Effect of grain size and domain state on thermal demagnetization tails

Thermal demagnetization of viscous remanence (VRM) and partial thermoremanence (pTRM) of 20- and 135-mu m natural magnetites reveals a broad spectrum f(T-UB) of unblocking temperatures T-UB, both > and < T-B, the blocking temperature. In contrast, 0.04-mu m single-domain (SD) grains demagnetize sharply at T-UB approximate to T-B. High- and low-T-UB tails of f(T-UB) were wider for 135-mu m multidomain (MD) grains than for 20-mu m PSD grains. 10-mT alternating-field demagnetization rendered the VRM of the 20-mu m magnetite more SD-like in subsequent thermal cleaning, selectively erasing both low- and high-T-UB tails of f(T-UB). Primary and secondary remanences are then cleanly separated, and non-linear Thellier paleointensity determination becomes linear. Our results agree well with (T-B, T-UB) data for 10(5)-year thermoviscous overprints in the Milton Monzonite over the range of log(time) common to the two studies. Because(T-UB)(av) approximate to T-B for PSD and MD as well as SD grains, the Pullaiah et al. paleothermometry method will work if (T-UB)(av) values are used instead of (T-UB)(max) values.