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a)
a)
Susceptibility
(×10 -3 SI)
10 2
10 2
30
Prograde
10 1
10 1
Königsberger
ratio
10 0
10 0
20
10 -1
10 -1
Ilm + Rt + Hae
Mt + Ilm + Hae
Ilm + Hae
J Induced
10
10 -2
10 -2
J Remanent
10 -3
Retrograde
Country
rock
Baked
contact
Dyke
Baked
contact
0
Andalusite-
cordierite
zone
Cordierite-
K-feldspar
z one
Biotite
zone
0
20
Pluton
TZ
Metres
b)
10 2
3000
2000
1000
0
Distance from pluton (m)
10 1
b)
0
Prograde
Retrograde
10 0
-1
4
10 -1
-2
5
3
1
2
10 -2
Baked contact
-3
Dyke
Country rock
10 -3
-4
200
400
600
800
10 -3
10 -2
10 -1
10 0
10 1
10 2
Induced magnetism (A/m)
Temperature (ºC)
Figure 3.50 Variations in magnetic properties through a doleritic
dyke intruding a metabasalt in the Abitibi area. (a) Profile across the
dyke, and (b) strengths of the induced and remanent magnetisms.
Based on data in Schwarz ( 1991 ) .
Figure 3.51 (a) Variation in magnetic susceptibility across different
zones comprising a contact aureole in California. (b) Schematic
representation of changes in temperature and oxygen fugacity within
the aureole. Paths 1
-
4 refer to the biotite, andalusite
-
cordierite,
transition, and cordierite
K-feldspar zones respectively. Path 5
represents retrogressive metamorphism. hae
-
-
haematite, ilm
-
increased magnetism during prograde metamorphism. In
the inner zone magnetite has been converted to ilmenite
and haematite, causing a decrease in magnetism. Retro-
grade metamorphism is magnetite destructive.
Figure 3.51b is a schematic illustration of the variation in
temperature and oxygen fugacity within the contact aure-
ole and relates the metamorphism to the stability of mag-
netite and haematite, as described in Section 3.9.3.1 ). Using
mineralogically derived estimates of physical and chemical
conditions, the appearance of magnetite is shown to be
associated with an increase in temperature. A further
increase in temperature and also oxygen fugacity leads to
destruction of magnetite in the inner zone of the aureole.
Retrograde metamorphism is associated with decreases in
both temperature and oxygen fugacity but conditions
remain outside the magnetite stability field. Figure 3.51b
shows how only small changes in oxygen fugacity and
ilmenite, mt
-
magnetite and rt
-
rutile. Redrawn, with permission,
from Kontny and Dietl ( 2002 ).
temperature across the haematite
magnetite boundary
cause signi cant changes in rock magnetism due to the
changes in magnetite abundance.
-
3.9.5.4 Serpentinisation
Serpentinisation is one of the most signi cant meta-
morphic processes in terms of its effects on physical prop-
erties. Density is inversely proportional to the degree of
serpentinisation (see Section 3.8.4 ), but magnetism
increases with serpentinisation owing to reactions that
produce magnetite at the expense of the paramagnetic
minerals olivine and orthopyroxene.
 
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