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growth of magnetite in the ambient geomagnetic
field.
The magnetite created is generally multidomain (see Ferro-
magnetically soft it carries a weak remanence which is
often parallel to the present-day
field. Since both suscepti-
bility and strength of remanent magnetism increase on
serpentinisation, the Königsberger ratio is fairly constant
and is normally about 2
a)
10
-1
10
-2
10
-3
10
-4
0
20
40
60
80
100
-
4(
Fig. 3.52d
).
The logarithmic relationship between degree of serpen-
tinisation and magnetic properties indicates that the rate of
production of magnetite increases as the rock becomes
more serpentinised. This is explained by Toft et al.(
1990
)
in terms of a multi-stage process involving a succession of
serpentinisation reactions.
If serpentinised ultramafic rocks are further meta-
morphosed to amphibolite facies or higher grades they
become progressively de-serpentinised. In terms of mag-
netic properties, the most important consequence is the
disappearance of magnetite as other elements substitute for
iron in its lattice, eventually resulting in paramagnetic
in metamorphic olivine, enstatite and iron
Degree of serpentinisation (%)
b)
10
-1
10
-2
10
-3
10
-4
0
20
40
60
80
100
Degree of serpentinisation (%)
c)
10
1
10
0
10
-1
chrome spinels.
Consequently, at granulite facies and above, the rock con-
tains only paramagnetic minerals and is comparatively
non-magnetic.
-
10
-2
0
20
40
60
80
100
Degree of serpentinisation (%)
d)
10
1
Q
= 1
10
0
3.9.5.5
Remanent magnetism of metamorphosed
and altered rocks
Remanent magnetism can be reset by the metamorphic
process if magnetic minerals are destroyed, or formed
and cooled through their Curie temperature. Low- and
medium-grade metamorphism can overprint a primary
remanence with a partial TRM. High-grade metamorph-
ism can remove the remanent magnetism by taking the
magnetic minerals above their Curie point, with the
remanence reset as the cooling passes back through the
Curie temperature of each magnetic mineral.
Alteration can superimpose a CRM on an earlier
remanence (
Fig. 3.46c
)
. When coarse-grained secondary
magnetite is produced, its multidomain character means
its remanent magnetism is easily changed and it will often
have a
10
-1
10
-2
0
20
40
60
80
100
Degree of serpentinisation (%)
Figure 3.52
Magnetic properties versus degree of serpentinisation
for some continental serpentinites. (a) Data from Josephine Creek,
Oregon (Toft et al.,
1990
). Data in (b), (c) and (d) from Red
Mountain. California (Saad,
1969
).
Figure 3.52
shows changes in magnetic properties with
degree of serpentinisation, for which density has been used
as a proxy (see
Section 3.8.4
). Susceptibility data from
Josephine Creek, Oregon, USA, and Red Mountain, Cali-
fornia, USA (
Figs. 3.52a
and
b
) show a broadly linear
increase in the logarithm of susceptibility with increasing
serpentinisation. The susceptibility of completely serpenti-
nised material is about 10 times that of unserpentinised
material. A similar relationship is indicated for remanent
magnetism in
Fig. 3.52c
.
This is a CRM caused by the
viscous remanent magnetism (VRM) parallel
to the present-day Earth
'
soft
'
s magnetic
field. Otherwise, rem-
anent magnetism is often parallel to metamorphic linea-
tions and in the plane of metamorphic fabrics, owing to
preferred alignment of mineral grains, especially when
pyrrhotite is the carrier.
'
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