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‐µm natural magnetites reveals a broad spectrum f(TUB) of unblocking temperatures TUB, both > and < TB, the blocking temperature. In contrast, 0.04‐µm single‐domain (SD) grains demagnetize sharply at TUB ≈ TB. High‐ and low‐TUB tails of f(TUB) were wider for 135‐µm multidomain (MD) grains than for 20‐µm PSD grains. 10‐mT alternating‐field demagnetization rendered the VRM of the 20‐µm magnetite more SD‐like in subsequent thermal cleaning, selectively erasing both low‐ and high‐TUB tails of f(TUB). Primary and secondary remanences are then cleanly separated, and non‐linear Thellier paleointensity determination becomes linear. Our results agree well with (TB, TUB) data for 105‐year thermoviscous overprints in the Milton Monzonite over the range of log(time) common to the two studies. Because (TUB)av ≈ TB for PSD and MD as well as SD grains, the Pullaiah et al. paleothermometry method will work if (TUB)av values are used instead of (TUB)max values.