Abstract
Here we report on a self-reversal of magnetization at cryogenic temperatures (25–60 K) due to N-type thermomagnetic behavior in samples of Cretaceous mid-oceanic ridge basalt recovered from Pacific Deep Sea Drilling Project Site 166. The magnetic carrier is highly oxidized single to pseudo-single domain titanomaghemite with a composition (0.57 ≤ x ≤ 0.66, 0.8 ≤ z ≤ 0.9) consistent with model predictions of ionic reordering. The observation of N-type magnetization provides further insight into the compositional field in which ionic reordering and self-reversal can occur in oceanic basalt. Specifically, our observations support the original models of Verhoogen [Verhoogen, J., 1956. Ionic reordering and self-reversal of magnetization in impure magnetites. J. Geophys. Res. 61, 201–209; Verhoogen, J., 1962. Oxidation of iron–titanium oxides in igneous rocks. J. Geol. 70, 168–181] and O’Reilly and Banerjee [O’Reilly, W., Banerjee, S.K., 1966. Oxidation of titanomagnetites and self-reversal. Nature 211 (5044), 26–28] indicating that self-reversal above and below room temperature is limited to relatively high titanium contents ( x > 0.6) and high oxidation states.
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