A method to study alterations of magnetic minerals during thermal demagnetization applied to a fine-grained marine marl (Trubi formation, Sicily)

The method presented is based on monitoring changes in the coercivity spectrum of an isothermal remanent magnetization (IRM) during stepwise thermal demagnetization. The procedure allows one to determine alteration temperatures and the coercivities and blocking temperatures of the magnetic minerals...

Full description

Saved in:
Bibliographic Details
Published in:Geophysical journal international 1992-07, Vol.110 (1), p.79-90
Main Authors: van Velzen, A. J., Zijderveld, J. D. A.
Format: Article
Language:English
Subjects:
alteration
Earth sciences
Earth, ocean, space
Exact sciences and technology
Geophysics: general, magnetic, electric and thermic methods and properties
Internal geophysics
magnetic minerals
Marine
marine marls
palaeomagnetism
thermal demagnetization
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:The method presented is based on monitoring changes in the coercivity spectrum of an isothermal remanent magnetization (IRM) during stepwise thermal demagnetization. The procedure allows one to determine alteration temperatures and the coercivities and blocking temperatures of the magnetic minerals involved in the alterations. It distinguishes between thermal decay due to unblocking of the remanence and demagnetization due to alterations of magnetic minerals. As a result, the initial magnetic mineralogy can also be determined. The method is demonstrated with samples of fine-grained marine marls from the Pliocene Trubi formation in Sicily (Italy). Single domain (SD) magnetite is the dominant remanence carrier in this sediment. A first alteration during heating to only 145°C is accompanied by a considerable reduction of coercivities higher than 0.1 T. This alteration is thought to be due to the reduction of stress in superficially maghemitized SD magnetite grains. Between 390 and 480°C grains with magnetite-like properties are produced, most likely by oxidation of pyrite in the sediment. At about 560°C the breakdown of iron-bearing silicates also produces magnetic grains. The magnetic grains formed during both alteration processes have low blocking temperatures. Demagnetization above 510°C is not only due to unblocking of the IRM. Alteration of magnetite to haematite proves to be an important demagnetization mechanism.
ISSN:0956-540X
1365-246X
DOI:10.1111/j.1365-246X.1992.tb00715.x