COMPARISON OF MAGNETIC HYSTERESIS PARAMETERS OF UNREMAGNETIZED AND REMAGNETIZED LIMESTONES

For white magnetite-bearing Mesozoic pelagic limestones from Italy which carry a ''primary'' magnetization, the values of Mrs/Ms and Hcr/Hc generally lie in the pseudo-single domain (PSD) field of the Day et al. (1977) plot. The logarithmic plot of Mrs/Ms against Hcr/Hc gives a straight line (R=0.814) with slope and intercept close to the empirical mixing line of Parry (1982) for single domain (SD) and multidomain (MD) magnetite. For one of the white pelagic limestone formations (Maiolica Formation), samples with hysteresis ratios closer to the MD field display increased parmagnetic susceptibility and are from the upper part of the formation characterized by increased detrital clay. We therefore associate the increased MD magnetite with increased detrital influx. For pinkish and reddish varieties of the Italian pelagic limestones, the presence of hematite is manifest by high saturation on fields, a wide range of Hcr/Hc, and ''wasp-waisted'' hysteresis loops attributed to the mixing of magnetite and high-coercivity authigenic hematite. The hysteresis ratios for a collection of Paleozoic and Mesozoic remagnetized magnetite-bearing limestones from Britain, Nevada, Alaska and the Appalachians lie mainly outside the PSD field and appear to follow a power law trend. Following Jackson et al. (1993), the high values of Hcr/Hc and the characteristically ''wasp-waisted' hysteresis loops can be interpreted in terms of a fine-grained subspherical high-coercivity SD magnetite mixed with a high proportion of superparamagnetic magnetite. The slope and intercept of the power law relationship for Mrs/Ms and Hcr/Hc in the remagnetized limestones are distinct from those observed for the Italian limestones, and may provide a means of fingerprinting magnetite of ''primary'' as opposed to diagenetic origin.