Determination of Magnetic Polarities of Dredged Basalts: A Limitation of the Method

Magnetic polarity determinations were attempted on 17 unoriented specimens of basalt dredged from 11 locations on Juan de Fuca Ridge and adjacent Heck and Heckle Seamount Chains (Northeast Pacific Ocean). Irving's method involving comparison of NRM and ARM demagnetization curves appears to work consistently only when the NRM has been demagnetized to about 2% or less of its initial intensity and when the ratio of intensity of initial ARM to intensity of residual NRM is high (in the order of 50). For 11 samples meeting these requirements, the polarity determined matches that predicted from the magnetic anomalies in the area of the dredge station. For the other specimens, it is shown that the position of the ARM demagnetization curve was not stabilized under the conditions of the experiments and normally magnetized samples cannot be reliably distinguished from reversed rocks.