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 rema gnetized 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 probab ly authigenic, Therefore we have studied the hysteresis properties of individual magnetic spherules from two formations, the remagnetized On ondaga (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, Tr enton (Ordovician, New York), and Helderberg (Devonian, New York) form ations and from the unremagnetized Wabash formation, The spherules stu died here vary from about 20 mu m to over 100 mu m in diameter. Nevert heless, on the basis of hysteresis behavior this spherule population s pans the full range of domain states, from truly multidomain (MD) to p seudo-single-domain (PSD) to single domain (SD). Because these spherul es are so large, it is remarkable that more than half display the high ly PSD-like behavior typical of many remagnetized carbonate rocks. Equ ally as remarkable is the lack of dependence on spherule size of vario us hysteresis properties such as coercive force (H-c), remanent coerci ve force (H-cr), the ratio of saturation remanence to saturation momen t (M(rs)/M(s)), and the ratio of remanent coercive force to coercive f orce (H-cr/H-c). These findings dispel the preconception that large sp herules are too magnetically soft to be stable carriers of ancient mag netization. Surprisingly, Onondaga and Wabash spherules yield virtuall y identical suites of hysteresis properties. On the one hand, this hig h degree of similarity could indicate that spherules are irrelevant to remagnetization. More intriguing, however, is that this similarity co uld mean that the Wabash formation, although effectively unremagnetize d, nevertheless contains magnetic mineralogical traces of a regional r emagnetization event. It is significant that for these spherules a bil ogarithmic 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 decreas ing 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 fro m remagnetized carbonate rocks yields M(rs)/M(s) = 0.72 at the SD boun dary. These results could indicate that a large percentage of SD-like spherules, like the SD carriers in remagnetized carbonate rocks, are g overned by magnetocrystalline anisotropy. Finally, the present data po int to values of H-c near 350 Oe both for SD-like spherules and for th e SD grains thought to carry remanence in remagnetized carbonates. Cle arly, PSD- and SD-like spherules share several key rock magnetic prope rties with remagnetized carbonate rocks. Thus spherules still are like ly candidates for carrying the remagnetization signal.