THERMAL DEMAGNETIZATION OF VRM AND PTRM OF SINGLE-DOMAIN MAGNETITE - NO EVIDENCE FOR ANOMALOUSLY HIGH UNBLOCKING TEMPERATURES

Stepwise thermal demagnetization of viscous remanent magnetization (VR M) and partial thermoremanent magnetization (pTRM) of single-domain ma gnetite with mean grain size 37 nm reveals only minor deviations from the predictions of Neel theory. Three different initial states were us ed, following (1) alternating field demagnetization to 100 mT, (2) the rmal demagnetization to 610-degrees-C, and (3) modified thermal demagn etization, consisting of zero-field heating to 610-degrees-C followed by zero-field cooling to the VRM or pTRM acquisition or blocking tempe rature, T(B). VRMs were produced by applying a 0.1 mT field for 3.5 hr , starting from initial states 2, 3 and 1, in that order, at T(B)=283- degrees-C and from states 3, 2 and 1 at T(B)=404-degrees-C. In a final experiment, pTRM was produced by applying 0.1 mT during cooling from T(B1)=414-degrees-C to T(B2)=404-degrees-C, starting from state 3. In all but one case, VRM began to unblock significantly (90% of initial i ntensity) almost-equal-to 15-degrees-C below T(B), dropped to almost-e qual-to 50% at T(B), and then tailed off, dropping below 10% almost-eq ual-to 40-degrees-C above T(B)=283-degrees-C or almost-equal-to 25-deg rees-C above T(B)=404-degrees-C. Partial TRM behaved in the same way, except that demagnetization was essentially complete by T(B1)=414-degr ees-C. The maximum unblocking temperatures (T(UB)) observed are exactl y as predicted by the, theory of Pullaiah et al. (1975). Although very long acquisition times cannot be tested, we find no evidence for anom alously high T(UB)s of VRM in single-domain grains. In previously repo rted cases of rocks with unexpectedly high T(UB)s, the VRM was probabl y carried by multidomain grains.