Geology Reference
In-Depth Information
Table 3.1
Koenigsberger
ratios
of
selected
rocks
(From Hunt et al.
1995
)
Roccia
Q
Marine sediments
5
Red sediments
1.6-6
Siltstone
0.02-2
Silty shale
5
Granite
0.1-28
Ganodiorite
0.1-0.2
Dolerite
2-3.5
Diabase
0.2-4
Gabbro
1-9.5
Oceanico Gabbro
0.1-58.4
Intrusive rocks
0.1-20
Volcanic rocks
30-50
Fig. 3.15
Saturation magnetization vs
T
for hematite
(Fe
2
O
3
) (Redrawn from Hunt et al. (
1995
))
Subaerial basalts
1-116
Oceanic basalts
1-160
Seamounts
8-57
The curve
M
s
D
M
s
(
T
) shows a drop in corre-
spondence with the Curie temperature
T
c
.Note
that
T
c
Š
680
ı
C for hematite.
The relative importance of the remanent mag-
netization with respect to the induced magnetiza-
tion is measured by the
Koenigsberger ratio
:
Granulites
0.003-50
rocks arises from the presence of small grains of
ferromagnetic minerals dispersed within a matrix
of diamagnetic and paramagnetic minerals. The
size of these grains strongly affects the magnetic
behaviour of the rock, just because of its influ-
ence on the number of magnetic domains.
In geology, the most important magnetic
minerals are undoubtedly iron-titanium (FeTi)
oxides, whose ternary diagram is shown in
Fig.
3.16
. Two classes of solid solutions are
particularly important: the
titanomagnetites
series and the
titanohematites
, which represent
primary phases of crystallization of igneous
rocks (1-5 % vol). In titanomagnetites, Ti
C4
substitutes Fe
C3
as the Ti content increases.
The crystal structure of these minerals is the
spinel structure. The addition of Ti decreases
progressively the saturation magnetization
and the Curie temperature (Fig.
3.17
), to the
point that the ulvospinel is paramagnetic at
ambient temperature. The general formula of
titanomagnetites is: Fe
3
x
Ti
x
O
4
,where
x
varies
between zero (magnetite) and 1 (ulvospinel).
The
M
r
M
D
M
r
¦H
Q
(3.64)
Therefore,
Q
>>1 indicates that the rema-
nent magnetization dominates, which is usually a
desirable attribute in marine geophysics studies.
Va l u e s o f
Q
for several kinds of rocks are listed
in Table
3.1
. These values have been determined
assuming that the magnetizing field
H
coincides
with the Earth's magnetic field at the Earth's
surface (
H
D
24
48 Am
1
). In general, mafic
rocks have a larger spontaneous magnetization.
For example, basalts are generally more magnetic
than rhyolites, and gabbros are more magnetic
than granites. Furthermore, the chemical compo-
sition being equal, extrusive rocks have larger
remnant magnetization and lesser susceptibility
than intrusive rocks. Finally, sedimentary and
metamorphic rocks generally have low values of
remnant magnetization and susceptibility.
An important aspect of the process of forma-
tion of magnetic domains is the relation between
the number of domains within a mineral grain and
the
grain size
. In general, the magnetization of
!
Fe
C2
C
Ti
C4
. However, any titanomagnetite with
x
>0.8 is paramagnetic at ambient temperature.
Regarding titanohematites, these minerals have
a corundum structure and constitute a lesser
2Fe
C3
ionic
substitution
is
