Trying to Make Sense of Multidomain Arai Diagrams

Details Trying to Make Sense of Multidomain Arai Diagrams Trying to Make Sense of Multidomain Arai Diagrams

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmgp72a0997c&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #GP72A-0997

Mathematics

Logic

1522 Paleomagnetic Secular Variation, 1540 Rock And Mineral Magnetism

Scientific paper

Variable contributions from specimen to specimen of two types of multidomain (MD) tail can explain the concave-up, concave-down, and S-shaped Arai diagrams exhibited by samples of the 1915 Mount Lassen dacite treated by Thellier's method. The slope of the line joining the endpoints of each Arai diagram gives a good estimate of the known geomagnetic field intensity for the Mount Lassen region during the 1915 eruption. In other words, the NRM divided by the total TRM gives the correct slope for the Thellier experiment, a feature long recognized as characteristic of MD remanence that is stable to heating. Fabian (2001) has demonstrated very elegantly that the common concave-up shape of MD Arai diagrams can be well accounted for by the low-T tail of each pTRM segment acquired during cooling from the Curie point in the earth's field. By low-T tail we mean that part of the pTRM with the unblocking temperature less than blocking temperature. However, concave-down and more complex shapes cannot be explained by Fabian's phenomenological theory of TRM. We suggest that the reason is its inability to incorporated the experimentally observed tail of pTRM*, that is, the tail with unblocking temperature greater than the blocking temperature of the pTRM produced during each Thellier step by heating to an intermediate temperature and cooling back down to room temperature in the laboratory field. If the pTRM* tail is systematically greater than the corresponding low-T pTRM tail, the Arai diagram will be concave-down, whereas if it is systematically smaller the shape will be concave up. If the two tails always make equal contributions, the Arai diagram will yield the ideal straight line with slope corresponding to the correct paleointensity. Such could occur if the non-ideal MD remanence were distributed symmetrically about the T-unblocking = T-blocking line of Fabian (2001). Finally, if one tail is greater than the other tail at low temperatures and greater at high temperatures, the Arai diagram will be S-shaped. Fabian, K., 2001, A theoretical treatment of paleointensity determination experiments on rocks containing pseudo-single or multidomain magnetic particles, Earth Planet. Sci. Lett. 188, 45-55.

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