Thermoremanent magnetization
When an
History
As early as the eleventh century, the Chinese were aware that a piece of
In the early 20th century, a few investigators found that
In paleomagnetism
Demagnetization
It has long been known that a TRM can be removed if it is heated above the Curie temperature of the minerals carrying it. A TRM can also be partially demagnetized by heating up to some lower temperature and cooling back to room temperature. A common procedure in paleomagnetism is stepwise demagnetization, in which the sample is heated to a series of temperatures , cooling to room temperature and measuring the remaining remanence in between each heating step. The series of remanences can be plotted in a variety of ways, depending on the application.
Partial TRM
If a rock is later re-heated (as a result of burial, for example), part or all of the TRM can be replaced by a new remanence. If it is only part of the remanence, it is known as partial thermoremanent magnetization (pTRM). Because numerous experiments have been done modeling different ways of acquiring remanence, pTRM can have other meanings. For example, it can also be acquired in the laboratory by cooling in zero field to a temperature (below the Curie temperature), applying a magnetic field and cooling to a temperature , then cooling the rest of the way to room temperature in zero field.
Ideal TRM behavior
The Thellier laws
The ideal TRM is one that can record the magnetic field in such a way that both its direction and intensity can be measured by some process in the lab. Thellier showed that this could be done if pTRM's satisfied four laws. Suppose that A and B are two non-overlapping temperature intervals. Suppose that is a pTRM that is acquired by cooling the sample to room temperature, only switching the field on while the temperature is in interval A; has a similar definition. The Thellier laws are
- Linearity: and are proportional to when is not much larger than the present Earth's field.
- Reciprocity: can be removed by heating through temperature interval , and through .
- Independence: and are independent.
- Additivity: If is acquired by turning the field on in both temperature intervals, .
If these laws hold for any non-overlapping temperature intervals and , the sample satisfies the Thellier laws.[4]
A simple model for the Thellier laws
Suppose that a sample has a lot of magnetic minerals, each of which has the following property: It is
The Néel model for single-domain TRM
See also
References
- ISBN 0-444-41084-8.
- ISBN 0-671-62028-2.
- ISBN 0-8047-1119-4.
- ISBN 0-521-32514-5.
- .