Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 97

fi eld while single-domain grains can only rotate their
remanence out of its easy direction of magnetiza-
tion to respond to the applied fi eld in a susceptibility
measurement.
Multi-domain grains therefore have a stronger fer-
romagnetic susceptibility than single-domain grains.
It's obvious that from this description of different sus-
ceptibility mechanisms that susceptibility is a compli-
cated signal with many different possible sources in a
sample. Even if the susceptibility of ferromagnetic
minerals dominates a sample, the much larger sponta-
neous magnetization of magnetite with respect to
hematite means that it will override the contribution
of hematite to a sample's susceptibility, and make it
hard to detect the hematite magnetically. Furthermore,
it is not easy to deconstruct the susceptibility signal
into its different sources. A remanence measurement
can be deconstructed to a certain extent using the dif-
ferent coercivities of magnetic minerals and magnetic
mineral grain sizes. This type of experiment is not
possible for susceptibility. Susceptibility is still a valid
method for measuring magnetic mineral concentra-
tion, but best used when one ferromagnetic mineral
dominates the magnetic mineralogy or is the sole mag-
netic mineral of a sample.

a relatively long time (tens of minutes). In a hysteresis
measurement, a sample is cycled through a DC fi eld
that changes its intensity and direction along one axis.
The magnetization of the sample is measured while the
magnetic fi eld is being applied. A hysteresis loop results
(Evans & Heller 2003 , fi g. 2.3) and four important
parameters may be derived from the loop: the coerciv-
ity B c , the saturation magnetization J sat , the saturation
remanence J rs and the coercivity of remanence B cr . The
fi rst two parameters are measured when the fi eld is
turned on; the last two are remanence measurements
measured when the fi eld is off. The domain state, and
hence the magnetic particle size, can be roughly deter-
mined from ratios of these parameters.
In a Day plot (Day et al. 1977 ), J rs / J sat is plotted as a
function of B cr / B c. Single-domain grains are considered
to have J rs / J sat ratios greater than 0.5 and B cr / B c ratios
less than 4, while multi-domain grains have J rs / J sat
ratios less than 0.05 and B cr / B c ratios greater than 4.
In between are grains that are said to be pseudo-single
domain (PSD), multi-domain grains with just a few
domains that behave magnetically like single-domain
grains. They are typically around 1 micron in size
(more or less). The Day plot is somewhat controversial
to interpret, mainly because most samples fall in the
PSD grain-size range. It became clear with more study
that mixtures of different magnetic grain sizes affect
the interpretation of the hysteresis parameter ratios.
Another concern to keep in mind is that the Day plot
was originally developed for titano-magnetites, and
should only be used for magnetite and titano-magnetite
magnetic mineralogies. Tauxe et al . (2002) have sug-
gested that, instead of the Day plot to present the
hysteresis data, J rs / J sat (sometimes called ' squareness '
because it determines the 'fatness' of the hysteresis
loop) is plotted as a function of the coercivity B c or
coercivity of remanence B cr separately.
First-order reversal curve (FORC) diagrams, derived
from detailed measurements of hysteresis loops, are
the logical next step in measuring the hysteresis prop-
erties of a sample. These time-intensive plots result
from the measurement of many hysteresis loops for a
sample over a range of fi eld strengths. FORC diagrams
indicate the coercivities of the magnetic grains in a
sample (i.e. the magnetic grain sizes) and the degree of
magnetic interaction between the magnetic particles.
See Tauxe (2010) for an excellent coverage of hyster-
esis loops, FORC diagrams and the use of the hysteresis
parameters in rock and environmental magnetic
studies.

Magnetic mineral grain size

Magnetic particle size, also called magnetic grain size,
is measured by the ratios of different magnetic mineral
parameters. Typical ratios include the ARM/χ ratio
that quantifi es the relative amount of fi ne - grained
single-domain magnetite to the amount of coarse
multi-domain magnetite. All bets are off if the mag-
netic mineralogy is contaminated by paramagnetics,
diamagnetics or hematite. The ratio of the remanence
measurements ARM/IRM can also be used to quantify
magnetic particle size. ARM tends to respond more
effectively to the fi ner - grained single - domain grains,
particularly magnetite, while the IRM activates and
responds to all magnetic grain sizes. This ratio should
be used as a magnetic grain size measure when only
one ferromagnetic mineral dominates the magnetic
mineralogy of a sample.
Magnetic hysteresis parameters are used extensively
to measure magnetic grain-size variations, but are
more diffi cult to use on large suites of samples to
monitor magnetic grain-size changes throughout a
sedimentary section because each measurement takes