Drilling‐induced remanence in carbonate rocks: occurrence, stability and grain‐size dependence

A strong grouping of the directions of natural remanent magnetization in a collection of Ordovician limestones and dolomites, prior to correction for the in situ orientation of the samples, led us to suspect the presence of a substantial spurious magnetization acquired during sample collection and preparation. A close correspondence between the directions of the remanence vectors and the direction of the ambient magnetic field during sawing and drilling of the samples suggested that the remanence was dominated by a component acquired during cutting of the samples. Ten specimens of the Camp Nelson Limestone and Shakopee Dolomite were demagnetized to 100 mT, given an anhysteretic remanence along their axes, and then sawed again. A substantial magnetization parallel to the ambient field during cutting was acquired by all of the specimens, and the resultant directions deviated by 7‐70d̀ from the direction of the anhysteretic magnetization. Stepwise alternating‐field and thermal cleaning to 60 mT and 400d̀C respectively failed to remove preferentially the cutting‐induced magnetic contamination. Since the fraction of magnetite grains cut during drilling and sawing must be a linear function of grain size, modified Lowrie‐Fuller tests were carried out and the results are interpreted to indicate the presence of a multidomain magnetite fraction in the Shakopee Dolomite, Camp Nelson Limestone and Oregon Dolomite. Ratios of initial to anhysteretic susceptibility (X/XARM) correlate well with the angular deviation of magnetic directions produced by sawing. This indicates that acquisition of drilling‐induced remanence is a function of magnetite grain size, compatible with the notion that the drilling‐induced component resides in large magnetite grains which have been cut.