Авторы: Dunlop D.

2007 г.

JGR Solid Earth

    Insight into the size and morphology of assemblages of magnetite particles can be gained by comparing temperature variations of remanence or susceptibility after zero-field cooling (ZFC) and after field cooling (FC) through the Verwey transition around Tv = 120 K. At 10 K a sample is demagnetized following ZFC, but in the FC initial state before warming the sample has a transition cooling remanence (TrCRM) acquired in crossing Tv. The matching transition warming remanence (TrWRM) acquired as a result of heating a demagnetized sample from low temperature across Tv is often called inverse thermoremanent magnetization (ITRM). In TrCRM experiments, initially demagnetized samples were cooled in a 2 mT field from 300 K to 10 K. Magnetization M was measured at 1 K to 5 K intervals, the highest-resolution data being taken between 140 K and 90 K. The field was zeroed at 10 K, and the TrCRM was monitored during zero-field warming back to 300 K. The properties of TrCRMs are generally similar to those of TrWRMs produced by heating a ZFC sample in a 2 mT field from 10 K. In 10 of 12 samples (grain sizes from 0.6 to 135 mm), M of monoclinic magnetite produced by field cooling through Tv exactly equals M of cubic magnetite produced by field warming through Tv, even though the ultimate TrCRM and TrWRM values when H ! 0 are entirely different. Mirror-image symmetry was observed between in-field warming curves tracking the acquisition of TrWRM and zero-field warming curves of TrCRM between 10 and 300 K. The symmetry, with increases in the field-on M curves mirroring decreases in the field-off Mr curves, was almost perfect from 10 to 110 K. Approximate symmetry was also observed between in-field cooling curves tracking TrCRM production and zero-field cooling curves of TrWRM between 300 K and Tv. Detailed study of the properties and mechanism(s) of transition remanences will help clarify why the ZFC/FC method is diagnostic in some instances and not in others.

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