Geology Reference
In-Depth Information
energies U 12 and U 21 and pre-exponential
factor 1/£ 0 (Dunlop and Özdemir 1997 ):
blocking energy is small and the equilibrium
is attained is a short time interval. This situ-
ation is referred to as an unblocked or super-
paramagnetic condition. At room temperature,
where U B is large, the equilibrium is attained
very slowly, so that the non-equilibrium state
can be considered as frozen or blocked in the
system. For a given grain volume V ,thereis
a critical blocking temperature , T B ,atwhich£
becomes small (e.g., 60-100 s) during a heating
experiment. Conversely, during a rapid cooling
from above T B , the equilibrium value of mag-
netization M eq is frozen in the sample as TRM.
This is the main conclusion of Néel's theory of
thermal relaxation. On the basis of this model,
TRM can be removed only reheating a sample
to its original blocking temperature. In the next
section, we shall see that this feature furnishes a
method for removing secondary TRM from rock
samples.
0 exp
1
U 12
kT
K 12 D
exp "
2 #
1 C
1
0
0 VM S H c
2kT
H
H c
D
0 exp
1
U 21
kT
K 21 D
0 exp "
2 #
1
1
0 VM S H c
2kT
H
H c
D
(6.23)
wherewehaveused( 6.17 )and£ 0 10 9 -10 10
s is the atomic time interval of reorganization
between successive state transitions. This
quantity determines the frequency at which
the grains try to switch their direction of
magnetization. When H > 0, U 12 > U 21 and
rotations from ™ D 180 ı to ™ D 0 ı are favoured, so
that K 12 < K 21 .If H is sufficiently large, state 1
is strongly favoured and M eq ! M S . Similarly,
if H < 0 and sufficiently large, then state 2
is strongly favoured and M eq ! - M S .Inthis
instance, we can write:
6.2
Paleomagnetic Sampling
and Cleaning Procedures
Well before the discovery of marine magnetic
anomalies in the early 1960s, paleomagnetism
provided the strongest evidence for continental
drift, although not yet for plate tectonics in the
modern acceptation. This important branch of
Earth Sciences supplied a large number of data
from all continents since the 1950s, which proved
that both igneous and sedimentary rocks formed
at latitudes that did not generally coincide with
their present day latitudes. This conclusion was
supported by the anomalous inclination of NRM
vectors observed in rock samples of various ages.
Furthermore, these data showed that going back
to ages older than Pliocene, the declination of the
magnetization vectors was distributed differently
from what we expected on the basis of secular
variation. The strike of the horizontal component
of magnetization seemed to migrate away from
the Earth's spin axis in so far as the rock ages
increased. As pointed out by Irving ( 2005 ), until
the late 1960s the dominant belief among geo-
physicists and geologists, especially in the United
States, was a form of fixism , which assumed that
0 exp "
2 #
1 j H j
H c
1
Š
1
0 VM S H c
2kT
(6.24)
for j H j sufficiently large. Conversely, for H D 0
we have that U 12 D U 21 and Eqs. 6.21 and
6.23 give:
0 exp
1
D
2
0 VM S H c
2kT
(6.25)
According to ( 6.22 ), M ( t ) will be close to
the equilibrium value M eq after a time interval
coinciding with the relaxation time £. The factor
U B D ½ 0 VM S H c is usually referred to as the
blocking energy . Note that both M S and H c in
this expression depend upon T .From( 6.24 )to
( 6.25 ), we see that the relaxation time strongly
depends from the temperature T .Fortemper-
atures close to the Curie temperature T c ,the
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