Magnetic characterization using a three-dimensional hysteresis projection,illustrated with a study of limestones

Limestones provide an important source of palaeomagnetic information despit e their low content of submicroscopic remanence-bearing minerals. The chief sources of these minerals are thought to be clastic volcanic magnetite and titanomagnetite, and organic magnetite, the latter mostly from bacterial s ources. Chemically remagnetized limestones carry magnetite or pyrrhotite. T hree hysteresis properties prove useful in identifying and characterizing t hese mineralogical influences on limestones: the ratio of zero-field maximu m remanence to saturation remanence (M-r/M-s) in an applied field, coercivi ty of remanence (B-cr) and coercivity (B-c). To a lesser extent K-f/M-s may be useful, where K-f is the ferrimagnetic susceptibility. Traditionally, t hese have been plotted on a combination of 2-D graphs that of necessity onl y preserve two variables (Day et al. 1977; Wasilewski 1973). However, we fo und that magnetic discrimination and characterization of the limestones was much easier on a three-axis hysteresis projection that preserves the value s of B-cr , B-c and M-r/M-s as independent variables. Using logarithmic sca les, the regression surfaces through the data become almost planar and dist inguish pelagic, shallow marine, shelf and remagnetized limestones on the b asis of the slope and intercept of the associated regression surface. Clear ly, there are sensitive sedimentological, geochemical or organic influences that dictate the magnetic mineralogy through sedimentary environment. More over, the 3-D plot of hysteresis criteria affords easy recognition of remag netized limestones and may permit the rejection of material unsuitable for palaeomagnetic study. The 3-D hysteresis projection may be useful for the c haracterization of other rocks and magnetic materials.