Absolute Paleointensity


The evolution of geomagnetic field is of great significance to studying the geodynamics. Geomagnetic field is a vector field, including both direction and intensity variation. Compared to the ancient direction information, the paleointensity is more difficult to obtain because of its complicated theory and experiment.

Principles

Most of the archaeologic materials (brick, tile and pottery) and volcanics can get a remanent when cooling from high temperature in an external field, which is called TRM. There are some researches indicate that the obtained TRM is proportional to the applied field when cooling in a low field (the geomagnetic field, for example). That is to say, if we can simulate the ancient TRM obtaining procedure in a given field in the lab, the ancient field can be calculated from TRMlab/NRManc = Blab/ Banc.

Methods

A number of paleointensity methods have been proposed each with advantages and disadvantages. One of the original methods proposed was developed in 1950's by Emile and Odette Thellier[1] . The method, known as the Thellier method, progressively replaces a samples natural remanent magnetization (NRM) with a laboratory TRM acquired in a known field. This yields multiple estimates of TRMlab/NRManc, which allow a more robust estimate of B_anc to be made. The unique feature of the original Thellier method is that the NRM is replaced using a series of pairwise heatings to progressively higher temperatures. During the first heating to the set temperature an applied field (Blab) is present throughout. The second heating to the same temperature is also in an applied field, but in the opposing direction (-Blab). The vector sum of the two heating is twice the NRM remaining and the vector difference is twice the TRM gain.


Influence effects

There are many influence factors during the paleointensity experiments, including mineralogy alteration during heating[3, 18, 21, 22], grain size effects of MD or PSD[23-25], anisotropy of magnetic remanence[26-30] and cooling rate effects[31-37]. All of these effects may lead to bias of paleointensity estimation.


References

  1. ^ Thellier E., Thellier O. Sur I’intensité du champ magnétique terrestre dans le passé historique et géologique. Annales de Geophysique, 1959, 15: 285-378.


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34. Halgedahl S. L., R. Day, M. D. Fuller. The effect of cooling rate on the intensity of weak-field TRM in single-domain magnetite[J]. J. Geophys. Res., 1980, 85: 3690-3698.
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37. Ferk, A., F. W. v. Aulock, et al. (2010). "A cooling rate bias in paleointensity determination from volcanic glass: An experimental demonstration." J. Geophys. Res. 115(B8): B08102.

Further Reading


Can be removed if not needed.



See Also


Relative Paleointensity
Thellier-Thellier Paleointensity Method
Coe Paleointensity Method
Shaw Paleointensity Method
Microwave Paleointensity Method
Perpendicular Paleointensity Method
Single Crystal Paleointensity Method
Multispecimen Paleointensity Method