Geology Reference
In-Depth Information
primarily due to concentration variations. Either dependence could serve as
a climate proxy, so ARM can be a useful rock magnetic cyclostratigraphic
parameter in either case.
IRM can measure the concentration variations of high coercivity ferro-
magnetic grains. IRMs are applied to a rock sample simply by exposing it to
a constant (DC) magnetic field. This can be easily done with a large electro-
magnet, but maximum field strengths only several hundred mT are typically
possible. For high field strengths of 1000s of mT, impulse magnetizers are
used in which a capacitor is discharged through a coil. Even though the field
is only applied for less than a second, it allows efficient application of an IRM
in fields as high as 5000 mT that can easily magnetize high coercivity goethite.
However, IRM application is a different process than ARM application. In an
ARM, the magnetizations of individual magnetic grains are “tickled” into
alignment with the biasing magnetic field. An IRM is more of a “sledge-
hammer” approach with the magnetizations being forced into alignment
with the applied field. ARM is thought to mimic the natural thermal rema-
nent magnetization process that magnetizes the magnetic particles in an
igneous rock. In a thermal remanence, the high temperatures of the cooling
igneous rock reduce the relaxation times of the magnetic grains' magnetiza-
tion to only minutes or seconds causing a magnetic grain's moment to flip
back and forth along the grain's easy magnetization axis. IRM probably
mimics the natural magnetization caused by a lightning strike. SD magnetite
grains are no more efficient at acquiring an IRM than magnetite grains
in the 0.1-10 µm grain size range, so IRMs are not grain size dependent over
the 0.01-10 µm grain size range and a good measure of magnetic mineral
concentration variations. Magnetite grains larger than 10 µm are highly likely
to be MD grains and IRMs show grain size dependence for these larger grains
(Peters & Dekkers 2003). IRMs are typically used to magnetize rock samples
containing high coercivity hematite for rock magnetic cyclostratigraphic
studies. Most ARM application equipment cannot reach the coercivities of
hematite. Hematite grains in the 0.1-1 µm grain size range acquire IRMs
with slightly less efficiency than larger hematite grains (1-100 µm), so there
is a slight grain size dependence that should be considered in the interpreta-
tion of IRM rock magnetic cyclostratigraphic data.
The stepwise application of IRMs to a sample in progressively higher fields
until its magnetization saturates is an IRM acquisition experiment. The fields at
which different magnetic minerals reach saturation is a measure of the magnetic
mineral's mean coercivity and can be used for magnetic mineral identification.
Magnetite and the ferromagnetic sulfide, greigite, have mean coercivities in the
10s to 100s of mT. Hematite has coercivities in the 100s to 1000s of mT, and
goethite has mean coercivities of 1000s of mT. The different magnetization
coercivity components used for magnetic mineral identification can be
obtained from an IRM acquisition experiment by using Kruiver et al.'s (2001)
coercivity analysis modeling routine. The magnetization coercivity compo-
nents are assumed to be log normal distributions. These log normal distributions
are fit to the gradient of the IRM acquisition curve (Figure 2.6).
