Geology Reference
In-Depth Information
ancient magnetizations in the rock. But even the most ancient magnetiza-
tions in a rock sample are not necessarily as old as the rock, so other tests
must be done to constrain the age of the rock's magnetization. These tests
will be described in Section 3.2.4.
Thermal demagnetization is the most widely used technique because
experience shows it to be the most effective at removing secondary magne-
tizations, particularly for very ancient rocks or rocks with magnetizations
carried by hematite (red beds). Thermal demagnetization involves heating
rock samples in magnetically shielded ovens, cooling them, and measuring
the remaining remanence at higher and higher temperature steps until they
lose their ferromagnetism. This is the Curie temperature for magnetite
(580°C) and the Neel temperature for hematite (680°C). The disadvantage of
thermal demagnetization is that the heating can often cause new magnetic
minerals to grow during the experiment, obscuring the ancient remanence
in the rocks.
Alternating field demagnetization exposes a rock sample to peak alternating
fields that then smoothly decrease to zero. The rock is measured after each
exposure to the alternating field, the peak alternating field is increased in steps
until the rock's magnetization is completely removed. Alternating field demag-
netization is used on recent unconsolidated sediments that can't be heated
without drying out and physically destroying the sample; however, it is less
successful at removing secondary magnetizations from ancient rocks. It is also
ineffective for rocks with high-coercivity hematite as the main magnetic
mineral, i.e., red beds.
The demagnetization of iron sulfides (greigite) is problematic because
heating the samples during thermal demagnetization usually causes the
iron sulfide to oxidize to magnetite that carries a secondary magnetiza-
tion acquired during the demagnetization. Even though the Curie tem-
perature of the iron sulfide is lower than the Curie temperature of any
depositional magnetite that may also be in the sediments, the secondary
magnetization created by the heating swamps the primary magnetization
in the rocks. Alternating field demagnetization can be used on these
rocks, but the magnetization isolated should be checked to see if it is
primary or secondary.
All progressive demagnetization data, thermal or alternating field, is
presented in an orthogonal demagnetization diagram (Zijderveld 1967),
also known as a vector component diagram (Butler 1992) or a vector
endpoint diagram (Tauxe 2010). These important diagrams may appear
complicated at first, but they are a good way of presenting the decay of a
three-dimensional vector during demagnetization. They are important to
use in any paleomagnetic study because they show, at one glance, the
quality of the paleomagnetic data. In typical orthogonal demagnetiza-
tion diagrams, the solid symbols represent one end of the horizontal
component of the paleomagnetic vector at some point during demagneti-
zation. The vector's other end is fixed to the origin. The open symbols
are used to depict the endpoints of the vertical component of the
