Absence of natural viscous remanent magnetization in multidomain high-titanium magnetites: evidence for domain-wall interactions

Details Absence of natural viscous remanent magnetization in multidomain high-titanium magnetites: evidence for domain-wall interactions Absence of natural viscous remanent magnetization in multidomain high-titanium magnetites: evidence for domain-wall interactions

Jul 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003geoji.154..104m&link_type=abstract

Geophysical Journal International, Volume 154, Issue 1, pp. 104-116.

Astronomy and Astrophysics

Astronomy

1

Domain Wall Interactions, Multidomain Remanence, Natural Breccias, Ruapehu Volcano, Titanomagnetite, Viscous Remanent Magnetization

Scientific paper

We have studied viscous remanent magnetization (VRM) acquisition in lava clasts from volcaniclastic breccias from Ruapehu, North Island, New Zealand. The age of these deposits ranges from 2.5 to >65 ka BP VRM parallel to the direction of the present Earth's field is present in most clasts. However, VRM is not recorded in lava clasts in which high-titanium titanomagnetite of composition in the range x= 0.44-0.65 (~TM55) is the only or the dominant magnetic mineral present. Experiments demonstrate that these TM55-bearing samples have low blocking/unblocking temperatures, which would have been reset by viscous processes to give VRM if the material was in a single-domain state. Our experiments show that our TM55 samples can acquire low-temperature pTRMs in the laboratory. Analytical electron microscopy confirms the composition and demonstrates that these TM55 grains are up to 100 μm in size and that there are no observable exsolution structures. The grains are therefore in a multidomain state. Previous theoretical work has predicted that multidomain (MD) magnetic material will not acquire VRM if changes in remanence result in a complete reorganization of the whole-grain domain structure, as the activation energy of this process considerably exceeds the kinetic energy available at ambient temperatures (~20 °C) even over timescales greater than the age of the Earth. Previous laboratory experiments have been reported that show the acquisition of VRM in crushed and sized MD magnetite and titanomagnetite. These results have been used by several authors to conclude that MD VRM proceeds by motion of small segments of individual walls rather than by whole-grain domain reorganization. Our experiments suggest that the previously reported VRM effects in synthetic MD titanomagnetites may be due to contamination by a minute fraction of very fine and very viscous particles rather than due to inherent MD properties. We conclude that, on our natural material, MD TM55 does not acquire VRM. In this material, there is evidently no mode of remagnetization available for VRM acquisition other than thermal activation between whole-grain LEM states, and there is insufficient kinetic energy available at 20 °C to effect this activation. This is the first demonstration of this effect.

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