Geology Reference
In-Depth Information
grains having relaxation times corresponding
to micro-coercivity
H
c
j
H
(0)
j
. Then,
H
(0) is
increased and the procedure is repeated to remove
the next more stable component, which should
have a different direction of magnetization. If
the new component has the same direction of
magnetization of the one detected at the previous
step, then it is interpreted as the ChRM. AF
demagnetization is effective when the dominant
ferromagnetic mineral is a titanomagnetite. In
these rocks, secondary NRM is mostly associated
the ChRM is retained by single-domain grains
with higher micro-coercivity.
The second fundamental technique for the
removal of secondary NRM is
thermal demag-
netization
. This method requires heating of a
specimen to high temperature below the Curie
point of the constituent ferromagnetic minerals,
then cooling to room temperature in zero mag-
netic field. It is based on the thermal relaxation
of an assemblage of SD grains described in the
previous section. A real rock sample can be
considered as formed by multiple ensembles of
grains with different physical and geometrical
characteristics. Grains with different volumes
V
have distinct blocking temperatures
T
B
,sothat
when the rock cools and passes through the vari-
ous
T
B
, the relaxation times of the corresponding
grain assemblages increase quickly. Therefore,
different equilibrium magnetizations are frozen
during this process, so that any subsequent varia-
tion in the direction of the external field at lower
temperatures does not affect these components.
This means that the TRM is not acquired at
one time just below the Curie temperature, but
during a long time interval over a set of blocking
temperatures. Therefore, reheating a sample to
a temperature
T
<
T
c
implies unblocking of the
frozen magnetizations for all grain populations
with
T
B
<
T
and the consequent removal of these
components of TRM. Expression (
6.24
)shows
that the relaxation time of can vary over a wide
range. SD grains with small values of £ are
called
superparamagnetic grains
:theirremnant
magnetization decays quickly to zero after re-
moval of the magnetizing field. These grains are
those that more likely acquire VRM through the
geological time. For fixed temperature, relation
(
6.24
) shows that the relaxation time varies with
the grain volume
V
and the micro-coercivity
H
c
. Grains with low values of the product
VH
c
will have shorter relaxation time with respect to
grains with higher values of this parameter. Good
paleomagnetic recorders must have values of £
of the order of several hundred Myrs. However,
even superparamagnetic grains are converted into
stable grains at low temperature. We have seen
that the temperature at which this transition oc-
curs is the blocking temperature,
T
B
.AnySD
grain is superparamagnetic between the Curie
temperature and the blocking temperature.
In the previous section, we have shown that the
stability of the TRM acquired by an assemblage
of SD grains is expressed in terms of relaxation
time £. According to Eqs.
6.23
and
6.25
this
quantity increases rapidly when the temperature
T
decreases. Let us consider an SD grain assem-
blage with a fixed value of
VH
c
.IftheTRMof
these grains has a relaxation time £ of geological
length for some temperature
T
, it is possible to
determine the blocking temperature at laboratory
time scale, namely, the temperature at which a
sample must be heated to reset its TRM through
a zero external field in a short time interval, say
£
B
D
60-100 s. In fact, using (
6.25
)wehavethat:
T
B
lg .2£
B
=£
0
/
M
S
.T
B
/H
c
.T
B
/
D
T lg .2£=£
0
/
M
S
.T/H
c
.T/
(6.26)
Plots of the blocking temperature as a function
of the relaxation time are shown in Fig.
6.4
.
These plots can be used to determine the blocking
temperature as a function of the initial relaxation
time. For example, an assemblage of SD mag-
netite grains with relaxation time £
D
1Myrat
93
ı
C is expected to have acquired substantial
VRM if it has been kept at this temperature for
