Hysteresis properties of magnetic spherules and whole rock specimens from some Paleozoic platform carbonate rocks

It is widely established that many Paleozoic carbonate rocks were remagnetized during the late Paleozoic Kiaman reversed superchron. Yet the paleomagnetic recorders of this event have remained elusive. Magnetic spherules have been candidates for this role, because they are probably authigenic, Therefore we have studied the hysteresis properties of individual magnetic spherules from two formations, the remagnetized Onondaga (Devonian, New York) and the effectively unremagnetized Wabash (Silurian, Indiana). Also, we have compared the hysteresis properties of spherules to those of remagnetized limestones from the Onondaga, Trenton (Ordovician, New York), and Helderberg (Devonian, New York) formations and from the unremagnetized Wabash formation, The spherules studied here vary from about 20 mu m to over 100 mu m in diameter. Nevertheless, on the basis of hysteresis behavior this spherule population spans the full range of domain states, from truly multidomain (MD) to pseudo-single-domain (PSD) to single domain (SD). Because these spherules are so large, it is remarkable that more than half display the highly PSD-like behavior typical of many remagnetized carbonate rocks. Equally as remarkable is the lack of dependence on spherule size of various hysteresis properties such as coercive force (H-c), remanent coercive force (H-cr), the ratio of saturation remanence to saturation moment (M(rs)/M(s)), and the ratio of remanent coercive force to coercive force (H-cr/H-c). These findings dispel the preconception that large spherules are too magnetically soft to be stable carriers of ancient magnetization. Surprisingly, Onondaga and Wabash spherules yield virtually identical suites of hysteresis properties. On the one hand, this high degree of similarity could indicate that spherules are irrelevant to remagnetization. More intriguing, however, is that this similarity could mean that the Wabash formation, although effectively unremagnetized, nevertheless contains magnetic mineralogical traces of a regional remagnetization event. It is significant that for these spherules a bilogarithmic plot of M(rs)/M(s) versus H-cr/H-c does not define a simple linear trend. Instead, M(rs)/M(s) increases more rapidly with decreasing H-cr/H-c for spherules in the PSD range than in the MD range, When a power law is fit solely to the hysteresis ratios of PSD-like to SD-like spherules, extrapolation to the SD limit gives M(rs)/M(s) = 0.75. Similarly, a power law fit to the hysteresis ratios obtained here from remagnetized carbonate rocks yields M(rs)/M(s) = 0.72 at the SD boundary. These results could indicate that a large percentage of SD-like spherules, like the SD carriers in remagnetized carbonate rocks, are governed by magnetocrystalline anisotropy. Finally, the present data point to values of H-c near 350 Oe both for SD-like spherules and for the SD grains thought to carry remanence in remagnetized carbonates. Clearly, PSD- and SD-like spherules share several key rock magnetic properties with remagnetized carbonate rocks. Thus spherules still are likely candidates for carrying the remagnetization signal.