Geoscience Reference
In-Depth Information
3.9.3.4
Summary and implications for magnetic data
Overall, widely variable levels of induced and remanent
magnetism occur in igneous rocks. Strong remanent mag-
netism can affect the polarity of associated anomalies.
Large intrusive masses often exhibit internal magnetic
zonation. Flat-lying
Susceptibility
(× 10
-3
SI)
0.0
1.0
2.0
3.0
a)
4.0
character
due to the cooling-related magnetic properties varying
throughout the
flow. The magnetic expression of some
igneous rocks is illustrated in
Sections 3.11.3
and
3.11.4
.
flows often have a
'
noisy
'
3.0
2.0
b)
3.9.4
Magnetism of sedimentary rocks
n
=104
In general, sedimentary rocks are weakly magnetic because
magnetite is not usually a significant constituent, although
an obvious exception is banded iron formation (BIF)
(
Fig. 3.42
).
The magnetic properties of clastic sediments re
1.0
10
-4
10
-3
10
-2
Susceptibility (SI)
ect the
mineralogy of their source. Quartz-rich units from a
mature source area have lower susceptibilities than sedi-
ments sourced from immature volcanic terrains, although
it is sometimes uncertain whether the magnetite within
these sediments is detrital or due to decomposition of
ma
c minerals.
Iron is most abundant in
fine-grained sedimentary
rocks, often associated with clays into which it is absorbed.
Waters that are in contact with the atmosphere are oxidis-
ing, eventually causing minerals containing ferrous iron
(e.g. magnetite) to be oxidised to ferric iron minerals (e.g.
haematite). Reducing environments occur in stagnant
water-logged environments, particularly those rich in
organic matter. Magnetite and pyrrhotite are stable in
these environments, but only form when there are low
levels of sulphur (otherwise iron is taken up by pyrite) or
carbonate (otherwise iron is taken up by siderite).
Variations in susceptibility can be correlated with grain
size in clastic sediments, graded bedding being mimicked
by decreases in susceptibility (
Fig. 3.47
). This is interpreted
as due to density strati
cation causing iron-rich grains to
occur mainly at the bottom of units, and/or a systematic
upward decrease in the number of larger (multidomain)
magnetite grains.
The iron-de
cient carbonates have very low suscepti-
bility. In contrast, sediments deposited in iron-rich solu-
tions associated with volcanogenic activity or Precambrian
chemical precipitates, can contain appreciable magnetite or
pyrrhotite. These sediments may be transitional to synge-
netic massive mineralisation or BIFs. These rocks are
0.0
Figure 3.47
Variation in grain size and magnetic susceptibility of a
greywacke sequence in southern Scotland. (a) Grain size distribution
within lenses of the sequence. (b) Frequency histogram of the
susceptibility measurements, which are the arithmetic means of 12
measurements. The correlation between the two parameters is quite
highly magnetic, generally remanently magnetised and
are characterised by strong anisotropy of susceptibility. In
the Hamersley iron-ore province of Western Australia, the
susceptibility of BIFs parallel to bedding exceeds the sus-
ceptibility normal to bedding, typically by a factor of 2 to 4
are rarely signi
cantly magnetised, since the colouring
attests to the dominance of haematite.
3.9.4.1
Remanent magnetism of sedimentary rocks
Detrital magnetic mineral grains acquire a primary detrital
remanent magnetism when they align themselves in the
Earth
s magnetic
field as they settle through water
(
Fig. 3.46b
). Its direction is sub-parallel to the direction
of the Earth
'
s
field, owing to gravity causing elongated
grains to lie
flat on the bottom. Subsequent rotation of
magnetic grains can occur in pore spaces. Diagenesis,
alteration and weathering of sediments can produce phys-
ical and chemical changes to magnetic minerals, below the
Curie point, to produce a secondary crystallisation
remanent magnetism (CRM), which is parallel and pro-
portional to the strength of the ambient field.
'
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