grains.
The observed DIRM is explained as having been produced during the initial drilling and can be modelled as a pure Isothermal Remanent Magnetization(IRM) acquired in a field of the order of 10 mT and by a strong nonuniform field concentrated near the cutting rim of the drill string.
In ODP Leg 118 cores, DIRM was more effectively cleaned by alternating fields(AF) than by thermal demagnetization, and judicious AF demagnetization was usually successful at defining the primary remanence. The use of a non-magnetic drill string would further reduce, and might possibly eliminate, DIRM production.
