Mathematics – Logic
Scientific paper
Jun 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003georl..30k..36d&link_type=abstract
Geophysical Research Letters, Volume 30, Issue 11, pp. 36-1, CiteID 1582, DOI 10.1029/2003GL017268
Mathematics
Logic
7
Geomagnetism And Paleomagnetism: Paleomagnetism Applied To Tectonics (Regional, Global), Geomagnetism And Paleomagnetism: Paleomagnetism Applied To Geologic Processes, Geomagnetism And Paleomagnetism: Rock And Mineral Magnetism, Geomagnetism And Paleomagnetism: Instruments And Techniques
Scientific paper
Magnetites with sizes from 1 μm to 135 μm were cooled in zero field and their magnetizations M(T) measured continuously. M(T) changed reversibly in cooling from T 0 = 300 K to 200 K, and in subsidiary warming-cooling cycles T i -> T 0 -> T i for any T i. Changes in M(T) in cooling from 200 K to 130 K were largely irreversible due to decreasing magnetocrystalline anisotropy which promotes wall unpinning and domain nucleation. Low-temperature demagnetization (LTD) is almost complete by 130 K in 20-135 μm magnetites but in 1-14 μm magnetites further LTD occurs on cooling to 120 K as magnetocrystalline easy axes change and domains reorganize at the Verwey transition. The observed irreversible changes are the basis of stepwise LTD as a method of paleomagnetic ``cleaning.'' Decrements ΔM in remanence due to cooling are most accurately measured at T 0, requiring a set of warming-cooling cycles T i -> T 0 -> T i. A less accurate method, continuous LTD, measures decrements M(T i) - M(T i-1) from the main cooling curve below 200 K, without intermediate warming-cooling cycles; this requires remanence measurements at T i < T 0. Stepwise or continuous LTD curves M(T i) discriminate among remanence types and grain sizes. The signal of finer (PSD) grains is enhanced compared to coarser (MD) grains. Analogous to the Lowrie and Fuller [1971] test, the inverse thermoremanence (ITRM) of 1-14 μm grains is harder to stepwise LTD than saturation remanence (SIRM), while anhysteretic remanence (ARM) is harder than either; for 20-135 μm multidomain grains, ITRM is softer than SIRM.
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