High-temperature susceptibility of magnetite: A new pseudo-single-domain effect

Abstract

Remanent magnetizations of magnetites between single-domain (SD) threshold size (≈0.1 μm) and ≈20 μm have SD-like intensities and coercivities. This paper shows for the first time that magnetite's induced magnetization also has pseudo-single-domain behaviour. The first part of the paper reports temperature-dependent initial susceptibility data, k0(T), of sized magnetites and assesses their granulometric potential. The second part transforms coercive force data, Hc(T), for the same magnetites into simulated k0(T) curves. The third part considers k0(T) results of coarse-grained mafic rocks as candidate sources of deep-seated magnetic anomalies. High-temperature susceptibility k0 measured with a Kappabridge for eight fractions of crushed natural magnetites (median sizes of 0.6, 1, 3, 6, 9, 14, 110 and 135 μm) shows a progressive increase in the height of the Hopkinson peak below the Curie point as grain size decreases. The trend is systematic and has granulometric potential in the 1-14 μm range. Self-demagnetization should produce almost flat k0(T) in grains larger than SD size but experimentally, well-defined Hopkinson peaks are not limited to the finest grains. 1-μm magnetites have a peak 1.5 times k0 at 20 °C and 14-μm grains have a peak of 1.25. Only 110 and 135 μm grains have T-independent k0. Using an empirical relationship between coercive force Hc and k0, Hc(T) data for the sized magnetites were used to simulate k0(T) results. A hump in the k0 heating curve around 250 °C was traced to annealing out internal strains, evident in Hc data measured in first heatings. For sizes ≤6 μm, observed Hopkinson peaks were smaller than predicted, possibly because of a previously unrecognized grain-size dependence of the empirical constant relating Hc and k0. Two crystalline rocks, a gabbro and a diabase, combine SD-like Hopkinson peaks and multidomain (MD) flat ramps in their k0(T) data. In the diabase, a Hopkinson peak is prominent in separated plagioclase grains containing submicron magnetite, but is masked in whole-rock data. The gabbro has a clear superposition of SD and MD k0(T) functions in its whole-rock data, with a Hopkinson peak of 1.35. If oceanic layer-3 gabbros have similar susceptibility enhancement above 500 °C, they could be more important magnetic anomaly sources than room-temperature k0 measurements on dredged or fault-uplifted samples would suggest.