Geology Reference
In-Depth Information
remanent magnetization (HIRM)
or hard IRM which is used as a mea-
sure of the contribution of high coercivity minerals to the IRM. It is
calculated by
SIRM IRM
+
HIRM
=
−03
.
T
(2.7)
2
2.6 Identification of Magnetic Mineralogies
and Choosing a Rock Magnetic Parameter
for Cyclostratigraphy
In planning a rock magnetic cyclostratigraphic study, it is important to first
identify the magnetic minerals that characterize the sedimentary sequence.
Of course, a standard paleomagnetic study would provide that information
and also the information about whether the magnetic minerals are primary
or not, an absolute necessity for a meaningful cyclostratigraphic study. The
coercivity of the magnetic minerals in a rock and reference to Table 2.2 can be
used for identification. IRM acquisition measurements of a subset of samples
from the rocks will help determine the different coercivity components in a
rock. It is not uncommon for sedimentary rock samples to contain more than
one magnetic mineral. The rock magnetic parameter for the cyclostrati-
graphic study can be chosen to target variations in the concentration or grain
size of just one of these minerals, particularly if evidence suggests that the
mineral is most likely primary.
If magnetite is present, it is likely to be a primary, depositional magnetic
mineral, and ARM is a good way of determining concentration/grain size
variations throughout the sedimentary sequence. Even if a higher coercivity
mineral like hematite or goethite is also present in the rock, it is unlikely that
the 100 mT peak alternating field typically used to apply an ARM will acti-
vate and magnetize the higher coercivity minerals. If hematite is the main
magnetic mineral in a rock, typical of red beds, IRM may be the only way to
determine its concentration variations. IRM acquisition experiments can be
used to decide the type of DC magnetic field that should be used to apply the
IRM. If magnetite and hematite are both present in a rock, IRM that has
been alternating field demagnetized at about 100 mT can be used to isolate
just the hematite concentration variations.
Susceptibility is a quick and easy measurement and has been used suc-
cessfully for rock magnetic cyclostratigraphic studies, but it will be impor-
tant to determine what mineral is contributing, or dominating, the
susceptibility measurement since it could be diamagnetic, paramagnetic,
and/or ferromagnetic minerals. One aid to identification is to measure the
susceptibility of a suite of samples at room temperature and after they
have been cooled to liquid nitrogen temperature (77 K) by immersion for
a minute or two in liquid nitrogen. Paramagnetic minerals have their
