Earth and Planetary Science Letters, 57 (1982) 173-181
173
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[4]
Magnetic characterization and M6ssbauer spectroscopy of
magnetic concentrates from Greek lake sediments
S. Papamarinopoulos
P.W. Readman 2
Department of Geophysics, University of Edinburgh, Edinburgh (Scotland)
Y. Maniatis and A. Simopoulos
N.R.C. "Demokritos", A ghia Parasket;i, Athens (Greece)
Received August 27, 1980,
Revised version received September 28, 1981
Bulk magnetic properties of three Greek lake sediments indicated that the main magnetic constituent was predominantly pseudo-single-domain magnetite. This was confirmed by successfully extracting and directly identifying the
magnetite. The majority of the magnetite in the magnetic concentrates is present as grains of about 2 3/~m in size, the
overall size ranging from < I to ~ 10 ~ m with occasional grains up to - 2 0 p,m. The grains are roughly equidimensional
and angular in form indicating that they are of primarily detrital origin, although an authigenic origin for the very' fine
grains must be a possibility. Mbssbauer spectra of the magnetic concentrates suggest that either the magnetite is slightly
non-stoichiometric to a similar degree in the three lakes, or contains impurity ions. Haematite was also found in the
concentrates but its abundance is o n l y - - 1 0 - 1 5 % that of magnetite and it is not detectable in any of the magnetic
properties of the sediments.
1. Introduction
Since the observation of a coherent pattern of
secular variations in declination in a sediment core
from Lake Windermere, England [1] there have
been several similar investigations on sediments
from other parts of the world. The same effort,
however, has not gone into the identification of
the magnetic minerals and remanence carriers in
the sediment, and most such investigations have
relied upon remanence measurements made on
whole sediment samples.
In this paper we report a simple magnetic extraction technique that has been used to obtain
I Present address: Institute of Geology and Mineral Exploration. 7(1 Messogion Street, Athens, Greece.
2 Present address: Geophysics Section, Dublin Institute for
Advanced Studies, 5 Merrion Square, Dublin, Ireland.
0012-821X/82/0000-0000/$l)2.75
magnetic concentrates from cores of sediment
taken from three Greek lakes, viz. Trikhonis
(Aetolia), Vegoritis (west Macedonia) and Volvi
(east Macedonia). The magnetic concentrates have
been investigated and identified directly using Xray diffraction, optical reflection microscopy, thermomagnetic analysis and MOssbauer spectroscopy.
2. Magnetic properties of the sediments
2.1. Palaeomagnetic measurements
Sediment cores were taken primarily for
palaeomagnetic measurements to determine detailed secular variations of the geomagnetic field
during the Holocene. The palaeomagnetic results,
which are of high quality, are reported elsewhere
[2] and here we give only a summary of the results
1982 Elsevier Scientific Publishing Company
174
relevant to the magnetic properties of the sediments.
Intensities of natural remanent magnetization
(NRM) J0 are generally in the range 10-50 #G
(I#G=10-9A
m-n). Initial susceptibilities k
range from I0 to 20 #G O e - n (1/~G O e - n = 4~r X
10-6 SI units) and modified Q ratios ( J o / k ) range
up to 5, with the majority of values between 1 and
4. Directional changes of the NRM on alternating
field (AF) demagnetization are limited to a few
degrees up to peak fields of 500-600 Oe (1 Oe = 0.1
mT). The N R M is generally reduced to half its
original value (the median destructive field) by
fields of 250-400 Oe.
2.2. Isothermal remanence
The isothermal remanence (IRM) produced by
a field of 10 kOe ranges from 0.4 X 103 to 2 X 103
# G for samples from Lakes Trikhonis and Volvi,
and from 2 X 10 3 to 8 X 10 3 # G for samples from
Lake Vegoritis. Saturation is obtained in fields
above 2 kOe and the coercivity of IRM, i.e. the
reverse field required to reduce the remanence to
zero is generally between 300 and 400 Oe. Typical
results from the three lakes are shown in Fig. 1.
These results suggest that magnetite is the major
carrier of the IRM, but by themselves do not
preclude the presence of haematite. Although fine
grained haematite has a much higher coercivity
than magnetite, since the IRM produced by high
fields ( ~ 10 kOe) is unlikely to exceed 5 ~ 10%
that of magnetite grains up to ~ 20/~m in size [3],
and will be less than 1% for single-domain grains,
it obviously could be present in proportions many
times that of magnetite and remain undetected in
IRM measurements. Coarse-grained haematite
could be present in still higher proportions since
its coercivity is nearer to that of magnetite. However, haematite can be ruled out as a major carrier
of the IRM simply by estimating the quantity
required in the sediments to produce the observed
IRM, which would give a haematite content of
more than about 0.4-0.8%, i.e. equivalent to ~ 8160% of the total iron content of the sediments.
The fact that this is unlikely is clearly confirmed
by M/3ssbauer Effect experiments on sediments
from these lakes [4] which show that ordered magnetic phases account for less than about 5% of the
total iron. Therefore although it is possible that
coarse-grained haematite could account for some
of the remanence in these sediments when the
IRM is low (i.e.< 0.5 X 103/~G), it certainly could
not be a major carrier in most of them.
2.3. Low-temperature remanence
Another method of identifying magnetic minerals in a sediment is to look for low-temperature
magnetic transitions, i.e. the Morin transition in
haematite at T ~ 260 K and the magnetite transi-
(A)
o~,~_~,.-.~ m~ •
J
1.C
a
~0
(c)
f
~
IRM
3xi0 3
HG
I
0.~
Vegoritis
a
(O)
a I~
2
/
a°
0.6
~,,_~.
•
Volv_i
•
•
- ~a
a
o.~
,
a
6
I O
o
. . . .
~
2 0
Tern peroture
0'5
0
0"5
1
1"5
Ja
aa
2
3
4
6
10
H ~ kCel
Fig. 1. Isothermal remanence as a function of applied field for
typical sediments from Lakes Vegoritis, Volvi and Trikhonis.
.
.
.
.
300
IK]
Fig. 2. Variation of remanence with temperature on cooling
sediments from Lake Volvi: curves A and B are IRM given in a
10 kOe field at room temperature to dried samples; C is a
similar IRM given to a wet sample; and D is the N R M
variation of a wet sample. Open circles show the curve on
rewarming from 80 K for D.
175
Dry
10f
ments with wet and dried samples of non-magnetic
lake sediment containing synthetic magnetite and
haematite powders (see Fig. 3).
Some typical results of low-temperature remanence experiments of both wet and dried samples
of sediments are shown in Fig. 2. The presence of
magnetite is suggested in some samples by slight
decreases in remanence at around T ~ 120 K, but
many samples which clearly contain "magnetite"
do not show conclusive transitions, presumably as
a result of small grain size, impurities or nonstoichiometry in the "magnetite".
No samples were found to show a Morin transition. This does not of course prove the absence of
haematite as the transition is easily masked by the
magnetization of a more strongly magnetic mineral,
or it may be suppressed, as in the case of the
magnetite transition, by impurity or grain size
effects.
Magnetite
0.8
1.0
::=="-
Dry
o.8 Haematite
//~
f
13
0.6
.~
a
E
0.4
o
z
0.2
0
i
L
100
150
i
,
,
,
. . . .
,
200 220 240 260 280'300
T e m p e r a t u r e
K
Fig. 3. Variation o f isothermal remanent magnetization with
temperature on cooling for wet and dried non-magnetic lake
sediments containing synthetic magnetite and haematite. The
IRM was given at room temperature in 12 kOe and measurements taken on cooling. The cooling rate used was about 5 °
min ~ and has resulted in considerable thermal lag in these
samples as indicated by the apparently broad Morin transition
a t ~ 2 2 0 K compared with a more precise determination of
2 6 0 ± 1 K obtained using a translation balance. The samples
consist of about 5 mg magnetite and 100 g m g haematite each
mixed with about 6 g sediment. The synthetic powders used
were from Columbian Chemicals Mapico series, Black (magnetite) and Red 110 (haematite).
tion at T-- 120 K. Since the N R M of these sediments is in general too weak to obtain reliable
results with the Digico low-temperature magnetometer, samples were given an IRM at room
temperature and its variation measured on cooling
to liquid nitrogen temperature. Undried samples
(Fig. 2, curves C and D) show a steplike decrease
at around room temperature, and in early papers
on lake sediments [5,6] such behaviour was incorrectly identified with the Morin transition in
haematite. We now attribute it to disorientation of
magnetic grains during freezing of water in the
sediment as is clearly demonstrated by our experi-
2.4. Lowrie-Fuller test
Some idea of the magnetic domain state can be
obtained using a modified Lowrie-Fuller test [7] in
which AF demagnetization curves of saturation
IRM are compared with those of saturation anhysteretic magnetization (ARM). Results are similar
1.0
I R M ~ ~
O0 200 &O0 600 800 1000
a
,
i
,
Haf (De
Fig. 4. Comparison of AF demagnetization of I R M and A R M
for sediments from Lake Trikhonis. I R M (open symbols) is
given in l0 kOe and the A R M (closed symbols) in an alternating field of 1000 Oe at 300 Hz with a parallel steady field of 0.4
Oe.
176
for the three lakes: ARM demagnetization curves
usually have significantly higher median destructive fields than those of IRM, as shown by the two
typical results from Lake Trikhonis in Fig. 4. This
implies that the majority of the remanence carriers
are fine-grained, i.e. single-domain (SD) or pseudo-single (PSD) size, that is to say below about
15-20 #m, the generally accepted threshold for
true multidomain (MD) behaviour [8,9].
2.5. High-field magnetization measurements
The ratios of saturation remanence J~s to saturation magnetization J~, and of coercivity of remanence Hc~ to coercive force Hc are another source
of information about the domain state of grains
(e.g. [8-10]). For non-interacting SD grains with
uniaxial anisotropy the ratio Jr~/J~ is 0.5, whereas
it is ~< 0.05 for MD magnetite grains. The coercivity ratio H~r/Hc, although not so diagnostic has
theoretical values between 1.1 and 2.02 for SD
grains [11,12] and is above 4 - 5 for MD grains.
Pseudo-single-domain grains, and samples containing grains of more than one domain state, have
intermediate J r s / ' I s and H c r / H c ratios.
Hysteresis curves up to-+ 17 kOe were obtained
for about ten samples from each lake and average
values (excluding thin volcanic ash horizons) of J~,
J~s/Js, Hc and H¢~/H c are shown in Table 1. Two
samples have very high Jr~/'Is values ( > 0.4) and
therefore must have average magnetic grain sizes
quite close to the PSD-SD threshold, so might be
expected to contain some SD grains in addition to
PSD grains. However the presence of significant
proportions of MD or superparamagnetic grains,
even for Lake Vegoritis which has the lowest J~s/Js
values appears to be unlikely because of the low
H,:~/H c values. Thus we interpret these results as a
whole to reflect the presence of predominantly
PSD grains.
3. Magnetic extraction
Identification of the magnetic minerals in lake
sediments using X-ray diffraction is not possible
using whole sediment samples because of the predominance of non-magnetic minerals. Thermomagnetic curves of whole sediments are impossible
to interpret due to complex chemical changes in
the sediment on heating which produce secondary
magnetic minerals whose magnetization swamps
that of the primary magnetic minerals. Therefore a
technique to extract magnetic concentrates of high
purity was developed [13].
A peristaltic pump is used to circulate a dispersed suspension of sediment through an inclined
narrow tube between the tapered polepieces of a
3.5-kOe permanent magnet. The method is an
improvement on previous ones as the residue after
passing the magnet is returned to the bath of
water/sediment mixture which is continuously
stirred by an electric stirrer, and extraction can
take place continuously over long periods. The
highest extraction efficiency, expressed as a percentage reduction in the saturation IRM after
extraction was 80%, although typically 20-50% is
obtained depending largely on the patience of the
researcher. Although we are sure that this technique would extract haematite if present in signifi-
TABLE 1
Average hysteresis properties of Greek lake sediments (the range of values for each parameter is given in brackets)
Volvi
Vegoritis
Trikhonis
Js X lO 2
(emu g - I )
Jrs/L
2.3 (1.1 -4.0)
2.5 (2.0 -3.6)
1.2 (0.96-1.49)
0.32 (0.22-0.43)
0.24 (0.21-0.27)
0.36 (0.32-0.44)
llc
II':r/]]c
(Oc)
200 (146-234)
130 ( 115-153)
204 (186-221 )
1.59 (1.52-1.78)
2.30 (2.09-2.37)
1.87 ( 1.82-1.92)
The "Is, JrJJs and H c values are based on between 8 and 11 samples depending on the lake. The Hcr/H ~ values are based on only 3 to
5 samples (for Volvi samples below the depth where ferrous carbonate occurs [4] are not included) but using values of Her for samples
close in depth to the samples used for the H c measurements generally give similar results.
177
cant proportions because experiments with other
sediments have yielded concentrates containing
predominantly haematite with smaller amounts of
magnetite, we might expect that the composition
of the concentrates be biased towards the more
strongly magnetic minerals in the sediment. This
tendency will decrease with longer extraction times
and will be less pronounced when the sediment is
not allowed to dry prior to the extraction (since
after drying it is difficult to separate the grains).
It was only possible to obtain small quantities
(several milligrams) of extract, as can be appreciated by the fact that the magnetite content of 20 g
of sediment inferred from its saturation magnetization (Table 1) is only about 2 - 8 rag. The extracts were room temperature air-dried prior to
further experiments; oven-drying at around 80°C,
as sometimes used, being avoided because finegrained magnetite can oxidize at t e m p e r a t u r e s 100°C [14,15] and hydroxides and gels [16] may be
unstable at low temperatures.
4. X-ray diffraction and optical microscopy
X-ray powder photographs of the magnetic concentrates were obtained using a Debye-Scherrer
camera. Magnetite was the strongest pattern in all
three concentrates. Back reflections were quite
blurred which made it impossible to obtain reliable estimates of the cell size, and suggesting that
either a range of compositions, highly strained or
very fine ( < 0 . 1 /~m) grains are present. A few
additional lines were also present and identified as
follows. In the Volvi concentrate, lines at 2.69 and
1.69 A were indexed as the (104) and (116) lines in
the haematite pattern, the intensity of the (104)
line (by visual inspection) being ~< 10% that of the
(311) magnetite line. A line at 3.33 ,~ was found in
all concentrates and was identified as the strongest
quartz line (101). This was moderately strong ( 50% the intensity of the (311) line in magnetite) in
Vegoritis and the next three strongest lines
(100, 112, 211) were also detected for this lake. A
line at 3.03.& was identified as the (104) calcite
line in Vegoritis and Trikhonis, and a broad diffuse line at 3.20 A in Volvi and Trikhonis as the
plagioclase (040) and 202) lines: additionally a
TABLE 2
Minerals detected in magnetic concentrates by X-ray diffraction
Magnetite
Haematite
Quartz
Plagioclase
Calcite
Other *
Volvi
Vegoritis
Trikhonis
x × ×
x x x
x × ~<
x×
x
×
x
×
×
x
×
x
>~
* Reflection ate2.03 ,~
The number of crosses indicate relative abundances (estimated from visual inspection of the strongest reflections):
Xx×,
~60%; X × ,
~30%; × , <~10-20%.
faint line at 4.03,~ was interpreted as the plagioclase (201) line in Trikhonis. A faint line at 2.03 A
was observed in Volvi and Vegoritis which was not
identified. The results are summarized in Table 2
where the number of crosses are used to indicate
relative intensity of strongest lines in the pattern.
The concentrates were set in polyester resin and
polished sections viewed under reflected light.
Again for all three concentrates the dominant
mineral was identified as magnetite. The grain size
varied from < 1 to about 10 # m with many grains
in the 2 - 3 ~ m range. The grains were more or less
equidimensional and angular in form, and therefore considered to be primarily of detrital origin.
A few larger grains up t o - 2 0 /~m were also
found. Haematite was also present in all three
concentrates but in much smaller proportions than
the magnetite, either as surface coatings on quartz
grains or as partially oxidized magnetite.
5. Thermomagnetic curves
Thermomagnetic curves for the concentrates
from the three lakes are similar and that for Lake
Trikhonis, heated in air, is shown in Fig. 5. An
irreversible decrease in magnetization starts at
300°C and continues up to 500°C. Heating to
700°C results in further chemical change as is
shown by the reduced magnetization on cooling.
The magnetization versus field curve at room
178
15
.~
e~ 10
E
~
Lake Trikhonis
~
E
0
magnetic concentrate
~
100
200
300
400
Temperature
500
°C
600
700
Fig. 5. Thermomagnetic curve in air for the magnetic concentrate from lake Trikhonis. The applied field is 4.7 kOe and
the heating rate~10 ° min - l . The sample was first heated
t o ~ 4 5 0 ° C (A ~ B) then cooled to below 200°C ( B ~ C) before
finally heating to 700°C and cooling to room temperature
(C~D---,E).
temperature before heating shows saturation for
fields a b o v e ~ 2 kOe. After heating to 700°C the
magnetization at room temperature is reduced to
4 emu g -1 at 14 kOe and shows an increase of ~ 0.1
emu g - ~ between 2 and 14 kOe suggesting that a
substantial quantity of haematite has been produced.
Although it is not possible to give a unique
explanation of the thermomagnetic curves the general characteristics are those shown by both finegrained iron-rich titanomagnetites and titanomaghaemites [14]. The observation of sharp peaks
in the Mrssbauer spectra (section 6) rule out the
presence of titanomagnetites, Fe3_xTixO4, with
significant Ti content, i.e. x~>0.2 [17]. The decrease in magnetization above about 300°C may
be caused by oxidation of "magnetite" towards
"maghaemite" or by oxidation of part of the sample, perhaps determined by grain size or composition, directly to haematite. The further decrease in
magnetization after heating to 700°C can be caused
by inversion of the maghaemite or by further
oxidation of the remaining portion of the sample
to haematite. It is likely that the magnetization
remaining after heating to 700°C is due to coarsergrained magnetite and that the magnetization lost
during the heating (i.e.-70%) is due to finergrained magnetite.
6. MOssbauer spectroscopy
Mrssbauer spectra were taken at room temperature with a constant acceleration spectrometer.
The source was 100 mCi 57Co (Rh). Only small
quantities ( ~ 1 mg) of the concentrates were available from the three lakes so special holders with a
diameter of 7 mm were used for mounting the
absorber. The spectra are shown in Fig. 6 and were
least-square fitted with a computer program which
can accommodate five different iron sites. The
variable parameters for each site in this program
are the magnetic hyperfine field H, the quadrupole
interaction e2qQ/4, the isomer shift 8 and the
intensity and half width at half maximum £ / 2 of
each absorption peak which is assumed of
TABLE 3
Hyperfine parameters for S7Fe at room tempeature
a-Fe203
]7/2
Volvi
Vegoritis
Trikhonis
0.14
0.29
0.18
Fe304
6
0.32
0.27
0.42
H
522
521
508
e2qQ/4
0.10
0.08
0.08
I (%)
I1
×
6
A-site
F/2
8
fI
e2qQ/4
I (%)
0.20
0.22
0.27
0.23
0.24
0.35
496
493
489
0.0
-0.03
- 0.02
29
X
26
F / 2 = half width at half maximum, 8 = chemical shift relative to iron metal, H-effective field, e2qQ/4 =quadrupole splitting of the
magnetic components, A = quadrupole splitting of the paramagnetic components, and I = relative absorption in percent. Crosses ( × )
indicate values not obtained because fitting was not possible for the full spectrum (see text)
179
LIMNIC
i
,
i
~
i
MAGNETIC
,
~
,
'~'Ja",%'
i
.
,
.
i
i
I
o
.
Relative percentage abundances of the magnetic oxides
.
•
i ~-.~,
..
.."
i
~-Fe203
.
,.
.:."
.-
~
pa.,.,
,
"
: .
:
•
.
-2 •."
TA B LE 4
EXTRACTS
i
,~'x. ,.n
~"
•
-
i
-.
•
C
%
P
.
995
,',"~..,...,.,
~: ~..
,.
i,.m r-~
:,.. : .,,j.
,
E
I~
7-
, ~
";
:'x.,:
•. ' . ,
f
-
H
W
F~
CI
9P o
LJ
-
;"
Ill,,
.:
~.
•
*. •
,..
,
.
.at*
995~
• :(
(%
..
,,
:
,.
L.n
L,
~
,
la
t
8
,
t
.
6
t
-4
UEL~CI
,
14
12
8
37
37
40
49
51
52
Lorenzian shape. All these parameters were unconstrained and the results of the fits are shown in
Table 3.
The analysis of the spectra shows that all three
concentrates contain mainly magnetite (component H in Fig. 6). There is also some haematite
( c o m p o n e n t / ) and various paramagnetic phases
(components I I I and I V for ferric and ferrous,
respectively). The spectrum of the Vegoritis concentrate cannot be fully analysed due to the presence of a component with broad absorptions near
velocities - 2 and + 4 m m s - 1 which may possibly
arise from intermediate relaxation conditions of
one of the magnetic phases [18]. Relative proportions of the oxide phases are given in Table4
where it can be seen that the haematite fraction is
significantly less in the Trikhonis sample than in
the Volvi and Vegoritis samples.
In all three samples the magnetite may be
slightly non-stoichiometric, i.e. Fe 3 a [] aO4 where
the symbol [] represents vacancies. The degree of
non-stoichiometry ~ can be calculated from the
relative absorption of the A and B sites (more
<
C"
.
B-site
:
990
",
A-site
C
Volvi
Vegoritis
Trikhonis
U1
Fe304
•
i
i
i
i
i
z
o
z
a
6
TY
(MM/SEC)
.
t
a
,
i
io
Fig. 6. Room temperature MOssbauer spectra for magnetic
concentrates from Lakes Volvi, Vegoritis and Trikhonis. The
line positions of the components of the spectra are indicated:
l = a - F % O 3 ; I I = F e 3 0 4 : I l l = f e r r i c ion in clay minerals or
superparamagnetic oxides; I V = ferrous ion in clay minerals.
Paramagnetic
B-site
Fe 2+
Fe3 +
F/2
8
tt
e2qQ/4
I (%)
F/2
8
~
1(%)
r/2
0.26
0.35
0.41
0.60
0.70
0.62
465
465
451
0.0
0.0
0.06
39
×
34
0.29
x
0.36
0.27
x
0.32
0.72
×
0.80
21
×
28
.
x
0.25
a
.
.
x
1.2
±
I(%1
x
2.4
×
6
.
180
strictly, the A-site Fe 3+ and B-site Fe 3+ not involved in the B-site Fe 2+/3+ electron interchange,
and the B-site Fe2+/3+), i.e. A / B = (1 + 5 8 ) / ( 2
- 6 8 ) following Coey et al [19]. 8 is found to be
quite similar for the three lakes: 0.05 ± 0.02 for
Volvi and Vegoritis and 0.06 ± 0.05 for Trikhonis.
The errors are determined from one standard deviation of the intensity and linewidth, the larger
error for Trikhonis arising from the greater uncertainty in defining the B-site absorptions which
have larger linewidths (see Table4 and Fig. 8).
These 8 values must be considered very tentative
as they are only valid in the absence of cations
other than iron. We note that the hyperfine fields
for,Trikhonis are smaller than those for Volvi and
Vegoritis and factors which could cause this are a
variation in particle size [18] or, as is quite likely in
samples such as these, replacement of some Fe
with cations such as Ti [17,20], A1 or Mg.
The hyperfine parameters for the haematite
spectra agree with those for well crystallized aFe203 [21], except that for the Trikhonis sample
the effective field is somewhat lower than normally
expected.
The hyperfine parameters for the paramagnetic
part, which represents 21-34% of the total absorption, are close to those determined in whole sediment samples from these lakes [4]. Therefore the
paramagnetic part of the spectra is likely to be due
to iron in the residual clay minerals and paramagnetic hydroxides or oxyhydroxides. Superparamagnetic effects [18] could also contribute to
the paramagnetic part of the spectra but due to the
complex form of the Fe304 spectra below the
Verwey transition at T - - 1 1 0 K it is difficult to
determine quantitatively such contributions.
7. Conclusion and discussion
Magnetic measurements on bulk sediments from
three Greek lakes suggested that the dominant
magnetic mineral was fine-grained (i.e. ~< 20/~m)
magnetite for all three of the lakes, and subsequent identification in magnetic concentrates obtained from the lakes confirmed this conclusion.
Haematite was also observed in the concentrates
but the amount was found to be small compared
to the magnetite and although because of its possibly less efficient magnetic extraction it could be
present in larger proportions in the sediment, it
clearly does not significantly contribute to the
magnetic properties of the sediments. Indeed it
was not detected in any of the magnetic measurements on whole sediment samples which emphasises the value of obtaining magnetic concentrates,
and of using non-magnetic methods of identification.
Optical microscopy shows the magnetite to be
of detrital origin, at least for grains large enough
to be observed under the microscope. The source
of detrital magnetite in the sediments from Lakes
Volvi and Vegoritis is fairly clear. Around Lake
Volvi, sedimentary, basic eruptive and metamorphic rocks are clearly exposed close to the lake,
and possible sources of magnetite are a Mesozoic
granite and, in particular, Palaeozoic amphibolites
altered towards gabbro [23]. For Lake Vegoritis,
the most probable source of magnetic detritus are
the extensive exposures of ophiolites to the north
east of the lake [24]. For Lake Trikhonis, it most
likely originates from the erosion of Pliocene
palaeolimic and river alluvia deposits located to
the north west of the lake [25], but there is, however no, obvious source from which the detrital
magnetite can have originally been derived. The
results from the MOssbauer Effect measurements
suggest that possibly a different situation may
exist in this lake as the haematite content is less
and the hyperfine parameters of the spectra are
slightly, but significantly different to those from
Lakes Volvi and Vegoritis. One explanation for
this could be that the concentrate from Lake Trikhonis contains a higher proportion of very finegrained magnetite which is too small (i.e. < 0.1
/~m) to be detected by the optical microscope.
With the abundance of iron hydroxides (or
oxyhydroxides) in these sediments [4] it seems
posible that some of the magnetite, most likely to
be the finer grain size fraction, could be of authigenic origin.
Acknowledgements
This work was performed as part of an investigation of geomagnetic secular variations recorded
181
by lake sediments, funded by the Natural Environmental Research Council (grant GR3/2238) and
under the general direction of Professor K.M.
Creer, to whom we are grateful for help and
encouragement. Dr. R. Gill of the Geology Department, University of Edinburgh helped with
identification of minerals using the optical microscope. One of the authors (S.P.) gratefully
acknowledges financial assistance from the Russel
Foundation in Scotland.
References
I F.J.H. Mackereth, On the variation in direction of the
horizontal component of remanent magnetization in lake
sediments, Earth Planet. Sci. Lett. 12 (1971) 332-338.
2 K.M. Creer, P.W. Readman and S. Papamarinopoulos,
Geomagnetic secular variations in Greece through the last
6000 yr obtained from lake sediment studies, Geophys. JR.
Astron. Soc. 66 (1981) 193-219.
3 EH.M. Dankers, Magnetic properties of dispersed natural
ironoxides of known grain size, Thesis, University of Utrecht
(1978).
4 P.W. Readman, J.M.D. Coey, Ch. Mosser and F. Weber,
Analysis of some lake sediments from Greece, J. Phys.
C6-37 (1976) 854-848.
5 K.M. Creer, R. Thompson, L. Molyneux and F.J.H. Mackereth, Geomagnetic secular variation recorded in the stable
magnetic remanence of recent sediments, Earth Planet. Sci.
Lett. 14 (1972) 115-127.
6 R. Thompson, Stratigraphic consequence of palaeomagnetic
studies of Pleistocene and Recent sediments, J. Geol. Soc.
London 133 (1977) 51-59.
7 HP. Johnson, W. Lowrie and D.V. Kent, Stability of
anhysteretic remanent magnetization in fine and coarse
magnetite and maghemite particles, Geophys. J.R. Astron.
Soc. 41 (1975) 1-10.
8 L.G. Parry', Magnetic properties of dispersed magnetite
powders, Philos. Mag. 11 (1965) 303-311.
9 R. Day, M.D. Fuller and V.A. Schmidt, Hysteresis properties of titanomagnetites: Grain size and compositional
dependence, Phys. Earth Planet. Inter. 13 (1977) 260-267.
10 D.J. Dunlop, J.A. Hanes and K.L. Buchan, Indices of
multidomain magnetic behaviour in basic igneous rocks:
alternating field demagnetization, hysteresis and oxide petrology, J. Geophys. Res. 78 (t973) 1387-1393.
11 E.C. Stoner and E.P. Wohlfarth, A mechanism of magnetic
hysteresis in heterogeneous alloys, Philos. Trans. R. Soc.
London, Ser. A 240 (1948) 599-642.
12 P. Gaunt, A magnetic study of precipitation in a gold-cobalt
alloy, Philos. Mag. 5 (1960) 1127-1145.
13 S. Papamarinopoulos, Limnomagnetic studies on Greek
sediments, Thesis, University of Edinburgh (1978).
14 P.W. Readman, A magnetic study of non-stoichiometric
titanomagnetites, Thesis, University of Newcastle-uponTyne (1972).
15 H.P. Johnson and R.T. Merrill, Magnetic and mineralogical
changes associated with low temperature oxcidation of magnetite, J. Geophys. Res. 77 (1972) 334-341.
16 J.M.D. Coey and P.W. Readman, Characterization and
magnetic properties of natural ferric gel, Earth Planet. Sci.
Lett. 21 (1973) 45-51.
17 S.D. Jensen and P. Shive, Cation distribution in sintered
titanomagnetites, J. Geophys. Res. 78 (1973) 8474-8480.
18 T.K. McNab, R.A. Fox and A.J.F. Boyle, some magnetic
properties of Fe304 microcrystals, J. Appl. Phys. 39 (1968)
5703-5711.
19 J.M.D. Coey, A.H. Morrish and G.A. Sawatsky, A M6ssbauer study of conduction in magnetite, J. Phys. CI-32
(1971) 271-273.
20 S.K. Banerjee, W. O'Reilly, T.C. Gibb and N.N. Greenwood,
The behaviour of ferrous ions in iron-titanium spinels, J.
Phys. Chem. Solids 28 (1967) 1323-1335.
21 W. Kundig, H. Boemmel, G. Konstabaris and R.H. Lindquist, Some properties of supported small a-Fe203 particles
determined with M6ssbauer Effect, Phys. Rev. 142 (1966)
327-333.
22 R.S. Hargrove and W. Kundig, M6ssbauer measurements of
magnetite below the Verwey transition, Solid State Comm.
8(1970) 303 307.
23 A.A. Psilovikos, Palaeographic development of the basin
and the lake of Mygdonia (Langatha and Volvi area,
Greece), Thesis, University of Thessaloniki (1977).
24 D. Mondrakis and G. Soulios, Sur une formation schisteradiolaritique h des ophiolites, charri+e el la presence des
melanges ophiolitiques dans la region de Arnissa. Leur
signification pour l'evolution tectoorog+nique de la zone
pelagonienne, Bull. Geol. Soc. Greece XIII/2 (1978) 18-33
(in Greek with French abstract).
25 S. Leontaris, Geomorphological investigations on the basin
of the aetolo-akarmanic lakes, Thesis, University of Athens
(1967).