Geophys. J. Int. (1998) 134, 554–572
Possible mechanisms causing failure of Thellier palaeointensity
experiments in some basalts
Andrei A. Kosterov1,2 and Michel Prévot1
1 L aboratoire de Géophysique et T ectonique, UMR 5573 du CNRS, Case 060, Université de Montpellier II, 34095 Montpellier Cedex 5, France
2 Earth Physics Department, Institute of Physics, St Petersburg University, PetergoV, 198904, St Petersburg, Russia.
E-mail: [email protected]
Accepted 1998 March 13. Received 1998 March 13; in original form 1997 January 13
SU MM A RY
The normally magnetized zone of the Jurassic Lesotho basalts, although providing
apparently quite reliable palaeofield directions (Kosterov & Perrin 1996), shows
anomalous behaviour when studied in vacuum using the Thellier palaeointensity
method: typically the slope of the natural remanent magnetization–thermoremanent
magnetization (NRM–TRM) curves is very steep at intermediate temperatures (200
to 400–460 °C). In order to elucidate the reasons for such an anomalous behaviour,
six representative samples (from a total of 74 studied using this method) were
subjected to a variety of analyses. These experiments indicate that the magnetic
properties are dominated by pseudo-single-domain (PSD) magnetite grains some 1 mm
in size, resulting from high-temperature oxidation of titanomagnetite. Laboratory
heatings in vacuum up to the Curie point do not change significantly the roomtemperature hysteresis characteristics or the initial susceptibility k. Similarly, the k(T )
curves in vacuum are (with a single exception) rather reproducible. Since the
laboratory TRMs yield almost ideal NRM–TRM plots, the anomalous NRM–TRM
plot is presumably due to some peculiarity of the natural TRM. The partial TRM
(pTRM) acquisition capacity in the moderate temperature range (cooling from 200 to
20 °C) is generally very strongly reduced after heating to 270 °C, which indicates that
some magnetic alteration has already occurred at these temperatures. Hysteresis
measurements between room temperature and the Curie temperature T show that
c
some small (less than 10 per cent) but significant irreversible changes in hysteresis
characteristics also occur during heating. In particular, the coercive force H at room
c0
temperature is typically reduced after heating at a moderate temperature (175 °C) but
increases after treatments at 475 °C and, more pronouncedly, at 580 °C. The saturation
magnetization J remains unchanged, except for a very small decrease ( less than 5
s0
per cent) occurring in some samples after the two latter treatments. These changes
are most clearly seen on H (T )–J (T ) bilogarithmic plots, which show that the
c
s
moderate-temperature change in coercivity can extend up to 200–250 °C. Thus
hysteresis measurements as a function of temperature offer a promising tool for
sample pre-selection for Thellier experiments. Alternating-field demagnetization and
cycling of pTRMs at liquid-nitrogen temperature suggest that the blocking mechanism
is largely multidomain-like near room temperature but becomes less so as the Curie
point is approached. The main reason for the failure of the Thellier experiments is the
loss of a fraction of the NRM (natural TRM) at temperatures apparently lower than
the blocking temperatures in nature. It is suggested that this anomalous behaviour
results from the reorganization of the domain structure of the PSD grains during
heating. This transformation, which seems to be triggered by the coercivity decrease
observed at very moderate temperatures, can reduce the NRM intensity without
requiring any correlated pTRM acquisition.
Key words: magnetite, magnetization, palaeointensity, rock magnetism.
554
© 1998 RAS
T hellier palaeointensity experiments
1 IN TR O D UCT IO N
The strength of the Earth’s magnetic field in the historic and
geological past is of considerable interest in geophysics. In
their milestone paper, Thellier & Thellier (1959) outlined a
method to determine the intensity of the ancient geomagnetic
field from the characteristics of thermoremanent magnetization
(TRM) produced during the cooling of natural volcanic rocks
and archaeological artefacts. The Thellier method is still considered to be the most reliable of those proposed for this
purpose. However, several conditions have to be obeyed to
ensure the significance of the palaeointensity results:
(1) the primary remanent magnetization must be a TRM;
(2) the secondary components must be weak with respect
to the primary component and must be removed at relatively
low temperatures;
(3) the remanence carrier must be reasonably stable during
heating in the laboratory;
(4) the independence and memory laws of the partial TRMs
(Thellier 1938) have to be valid.
Condition (2) is easy to check and is fulfilled by a significant
fraction of volcanic rocks. The Thellier method provides the
most reliable way of checking the stability of the TRM
(condition 3), but this condition is, however, only exceptionally
observed to hold over the whole temperature range of the
investigation. Condition (4) holds true for single-domain (SD)
grains only, not for large grains (Shashkanov & Metallova
1972). The magnetic carrier in the volcanic rocks selected for
palaeointensity experiments is commonly a near-magnetite
spinel formed from spinodal decomposition of a former Ti-rich
titanomagnetite. The grain size of this spinel phase is generally
larger than the single-domain/pseudo-single-domain (SD/PSD)
threshold, which makes it possible that a significant number
of volcanic rocks do not rigorously meet condition (4). It is
commonly believed (Haggerty 1976) that this transformation
occurs above 600 °C. Thus, the primary remanence should be
a TRM (condition 1).
The present work is an effort to understand the reason for
the unusual thermal behaviour of the NRM of some of the
early Jurassic Lesotho basalts. These rocks seem to be almost
ideal recorders of palaeomagnetic field directions (Kosterov &
Perrin 1996). However, many of these lava flows yield quite
unacceptable results when studied with the Thellier method.
The objective of the work reported here was to investigate,
through a variety of rock-magnetic studies, the reasons for this
anomalous behaviour.
2 PA LA E OM A G NE TI S M, O PAQ U E
M I NE R A LO GY A N D R O C K -M A G NE TI C
C H A R A C TE R IS TI C S O F SA M P LE S
2.1 Palaeomagnetism
The samples used in the present study were tholeiitic basalts
from the Stormberg Formation, cropping out in Lesotho. The
Lesotho basalts present a quite simple magnetostratigraphy
with, from bottom to top, a reversed zone, a transitional zone
and a well-developed normal zone (Van Zijl, Graham & Hales
1962; Marsh et al. 1997). The localization and palaeomagnetic
characteristics of our entire collection of normal and reversed
flows were described by Kosterov & Perrin (1996). We found
© 1998 RAS, GJI 134, 554–572
555
that only a small fraction of these flows, all of reversed polarity,
provided apparently reliable palaeointensity data (Kosterov
et al. 1997). In contrast, all the flows from the normally
magnetized Mafika–Lisui Pass section (Kosterov & Perrin
1996) show, to various extents, a peculiar type of non-ideal
behaviour in Thellier experiments. From a total of 74 samples
from this section, studied using the Thellier method, we selected
six representative samples for the present investigation.
Initially, in order to assess the samples’ suitability for the
Thellier experiments, the following set of experiments was
carried out. Viscosity indices (Thellier & Thellier 1944; Prévot
1981) were determined from a two-week storage in the ambient
field followed by another two-week storage in zero field.
Palaeomagnetic directions for each sample were determined
using either thermal ( heating in air) or alternating-field (a.f.)
demagnetization.
The determination of the characteristic-remanence (ChRM)
directions from both thermal and a.f. cleaning (Fig. 1) was
straightforward. The ChRM of the selected samples was easily
isolated in the range 10–150 mT (a.f. treatment) or 200–600 °C
(thermal cleaning), with mean angular deviations of less than
1 ° (Table 1). For all the samples from a single flow, the ChRM
directions are very close to the mean. The secondary components were always weak compared to the primary component
and yielded scattered directions that always differed from the
direction of the present-day geomagnetic field at the sampling
locality. The viscosity indices did not exceed 3 per cent,
confirming that the viscous remanence is almost negligible.
2.2 Magnetic mineralogy
In order to identify the magnetic minerals, polished thin
sections were examined with an optical microscope, and the
temperature dependences of the initial magnetic susceptibility
k and saturation magnetization above room temperature were
measured under vacuum or in helium, respectively. In addition,
low-temperature measurements of SIRM (saturation isothermal remanent magnetization) were carried out at the IRM
(Institute for Rock Magnetism), Minneapolis, using a Quantum
Design magnetic properties measurement system (MPMS).
Microscopic observations of polished thin sections indicated
that in all samples the opaque mineral was mainly titanomagnetite, transformed by spinodal decomposition into Ti-poor
titanomagnetite remnants delimited by exsolutions along (111)
planes. This suggests that the dominant (and sometimes exclusive) magnetic mineral is near magnetite in composition. With
only few exceptions, the ‘magnetite’ crystals were not affected
by any subsequent lower-temperature alteration such as granulation. Similar observations hold true for the entire Lesotho
basalt formation (Kosterov et al. 1997; Prévot et al. in preparation). The size of the original titanomagnetite crystals varies
greatly from sample to sample. Samples 21 and 86 (the latter
sample was drilled only 10 cm above the bottom contact of a
3–6 m thick flow) show elongated, skeletal titanomagnetite
crystals, sometimes reaching several tens of microns in length,
but not more than a few microns wide in their widest sections.
In contrast, samples 47 and 244 ( both drilled from flow
interiors) contain euhedral crystals reaching a few hundreds of
microns in size. The titanomagnetite crystals of sample 117
have intermediate characteristics. They are typically several
tens of microns in size and vary from anhedral to euhedral
shapes.
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A. A. Kosterov and M. Prévot
10-4 Am2kg-1
10-4 Am2kg-1
10-4 Am2kg-1
Figure 1. Orthogonal diagrams of thermal and alternating-field
demagnetization of the NRM of the six selected samples of Lesotho
basalt. The open and solid symbols correspond to the vertical and
horizontal planes, respectively. Each scale division corresponds to
10−4 A m2 kg−1.
The exsolutions are ilmenite or, less commonly, metailmenite. In samples 21 and 86, the ilmenite is replaced by unidentified non-opaque phases. Titanohaematite exsolutions are
sometimes present as a minor constituent (samples 117 and
244). No pseudo-brookite exsolutions were observed. Thus we
have no specific evidence that the spinodal decomposition
during cooling in nature stopped above the Curie temperature
of magnetite.
In contrast to the titanomagnetite crystals, the size of the
magnetite crystals resulting from spinodal decomposition
seems rather constant. Their typical width varies from 0.5 mm
to 1 mm from sample to sample. Their shape seems more
equant when the magnetite was formed from skeletal crystals
rather than from large, euhedral crystals. In the latter case, the
length-to-width ratio of these magnetite rods can reach several
units (Davis & Evans 1976). These dimensions and shapes
correspond to small PSD grains.
The temperature dependences of the initial susceptibility
[k(T ) curve] and the saturation magnetization [J (T ) curve]
s
of three representative samples are shown in Fig. 2. Sample
244 and, to a lesser extent, sample 47 (not shown) yield
reversible curves with a single Curie point, indicating the
presence of magnetite. In contrast, sample 86 yields an irreversible k(T ) curve, with two inflection temperatures visible on
both the heating and the cooling curves. This suggests, in
agreement with the J (T ) curve, that in addition to magnetite,
s
a second spinel phase with a Curie point near 400 °C is present.
This latter phase, which is rather unstable upon heating, is
presumably titanomagnetite (or titanomaghemite). Magnetite
is the dominant phase in the other three samples. Sample 199
(and similarly samples 21 and 117) exhibits some indication
[in the k(T ) heating curve] that this second phase is also
present here, but in a much smaller proportion than in
sample 86.
In order to investigate further the magnetic mineralogy of
the samples, we measured, in zero field, the thermal decay of
the saturation remanence (SIRM) acquired at 15 K in a d.c.
magnetic field of 2.5 T. All curves are characterized by a major
SIRM loss between 90 and 130 K (Fig. 3a). An examination
of the derivatives of the SIRM
(T ) curves (Fig. 3b) shows
15 K
that the transition temperature, if defined as the temperature
at which the rate of the SIRM decay is maximal, varies from
115 K for sample 199 to 100 K for samples 021 and 086, which
exhibit less pronounced peaks. These temperatures are somewhat lower than the temperature of 121–122 K reported for
the Verwey transition of stoichiometric magnetite, indicating
some degree of non-stoichiometry due either to vacancies or
to substitution of minor elements for iron (Aragón et al. 1985;
Kakol & Honig 1989a; Aragón 1992). The broadness of the
transition zone probably reflects some variance in chemical
composition from one crystal to the other. An important
conclusion of these experiments is that near-magnetite is
present as a primary phase in all our samples.
The thermal demagnetization of SIRM
should also pro15 K
vide some indications about the grain size of the near-magnetite
phase. For stoichiometric magnetite in the range 37–220 nm,
the fraction of SIRM surviving the warming-up to room
temperature (SIRM memory) has been found to be grain-sizedependent (Özdemir, Dunlop & Moskowitz 1993). However,
non-stoichiometry reduces or even suppresses the SIRM loss.
For sample 199, which seems to contain almost-stoichiometric
magnetite, the SIRM memory is nearly 40 per cent. According
© 1998 RAS, GJI 134, 554–572
T hellier palaeointensity experiments
557
Table 1. Palaeomagnetic and rock-magnetic characteristics of the samples used in the present study. In ‘Paleodirectional results’, ‘Flow mean’ is
the mean palaeomagnetic direction for each particular flow, determined by Kosterov & Perrin (1996), and (Inc, Dec, MAD) are the inclination,
declination and mean angular deviation determined by the demagnetization procedure given in the corresponding column heading. In ‘NRM–TRM
plot characteristics’, T is the upper limit of the temperature interval in which an anomalous loss of NRM was observed, % NRM lost is the
fl
fraction of the total NRM lost in this temperature interval, and ‘Slope’ is the slope of the straight line best fitting the experimental points of the
NRM–TRM plot within this temperature interval. F is the laboratory field.
l
Flow/
sample
Mafika 009/
92M021
Mafika 018/
92M047
Mafika 033/
92M086
Mafika 043/
92M117
Mafika 082/
92M199
Mafika 101/
92M244
Palaeodirectional results
NRM–TRM plot characteristics
Flow mean
(Inc, Dec, MAD)
Thermal
(Inc, Dec, MAD)
A.f. demagnetization
(Inc, Dec, MAD)
Thellier
(Inc, Dec, MAD)
T , °C
fl
% NRM
lost
‘Slope’
(F =20 mT)
l
−53.5, 340.2, 5.3
−50.1, 340.6, 0.6
−51.5, 339.2, 0.4
−49.7, 344.8, 1.5
463
29.1
−3.6
−48.8, 341.4, 5.0
−46.8, 346.5, 0.3
−47.5, 344.8, 0.4
−49.2, 348.3, 1.7
463
44.4
−3.4
−61.2, 336.1, 3.7
−61.6, 333.7, 0.6
−65.0, 334.2, 0.5
−58.8, 339.0, 2.3
394
24.0
−3.5
−54.8, 357.7, 4.1
−54.6, 352.7, 0.5
−55.4, 351.1, 0.7
−54.5, 355.2, 1.1
—
—
—
−52.8, 333.1, 2.9
−53.2, 334.0, 0.4
−52.0, 337.0, 1.0
−56.7, 331.3, 3.3
463
42.5
−4.3
−74.9, 328.1, 3.0
−77.0, 323.8, 0.8
−78.0, 328.5, 0.5
−76.8, 318.5, 0.8
—
—
—
to the data of Özdemir et al. (1993), a very fine magnetic grain
size (several tens of nanometres here) is expected. Qualitatively,
a similar conclusion might be drawn for most of the other
samples too. Compared to the microscopic observations and
hysteresis characteristics (Section 2.3 below), this estimate
seems too low by an order of magnitude, probably because
the SIRM memory may depend also on other characteristics
of the magnetic grains, such as the kind of anisotropy.
All five crystalline anisotropy constants of monoclinic magnetite are almost temperature-independent in the range 5–75 K
(Kakol & Honig 1989b). Thus, any large SIRM decay in this
temperature range is commonly attributed to the presence of
superparamagnetic grains (e.g. Özdemir et al. 1993). According
to Fig. 3a, such grains represent a significant fraction of the
whole magnetite population in all samples except sample 199.
grain as a whole, the J /J ratio of the SD magnetite will be
rs s
reduced from 0.5 to about 0.3, depending on the geometry of
the magnetite grains (Davis & Evans 1976). The average J /J
rs s
ratio of our samples being approximately 0.2 (Table 2), a PSD
structure can be inferred from the hysteresis data, in good
agreement with the microscopic observations. Rock samples
having such hysteresis characteristics are commonly used in
palaeointensity experiments (e.g. Prévot et al. 1985; Garnier
et al. 1996). Note also that the hysteresis characteristics of the
present samples are not significantly different from those of
the samples from the Lesotho basalts which provide apparently
reliable palaeointensity results (Kosterov et al. 1997).
3 PA L A EO I NT ENS I TY EX P ER I M EN TS
3.1 Experimental procedure
2.3 Hysteresis characteristics
Measurements of hysteresis at room temperature (Table 2) were
carried out using a laboratory-built translation inductometer at
the Laboratoire de Géomagnétisme, Saint-Maur, France, in a
maximum d.c. field equal to 800 mT. Sample 86 has the lowest
coercive force (10.5 mT), samples 21, 47, 117 and 244 have an
intermediate coercive force (16–18 mT), and that of sample 199
is 29.1 mT. The same tendency holds for the coercivity of the
remanence, which is at its minimum (24.8 mT) for sample 86
and at its maximum (51.4 mT) for sample 199. This suggests that
the phase with an intermediate Curie point, which is present in
a considerable amount in sample 86, has a lower coercivity than
the near-magnetite phase (see Fig. 2).
When plotted on a Day plot (Day, Fuller & Schmidt 1977)
(Fig. 4), all samples fall into the pseudo-single-domain range,
in agreement with the microscopic observations. It must be
pointed out, however, that the SD/PSD limits shown in Fig. 4
refer to non-interacting particles, which is probably not the
case here. As shown above, the magnetic grains are packed
into host crystals which were formerly high-titanium titanomagnetite. Owing to the net demagnetizing field of the host
© 1998 RAS, GJI 134, 554–572
Palaeointensity experiments were carried out using the Thellier
method in its classic form (Thellier & Thellier 1959), i.e. by
double stepwise heatings in an arbitrarily positive/negative
laboratory magnetic field, applied throughout the whole heating–cooling cycle. All heatings were performed in a vacuum
better than 10−2 mbar. The intensity of the applied field was
20.0±0.1 mT. 17 or 18 temperature steps were used, in a range
from 100 to 570–580 °C. The temperature reproducibility
between any two heatings to the same nominal temperature
was normally within 2 °C. In order to control for magnetic
changes produced by heating, partial TRM (pTRM) checks
were performed after the third heating step, and then after
each subsequent step throughout the whole experiment. Also,
the room-temperature magnetic susceptibility was measured
after the second heating at each temperature step.
3.2 Thellier experiments on NRM
Examples of typical NRM–TRM plots, together with the
corresponding orthogonal diagrams, are given in Fig. 5.
Despite the low viscosity of the samples, the initial part of the
558
A. A. Kosterov and M. Prévot
Figure 2. Susceptibility and saturation magnetization versus temperature for three selected samples. The solid and dashed curves indicate heating
and cooling, respectively. In the plots of saturation moment, the open symbols refer to cooling curves: the open circles indicate cooling from 175 °C
to room temperature, the open diamonds indicate cooling from 275 to 175 °C, the open triangles indicate cooling from 475 °C to room temperature,
and the open squares indicate cooling from 580 °C (595 °C in some cases) to room temperature. The solid squares indicate the values measured
during heating.
NRM–TRM plots, corresponding to temperatures up to 200–
250 °C, is affected by a secondary magnetization. Thus, as
usual, this part of the diagram cannot be used for palaeointensity determination. After this part of the plot, decrease of the
NRM is observed, which is not matched by a corresponding
pTRM acquisition. This decrease extends up to 400–460 °C,
depending on the sample. At this upper temperature, the NRM
loss can be as large as nearly 50 per cent (Table 1 and Fig. 5).
At higher temperatures, almost linear segments, extending at
least up to 550 °C and covering about 40 per cent of the total
NRM intensity, are observed for most samples. Up to 522 °C
the pTRM checks are shifted to the left, which indicates a
progressive decrease of pTRM acquisition capacity in the
blocking-temperature range below 460 °C, due to the successive
heatings. For most samples, the last heating steps, in the
vicinity of the Curie point, seem to be affected by an increase
© 1998 RAS, GJI 134, 554–572
T hellier palaeointensity experiments
559
Figure 4. Day plot (Day et al. 1977) of all rock samples from the
Mafika section (normal polarity). The data corresponding to the six
samples selected for the present study are shown by solid symbols.
palaeointensity. Such an anomalous behaviour during the
Thellier experiment could not have been foreseen, bearing in
mind the rather encouraging palaeomagnetic and rockmagnetic properties of the samples.
3.3 Thellier experiments on laboratory TRM
Figure 3. SIRM
decay curves measured during warming up in
15 K
zero field (a), and their first derivatives ( b).
in pTRM acquisition capacity, as is commonly observed in
most lavas.
None of these six samples yielded a convincing palaeointensity result. All the samples, except perhaps sample 244, would
provide implausibly large palaeointensity estimates if the generally preferred, low temperature range (say 200 to 400–460 °C)
was used; in fact these strongly curved or even kinked
NRM–TRM plots cannot be safely interpreted in terms of
Three samples (samples 47, 117 and 199) were selected for this
study. All of them are characterized by a large, steep decrease
in the NRM–TRM plot at intermediate temperatures (Fig. 5)
and show only small changes in their hysteresis parameters
after the heating experiments (see Figs 13 and 14 below). After
the completion of the first Thellier experiment, a total TRM
was created by cooling down from 600 °C in a 20 mT magnetic
field. These laboratory TRMs were then subjected to a Thellier
experiment under conditions similar to those of the first
experiment, except that the laboratory field intensity was 30 mT.
The second laboratory TRM (TRM ) is equal to the first
2
one (TRM ) (once the scaling factor of 1.5 is taken into
1
account) for sample 117, and is some 3 per cent higher for
samples 47 and 199, revealing that only minor alteration
Table 2. Room-temperature hysteresis parameters of virgin samples.
Sample
number
J,
s
mA m2 kg−1
J ,
rs
mA m2 kg−1
H , mT
c
H , mT
cr
J /J
rs s
H /H
cr c
021
047
086
117
199
244
627
400
805
981
618
1007
133
72
144
184
174
154
15.7
17.7
10.5
16.8
29.1
16.9
31.0
37.4
24.8
37.1
51.4
37.3
0.212
0.180
0.179
0.188
0.282
0.153
1.97
2.11
2.36
2.21
1.77
2.21
J , saturation magnetization; J , remanent saturation magnetization; H , coercive force;
s
rs
o
H , remanent coercive force.
cr
© 1998 RAS, GJI 134, 554–572
560
A. A. Kosterov and M. Prévot
Figure 5. Results of Thellier experiments for the six selected samples: NRM–TRM plots and associated orthogonal NRM demagnetization
diagrams. In the NRM–TRM plots the open squares denote the pTRM checks; in the orthogonal diagrams the open and solid symbols correspond
to the vertical and horizontal planes, respectively. The units are A m2 kg−1.
continued during the second experiment. However, for all three
samples the diagrams are slightly concave upwards (Fig. 6). It
is interesting to estimate an apparent ‘palaeointensity’ from
the ‘most linear’ segments of the TRM –TRM diagrams, as
1
2
is done commonly for natural TRM. If we take this approach,
the results vary somewhat from sample to sample. For sample
47 the segment from 301 to 567 °C yields the best ‘palaeointensity’ estimate of 19.2±0.2 mT, only 0.2 mT lower than the value
obtained from the TRM /TRM ratio. For sample 199, a fairly
1
2
straight line can be fitted through all points except the last
one, yielding an estimate of 19.2±0.1 mT, again just 0.2 mT
lower than that obtained from the TRM /TRM ratio. In
1
2
contrast, the apparent palaeointensities from sample 117 may
be as high as 25.2±0.5 mT and as low as 18.5±0.2 mT,
depending on which points are used to fit a straight line. Thus,
at most, the field palaeostrength can be overestimated by 25
per cent.
Despite the fact that we are inevitably dealing here with
© 1998 RAS, GJI 134, 554–572
T hellier palaeointensity experiments
561
Figure 5. (Continued.)
samples somewhat altered by previous heatings, the results of
these Thellier experiments on artificial TRMs provide a fundamental constraint upon the interpretation of the non-ideal
behaviour of these samples as observed during the NRM–TRM
experiments: this behaviour is a property of the NRM, and is
not observed for the laboratory TRM.
3.4 Unblocking versus blocking spectra
For the purpose of analysing our data, it is convenient to
examine separately the blocking- and unblocking-temperature
spectra. Fig. 7 compares, for samples 47, 117 and 199, the
© 1998 RAS, GJI 134, 554–572
NRM unblocking spectrum (solid squares), the TRM blocking
spectrum (open squares), and the unblocking and blocking
spectra of TRM (solid and open circles, respectively). The
1
NRM and TRM unblocking spectra were normalized by the
NRM and TRM intensities, respectively. All spectra have been
reduced to values per one degree, in order to provide a
common scale for the pTRM differences calculated from the
variable temperature increments.
Examination of these spectra reveals that the NRM unblocking spectrum (solid squares in Fig. 7) is strongly dissimilar to
the other spectra. Even for sample 244, which shows the best
similarity of the spectra, there is still a small part between 250
562
A. A. Kosterov and M. Prévot
Figure 7. Blocking (open symbols) and unblocking (solid symbols)
spectra as calculated from the Thellier experiments. The squares refer
to the first experiment (solid symbols, NRM unblocking; open symbols,
TRM blocking), and the circles to the second experiment (with
1
laboratory TRM used as the ‘NRM’).
Figure 6. Results of Thellier experiments on laboratory TRM. F =
lab
30 mT. Notation as in Fig. 5.
and 330 °C where NRM unblocking is more effective than
pTRM blocking, resulting in a small but noticeable ‘kink’ in
the NRM–TRM plot. Unlike the NRM, the unblocking curves
of the laboratory-created TRM closely follow the blocking
1
curve. This emphasizes the uniqueness of the NRM as a specific
magnetic state that cannot be replicated by laboratory TRM
for our samples.
Three main hypotheses can be put forward to explain the
© 1998 RAS, GJI 134, 554–572
T hellier palaeointensity experiments
failure of the Thellier experiments: (1) a difference between the
blocking and unblocking temperatures inherent in the samples,
due to the presence of multidomain grains (Levi 1977;
Bol’shakov & Shcherbakova 1979; Worm et al. 1988;
Shcherbakov, McClelland & Shcherbakova 1993; McClelland,
Muxworthy & Thomas 1996; McClelland & Briden 1996);
(2) destruction of parts of the remanence carriers or modification of their chemistry during heating in such a way that
the low-T blocking temperatures are increased; and (3) irreversible physical changes during heating, resulting in the unblocking of the NRM at temperatures lower than the blocking
temperature. The rock-magnetic experiments described below
were carried out to try to identify the most plausible
explanation.
4 PR O P ER T IE S O F PAR T IA L T R M S A N D
E FFE CT O F H EAT IN GS
4.1 Experimental procedure
After stepwise alternating-field demagnetization (up to 150
mT) of the NRM, an ‘initial’ pTRM was given to the samples
by cooling from 200 °C to room temperature in a 20 mT
magnetic field parallel to the Z direction. This pTRM
[pTRM(200–20)] was then a.f. demagnetized in the same way
as the NRM. A second ‘initial’ pTRM was given by cooling
the samples from 270 to 200 °C in a 20 mT magnetic field
parallel to the +X direction, followed by cooling to room
temperature in zero field. After a.f. demagnetization of this
pTRM [pTRM(270–200)], the first two-component control
pTRM (pTRM1) was acquired so that during cooling, the
c
magnetic field was along the +X direction during the interval
270–200 °C and then along the +Z direction during the
interval 200–20 °C (Fig. 8). This control pTRM was then a.f.
demagnetized. After acquisition (and subsequent demagnetization) of ‘initial’ pTRMs in the intervals 330–270 °C, 460–
330 °C and 520–460 °C, a second two-component control
pTRM (pTRM2) was given during cooling in a 20 mT magnetic
c
field parallel to +X during the interval 520–460 °C and then
parallel to +Z during the interval 460–330 °C, which was
followed by cooling in zero field to room temperature. After
acquisition and subsequent demagnetization of pTRMs in the
intervals 560–520 °C and 610–560 °C, a third two-component
Figure 8. Experimental procedure for pTRM acquisition. The temperature intervals in which the pTRMs were acquired are indicated
along the horizontal axis; the letters within each interval indicate the
direction of the magnetizing field, in sample coordinates. The intervals
in which the control pTRMs were acquired are indicated by horizontal arrows.
© 1998 RAS, GJI 134, 554–572
563
control pTRM (pTRM3) was given, following a procedure
c
similar to that used for pTRM2 (Fig. 8).
c
All heatings were carried out in vacuum, under the same
conditions as for the palaeointensity runs. The magnetic susceptibility was measured at room temperature after each pTRM
acquisition. For samples 21, 47 and 199, sister specimens were
subjected to the same procedure as the regular specimens but,
in order to check the low-temperature (LT) memory of these
remanences, these specimens were cycled to the liquid-nitrogen
temperature in zero magnetic field prior to a.f. demagnetization.
4.2 ‘Initial’ pTRMs
Because it was not possible to demagnetize the samples completely using a 150 mT alternating field (the highest value
available in the laboratory), the pTRM intensity is defined
here as the total amount of magnetization removed by a.f.
demagnetization to 150 mT. This may introduce some error
into the numerical values of the pTRMs but this error is quite
small: the pTRM values obtained in this way agree well with
those of the pTRMs measured for the sister specimens in a
regular Thellier experiment.
The a.f. demagnetization curves of the ‘initial’ pTRMs,
normalized by their initial values, are shown in Fig. 9. They
all follow the trend of becoming harder with an increase of
the blocking-temperature range, thus supporting the intuitive
expectation (and the prediction of SD theory) that the stability
of the pTRM increases with its blocking temperature. The
shape of the demagnetization curves evolves from a typical
multidomain-like curve to a more single-domain-like one, with
a characteristic plateau at low alternating fields. The median
destructive field increases considerably with the blocking temperature: it is equal to 5–8 mT for pTRM(200–20) (15 mT for
sample 199) and reaches about 30 mT for pTRM(T –560)
c
(45 mT for sample 199). Such a three-fold increase in unblocking field suggests that these pTRMs were not blocked by the
same mechanism.
4.3 Control pTRMs
Owing to the experimental procedure used, six control pTRMs
were available for the 200–20 °C and 270–200 °C intervals,
three for the 460–330 °C and 520–460 °C intervals (see also
Fig. 10), and only one for the 560–520 °C and T –560 °C
c
intervals. The behaviour of the control pTRMs has much in
common for all the samples studied, particularly regarding the
pTRMs acquired in the two lowest-temperature intervals. The
magnitude of the control pTRM(200–20), acquired after heating to 270 °C, is reduced by a factor varying between 2 and 3
compared to the initial value. Similarly, the magnitude of the
control pTRM(270–200), acquired after heating to 330 °C,
decreases by about 50 per cent. It is worth noting that even
the first control pTRM(270–200) (acquired after heating to
270 °C) has already decreased compared to its initial value.
These results confirm the occurrence of irreversible changes in
magnetic properties at very moderate temperatures.
Upon the subsequent heatings to progressively higher temperatures (up to 520 °C), the pTRM acquisition capacity
corresponding to the low-temperature intervals continues to
decrease, although much less rapidly than before. A decrease
of acquisition capacity also takes place for pTRM(460–330)
after heating to 520 °C; however, it is far less pronounced in
564
A. A. Kosterov and M. Prévot
Figure 10. Evolution of (a) pTRM (200–20), ( b) pTRM (460–330),
and (c) pTRM (520–460) as a function of the temperature of the
preceding heating. The starting points of the plots denote the ‘initial’
pTRMs (see text); ‘treatment T ’ is the highest temperature reached
during the preceding heatings.
Figure 9. Alternating-field demagnetization of ‘initial’ pTRMs and
NRM (see text). Crosses, NRM; solid squares, pTRM (200–20); open
squares, pTRM (460–330); solid triangles, pTRM (520–460); open
triangles, pTRM (560–520); diamonds, pTRM (610–560).
its relative value than is the case for the low-temperature
pTRMs. The decrease in pTRM acquisition capacity measured
in this way matches quantitatively the diminution of pTRM
as checked during the routine Thellier experiments.
Beyond 560 °C, and especially at 610 °C, another process
starts that causes a significant increase of pTRM acquisition
capacity and affects, more or less uniformly, the whole blocking-temperature spectra of the samples. The only exception is
pTRM(610–560), which seems to decrease. However, a very
small irreversible decrease ( by just few degrees) of the Curie
point produced by heating might also account for the observed
decrease of the control pTRMs(610–560).
4.4 Low-temperature treatment of partial TRMs
Cycling to liquid-nitrogen temperature (77 K) in zero magnetic
field (LT treatment) is another way to gain insight into the
blocking mechanism of the remanences carried by magnetite.
In the course of an LT cycle, the sample passes through both
the isotropic point, at which K vanishes (around 130 K), and
1
the Verwey transition, from a cubic to a most probably
© 1998 RAS, GJI 134, 554–572
T hellier palaeointensity experiments
monoclinic crystal structure (around 120 K). It has long been
known (Kobayashi & Fuller 1968; Merrill 1970) that LT
demagnetization affects primarily the fraction of remanence
controlled by magnetocrystalline anisotropy, which is much
more important, in relative measure, in multidomain grains.
However, the multidomain remanence does not vanish fully
after LT demagnetization, and the single-domain remanence
does not remain unchanged (Halgedahl & Jarrard 1995;
Shcherbakova et al. 1996).
The effect of low-temperature treatment on the partial TRMs
was investigated only for samples 21, 47 and 199, using
specimens taken from locations directly adjacent to those used
for the study of the regular pTRMs. After the treatment at
liquid-nitrogen temperature in zero magnetic field, the lowtemperature memory of the pTRM was subjected to the same
a.f. demagnetization procedure as were the regular pTRMs.
The LT memory is always at its minimum for pTRM(200–20),
ranging from 50 to 60 per cent of the initial remanence, and it
increases gradually with the temperature of the blocking
interval of the pTRM (Fig. 11), approaching 90 per cent for
the highest temperature range; this agrees with the recent
results of Shcherbakova et al. (1996). However, in all our three
samples the LT memory of pTRM(560–520) is 10 to 15 per
cent lower than that of either pTRM(520–460) or
pTRM(T –560). We have no explanation for this discontinuity.
c
The whole data set suggests that almost half of the low-
565
temperature remanence is carried by MD particles with pinning
defects, whose coercivity is of magnetocrystalline origin.
The results of the a.f. demagnetization of the LT memory of
the pTRM are plotted in Fig. 12, together with those for the
original pTRMs. The low-temperature treatment removes
preferentially the softest part of the remanence, in accordance
with previous studies (Kobayashi & Fuller 1968; Merrill 1970;
Dunlop & Argyle 1991; Heider, Dunlop & Soffel 1992).
However, for the low-temperature pTRMs, although their LT
memory is more resistant to alternating-field demagnetization
than the original pTRM, their demagnetization curves remain
rather MD-shaped, in contrast with previous observations for
either SIRM or total TRM (Merrill 1970; Heider et al. 1992).
This suggests that even the part of the moderate-temperature
pTRMs which resists LT cycling can be of MD origin, through
internal stresses linked to crystal defects. In contrast, the high
values of LT memory and strong resistance to alternating fields
of the pTRMs acquired in the highest blocking-temperature
intervals suggest an SD-like blocking process.
5 E FFE C T O F LA B O R ATO R Y HE AT IN G O N
HY S T ER ES I S C H A R A C TE R IS TI C S
5.1 Room-temperature measurements
Hysteresis loops at room temperature were measured at SaintMaur, using a maximum field of 800 mT, on eight minicores
Figure 11. Histograms of pTRMs and their respective LT memories (shaded). The bottom right diagram shows the evolution of the LT memory
as a function of the upper temperature of the pTRM blocking interval.
© 1998 RAS, GJI 134, 554–572
566
A. A. Kosterov and M. Prévot
Figure 12. Comparison of alternating-field demagnetization of NRM
(crosses), and ‘initial’ pTRMs and their corresponding LT memories.
Squares, pTRM (200–20); circles, pTRM (460–330); triangles, pTRM
(520–460); diamonds, pTRM (610–560). The solid lines and solid
symbols indicate basic pTRMs; the dashed lines and open symbols
indicate LT memories.
a few millimetres in size cut from each sample. The first
minicore was not heated; the other seven were first heated in
vacuum to the various temperatures used for the experiments
on partial pTRMs (Section 4). Some of the data (Fig. 13),
particularly J and J , show a significant scatter, which we
s
rs
attribute to variation of the ferrimagnetic oxide content
between different minicores. Considering again the T –450 °C
0
interval where T is room temperature, it appears that heating
o
in vacuum did not change the room-temperature hysteresis
parameters by much, except for sample 86, which displays a
significant decrease in coercivity. For several samples, the
heatings to 560 °C and 610 °C resulted in significant or even
drastic (sample 86) changes. On the Day plot (Fig. 14), the
representative points shift towards the single-domain range.
These changes are probably caused by the further development
of ilmenite/magnetite intergrowths of much smaller size.
5.2 Measurements above room temperature
Measurements of hysteresis at high temperatures allow a more
rigorous examination of the changes produced by heating.
Furthermore, as we will see in the next section, these data
allow us to investigate the origin of the coercivity. These
measurements were carried out with a Princeton Measurements
vibrating-sample magnetometer at the Institute for Rock
Magnetism, Minneapolis. Two specimens from each sample
were measured, using a maximum d.c. field of 1.0 T. For the
first set of specimens the complete hysteresis loops were traced
at increasing temperatures from 25 to 610–630 °C, with a 25 °C
increment below 450 °C and a 10 °C increment above. Because
of the experimental set-up it was not possible to measure the
coercivity of the remanence. For the second set of specimens,
in order to distinguish between reversible and irreversible
changes, measurements were carried out during heating, and
also during cooling down to room temperature after reaching
successively higher steps (175, 475, and 580 or 595 °C). Also,
after the 275 °C step, the specimens were cooled to 175 °C
only. All measurements were performed under a helium
atmosphere.
The temperature dependence of the saturation magnetization
J (T ) is shown in Fig. 2 for some selected samples from the
s
second set of experiments. As pointed out above, and in
agreement with the room-temperature measurements after
heating in vacuum, the J (T ) curves are basically reproducible.
s
At most (samples 86, 117 and 199), a reduction of the order
of 5 per cent is observed. In contrast, the coercive force
(Fig. 15) exhibits marked irreversible changes. For all samples
except sample 244, an irreversible decrease occurs at very
moderate temperatures, which is documented by the H
c
measurements made during cooling from 175 and 275 °C. Note
that in this temperature range, absolutely no changes in J
s
occur (Fig. 2). This suggests that we are not dealing with
chemical changes but, rather, with purely magnetic ones. After
heating at 475 °C and above a different process occurs: the
coercivity exhibits an irreversible increase, accompanied for
most samples by irreversible changes in J .
s
6 O R I GI N O F CO E R C IVI TY
The interrelations between the temperature dependences of
hysteresis loop characteristics are an important source of
information on the mechanisms of coercivity and hence on the
remanence-blocking in a particular sample. The coercivity has
its origin in the potential barriers that have to be overcome to
enable the reversal of spontaneous magnetization in the case
of single-domain grains or the motion of domain walls in the
case of multidomain grains. In general, these barriers can be
due to shape, magnetocrystalline or stress-induced anisotropy,
the latter being governed in turn by magnetostriction. For
single-domain grains, H 3J in the case of shape anisotropy,
c s
H 3K /J (where K is the magnetocrystalline anisotropy
c
1 s
1
constant) for magnetocrystalline anisotropy, and H 3ls/J
c
s
(where l is the magnetostriction constant and s is the internal
stress) for stress-controlled anisotropy (Nagata 1961). In MD
particles, the coercivity is due to crystal defects or inclusions.
In this case, the coercivity is either of the form H 3K /J or
c
1 s
H 3ls/J (if internal stresses are present) (Hodych 1986).
c
s
Since J K and l for magnetite have very different temperature
s
dependences, the origin of the coercivity of a magnetite-bearing sample can be inferred from the relationship between
the temperature dependences of the coercive force and the
saturation magnetization.
A convenient way to visualize this relationship is to plot the
normalized coercive force against the normalized saturation
magnetization on a bilogarithmic scale. The plot will then be
a straight line if these two quantities are related through a
single power law. The degree n of this power law can range
from unity (coercivity due to pure shape anisotropy) to 6–9
(where magnetocrystalline anisotropy is dominant) (Fletcher
& O’Reilly 1974). A stress-controlled coercivity would exhibit
© 1998 RAS, GJI 134, 554–572
T hellier palaeointensity experiments
567
Figure 13. Evolution of the main hysteresis parameters measured at room temperature on separate fragments of samples after single heatings in
vacuum at various temperatures.
intermediate values of n. Between 20 and 500 °C, the magnetostriction constant l has been found to vary as J2.31 for pure
s
magnetite, while a larger power is found for Ti-substituted
magnetites (Moskowitz 1993). The existing experimental data
for magnetite-bearing rocks below room temperature (Hodych
1986, 1990, 1996), magnetite extracts (Dankers & Sugiura
1981) and synthetic magnetites (Levi & Merrill 1978; Dunlop
1987) all give n values between 1 and 2, which are interpreted
in favour of a mostly magnetostrictive origin of the anisotropy.
The value of n seems to be almost independent of grain size,
from the submicron range to several tens of microns. However,
the data of Heider, Dunlop & Sugiura (1987) for hydrothermally recrystallized magnetites deviate somewhat from this
trend, giving values of n between 2.5 and 4. This is readily
explained by the low dislocation density in these crystals,
which is hardly true for the exsolved magnetite grains typically
present in basalts.
The bilogarithmic plots of coercive force versus saturation
magnetization are shown in Fig. 16 for the first subset of
specimens (measured during heating only). For all samples
except sample 244, the initial part of the H –J plot, below
c s
200 °C or so, is characterized by a steeper slope than that
beyond this temperature. This feature is at least partly due to
the irreversible decrease in coercivity observed in this temperature range (Fig. 15), and the slope of this segment has therefore
no significance in terms of anisotropy. On the other hand, for
all samples, the slope of the curve is strongly and progressively
reduced at high temperatures (beyond 500–530 °C in general,
© 1998 RAS, GJI 134, 554–572
but at only 300 °C for sample 86). This feature correlates
reasonably well with the irreversible increase in coercivity
observed at high temperatures. Thus this high-temperature
portion of the curve, which is not linear anyway, also has no
significance regarding anisotropy.
Thus, only the linear, central part of the H –J curve, if
c s
present, can provide information about the origin of the
anisotropy. The most reliable data are provided by sample
244, which is not affected by a noticeable decrease in coercivity
at low temperatures (Fig. 15) and displays a linear dependence
of the H –J plot extending from room temperature to 500 °C.
c s
The coercive force thus follows a single power dependence on
the saturation magnetization, with n#1.2. No linear segment
is observed for sample 86, and there are some doubts regarding
sample 117, where the central part of the curve is slightly
S-shaped. The other three samples yield n values between 0.83
(sample 21) and 1.26 (sample 199, Fig. 16). Note that the small
irreversible increase in coercivity which is observed for all
samples after heating to 475 °C and beyond (Section 5) might
bias n towards low values. Considering all our data, we
conclude that the anisotropy is probably of mainly magnetostrictive origin. Magnetocrystalline anisotropy seems
marginal.
The relationship between coercive force and saturation
remanence has received little attention in the past. A common
belief is that the coercive force should be directly proportional
to the saturation remanence (e.g. Xu & Merrill 1990). However,
for all our samples except sample 244, the H (T )–J (T ) curve
c
rs
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A. A. Kosterov and M. Prévot
Figure 14. Day plot showing room-temperature hysteresis parameters as a function of the temperature of the preceding treatment (under vacuum).
The initial and final (610 °C) points are shown by solid symbols.
(on a linear scale) corresponding to temperatures up to 400 °C
is clearly concave upwards rather than linear (Fig. 17). Since
in the case of multidomain grains the saturation remanence
and coercive force are related through the demagnetization
factor N, the absence of linearity has to be ascribed to changes
in N. This conclusion agrees with the results of the calculations
of Xu & Merrill (1987), which show that for small multidomain
grains N varies with the number of domains and approaches
the ‘true’ multidomain limit of 4p/3 (for a sphere, in CGS
units) only for grains with at least 10 domains. For grains with
fewer than about 10 domains the demagnetization factor is
subject to change with any rearrangement of the domain
structure. Hence the non-linear relationship between the coercive force and the saturation remanence suggests that considerable changes in the domain structure of the magnetic mineral
occur in our samples in the interval 20–400 °C.
7 DI S C US S I O N
Among the various mechanisms which might produce a nonideal behaviour during Thellier experiments, two of them can
be readily discarded. Any effect from secondary magnetizations
is quite unlikely. The secondary overprints are fairly small and
are removed at 250 °C at the most, which is approximately the
temperature at which the anomalous decrease of the NRM
starts. We have no special reason to suppose that the primary
remanence is not a TRM: as already mentioned, spinodal
decomposition is commonly believed to occur above 600 °C
(Haggerty 1976). Thus, the magnetite in our samples is
expected to carry a TRM. Note, however, that in the
absence of pseudo-brookite in our rock samples, we have
no specific data which might prove this general statement for
certain.
As mentioned above, there are three main possible explanations of the behaviour of our samples:
(1) an intrinsic multidomain effect, involving no chemical
or magnetic change;
(2) some irreversible chemical changes;
(3) some irreversible physical changes.
These three possibilities are discussed below in the light of our
experimental data.
7.1 Intrinsic multidomain effect
Levi (1977) and McClelland et al. (1996) showed that magnetite
grains with a size of several microns fail to yield the correct
results in terms of palaeointensity, and always produce
‘NRM’–TRM plots that are concave upwards. Levi (1977)
showed that the degree of concavity increases gradually with
the mean grain size of the magnetite, which varies from about
0.1 to 2.7 mm for his samples. For the largest size, the ratio of
the slopes corresponding to the lowest- and highesttemperature parts of the NRM–TRM plots is about two. Thus,
this intrinsic effect of grain size seems insufficient to explain
the shape of the NRM–TRM plots of most of our samples.
This expectation is confirmed by the results of our
TRM –TRM experiments. These experiments were carried
1
2
out on three samples (samples 47, 117 and 119) which are
characterized by a large, steep decrease of the NRM–TRM
© 1998 RAS, GJI 134, 554–572
T hellier palaeointensity experiments
Figure 15. Temperature dependence of coercive force in the second
set of experiments (see text). Solid squares, values measured during
heating; open circles, cooling from 175 °C; open diamonds, cooling
from 275 to 175 °C; open triangles, cooling from 475 °C to room
temperature; open squares, cooling from 580 °C (595 °C in some cases)
to room temperature.
© 1998 RAS, GJI 134, 554–572
569
Figure 16. Bilogarithmic plots of normalized coercive force versus
normalized saturation magnetization for the first set of experiments
(measurements during heating only). The figures near the points denote
the temperature.
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A. A. Kosterov and M. Prévot
Figure 17. Normalized coercive force versus normalized saturation
remanence for the first set of experiments (measurements during
heating only).
plot at intermediate temperatures and show only small changes
in the hysteresis parameters after heating. Even for sample
117, which shows the most pronounced concavity of the
TRM –TRM plot (Fig. 6), the ratio of the two slopes men1
2
tioned above is less than 1.4. This is much smaller than that
observed on the NRM–TRM curve (Fig. 5). For the other
samples (47 and 199), the discrepancy between the NRM–TRM
and the TRM –TRM curves is even more blatant.
1
2
This behaviour is in agreement with our other magnetic
observations. The hysteresis parameters, resistance to alternating field and LT memory are all too large to be compatible
with the presence of a significant fraction of large MD grains.
At most, such grains could carry most of the low-temperature
pTRMs, as discussed above. But these pTRMs contribute only
a few per cent of the total TRM intensity, and thus cannot
seriously affect the results of the Thellier experiments.
Another important conclusion to be drawn from these
TRM –TRM experiments is that this peculiar behaviour is
1
2
specific to the natural TRM. Thus the observed changes are
irreversible, and occur during the first laboratory heatings in
the NRM–TRM experiments.
7.2 Irreversible chemical changes
These changes are most easily detected from the k(T ) and, to
a lesser extent, the J (T ) experiments. Let us first recall that
s
two magnetic minerals can be identified in our samples:
(1) almost pure magnetite, which is present in all samples and
is thermally stable (Fig. 2); and (2) a phase with a Curie point
of about 400 °C (probably titanomagnetite), which is unstable
upon heating in vacuum to about 600 °C (Fig. 2). This phase
presents hysteresis properties suggesting a larger magnetic
grain size than that of the magnetite. This second phase is
important only in sample 86.
Chemical changes can explain the shape of the NRM–TRM
plot if the main magnetic phase was strongly altered or
partially destroyed at intermediate temperatures during the
Thellier experiments. Figs 6 and 8 show that the temperature
range of this process is best identified in samples 47, 86 and
199, where it extends from approximately 200 to 400 °C. There
is no magnetic evidence for any large chemical changes occurring in this temperature interval: the saturation magnetization
measured at room temperature is not systematically modified
(Fig. 14) and the room-temperature susceptibility either
remains unchanged or decreases only slightly. In the second
case, the decrease is not specific either to the 200–400 °C
temperature interval or to the samples which are most affected
by the suspicious NRM decrease.
A more indirect but possibly more sensitive indication of
chemical alteration might be the development of some CRM
(chemical remanent magnetization) during heating. In this
respect, it is of interest to note that the representative points
of the ‘NRM’, drawn from palaeointensity measurements, do
not exhibit as perfect an alignment on the orthogonal diagrams
(Fig. 5) as that observed for conventional thermal demagnetization ( heating in zero field, Fig. 1). Between 200 and 353 °C
(329 °C for sample 199), the NRM directions are defined well
by straight segments passing through the origin, and closely
agree with the directions determined from thermal cleaning in
zero field. Above this temperature, the ‘NRM’ inclinations (in
sample coordinates) shift slightly towards the direction of the
laboratory field applied during the first heating, which suggests
that a small CRM may have been acquired. However, this
process of chemical alteration, if real, does not start at sufficiently low temperatures to be somehow linked to the anomalous decrease in NRM, which begins at 200 °C.
Thus we think that there is no evidence for a chemical
alteration occurring between 200 and 400–460 °C that is large
enough to destroy the remanence in a proportion that can
account for the large anomalous decrease in the NRM (Table 1)
observed during the Thellier experiments.
7.3 Irreversible physical changes
Evidence for some physical alteration (i.e. alteration of magnetic properties) during heating at moderate temperatures is
mainly provided by the coercivity data. The decrease observed
at low temperatures seems to operate up to 200–250 °C
(Fig. 15). Annealing is known to decrease the coercive force of
magnetite (Parry 1965; Lowrie & Fuller 1969; Smith & Merrill
1984), but the temperatures involved are much higher. We
tentatively suggest that some decrease in internal stress is the
cause of the coercivity decrease at low temperatures. The
increase in coercivity, which generally starts between 275 °C
and 475 °C, is an effect opposite to that of annealing. It can
be due to some increase of internal stress due to the formation
of microchemical heterogeneities, which, ultimately, will form
new exsolutions. In agreement with this suggestion, we
note that the hysteresis characteristics after the 610 °C heating
tend more towards those of SD grains; this is particularly
pronounced for sample 86 (Fig. 14).
Bearing in mind all the experimental results, we are obliged
to conclude that on heating our samples to moderate temperatures an irreversible process takes place that alters the samples’
magnetic properties, while the chemical composition of the
remanence carriers remains virtually unchanged. There are
essentially two magnetic changes: an irreversible decrease of
© 1998 RAS, GJI 134, 554–572
T hellier palaeointensity experiments
coercive force at low temperatures and an anomalous loss of
NRM between approximately 200 and 400–460 °C.
In the absence of a consistent theory for pseudo-singledomain grains, the exact physical nature of this process is
difficult to establish. Only a very crude and entirely qualitative
model will be considered here. Considering the six samples we
studied, we note that only one of them (sample 244) exhibits
no decrease in coercivity at low temperatures. This sample
also exhibits the smallest anomalous NRM decrease. Thus, the
low-temperature decrease in coercivity seems to be at the
origin of this anomalous behaviour. The coercivity of MD
grains is controlled by crystal defects in two different ways.
Defects are privileged sites for the nucleation of reversed
domains and, subsequently, they control domain wall progression by pinning/unpinning. We suggest that the decrease
in coercivity which occurs at low temperatures (up to 200–
250 °C) in our samples results in a decrease in the critical field
needed for nucleation. If, as we assumed above, this coercivity
decrease is due to a reduction in residual microstress, then the
total free magnetic energy is modified and new local energy
minima (LEM) can appear. Moreover, the increase in temperature itself can favour domain rearrangements into other LEM
states (Enkin & Dunlop 1987; Enkin & Williams 1994). Thus
we suggest that the decrease in coercivity and the increase in
temperature result in a rearrangement of the magnetic domain
configuration of some grains. This interpretation is supported
by the fact that the demagnetizing-field coefficient N, as
calculated from hysteresis data (Section 6), changes during
heating at moderate temperatures, except for sample 244.
As a result of domain rearrangement, these grains would
lose most, if not all, of their NRM. Obviously, the pTRM
acquired during the subsequent cooling to room temperature
would have absolutely no relation to the magnitude of the
NRM lost during heating. If this reorganization occurred at
moderate temperatures, the pTRM gain could be extremely
small compared to the loss of the NRM (which is a total
TRM). Such a process would result in an extreme steepness
of the NRM–TRM curves, as observed for most of our samples
at intermediate temperatures.
8 CO NCL US I O NS
Drastic non-ideal behaviour during Thellier experiments can
be observed in basalts. This anomalous behaviour is characterized by a large decrease in the NRM at intermediate
temperatures which is not accompanied by a mirror increase
in the TRM. This behaviour is observed in the absence of
significant chemical changes due to the laboratory heatings.
The presence of MD grains is not, in the present case, a
plausible explanation for this anomalous behaviour.
We suggest that this behaviour is due to the transformation
of the micromagnetic structure of PSD grains from a metastable configuration to a more stable one, as a result of an
irreversible decrease in coercivity which occurs at relatively
low temperatures (up to 200–250 °C). This decrease is probably
due to a reduction of internal stress during the laboratory
heatings. This irreversible physical change can lower the
nucleation field of some crystal defects, which, together with
the moderate temperature increase, can favour the rearrangement of magnetic domains. For individual crystals, such a
rearrangement occurring during heating would result in a
large, if not total, NRM loss, while the pTRM acquired during
© 1998 RAS, GJI 134, 554–572
571
the subsequent cooling could be quite small. The proposed
process is entirely decoupled from the blocking/unblocking
process: it can affect any fraction of the NRM regardless of its
‘true’ (i.e. caused by thermal fluctuations) unblocking
temperature.
We have also shown here that the measurement of complete
hysteresis loops as a function of temperature provides an
independent and apparently very sensitive method for testing
the magnetic stability of rock samples as a function of temperature. It seems to be of interest to use this type of investigation, as a complement to the classic tests of the Thellier
palaeointensity method, in order to ensure better the validity
of palaeostrength determinations.
A CKN O W LE DG M ENT S
This work was supported by CNRS-INSU under the DBT
programme ‘Terre Profonde’ (contribution CNRS-INSU-DBT
No. 82). A. Kosterov’s stay in Montpellier was made possible
thanks to a PhD grant from the French Government. We
thank James Marvin and Mike Jackson (Institute for Rock
Magnetism, Minneapolis, USA) and Maxime LeGoff
(Laboratoire de Géomagnétisme, Saint-Maur, France) for help
with the hysteresis measurements. The manuscript has benefited from the reviews of Buffy McClelland and two anonymous referees. Funds for the IRM operation were provided by
the Keck Foundation and the University of Minnesota.
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