Geology Reference
In-Depth Information
(a)
Day plot
SD
0.5
PSD
MD
0
1
2
3
4
B
cr
/B
c
5
6
(b)
Squareness plot
Figure 2.5
Day plot of
hysteresis parameter ratios: J
rs
/
J
sat
plotted as a function of B
cr
/
B
c
(a). “Squareness plot” of
M
rs
/M
sat
vs. H
c
for samples
from the Cupido Formation of
Mexico (b). This type of plot
is sometimes used instead of
the Day plot to indicate
ferromagnetic particle domain
state and hence grain size.
MD, multidomain; vortex is
a PSD state of Tauxe et al.
(2002). Source: Hinnov,
Kodama, Anastasio, Elrick &
Latta 2013. Copyright 2013 by
the Geological Society of
London.
0.7
0.6
0.5
0.4
Chico
0.3
Garcia
0.2
Vortex
0.115
0.1
MD
0
0
10
20
30
H
c
(mT)
40
50
60
remanent magnetization reaches saturation and no longer increases, the
SIRM or J
rs
has been reached. There may be different magnetic minerals and
populations of grains with different magnetic mineral grain sizes in the
sample. Each of these magnetic minerals or population of magnetic grains
will have a different mean coercivity of remanence (B
cr
) which can be
modeled by fitting Gaussian curves to the first derivative of the IRM acqui-
sition curve (Kruiver et al. 2001) (Figure 2.6). Magnetite has much lower
coercivities than hematite (Table 2.2) and the coercivity analysis of the IRM
acquisition curve can be used to distinguish between magnetite and hema-
tite in most cases. If there is the possibility that the iron sulfide greigite is
present in the samples, it cannot be easily distinguished by coercivity alone
and a thermal demagnetization experiment should be conducted to search
for the ~300°C Curie temperature of greigite.
