RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES3004, doi:10.2205/2005ES000173, 2005
Methods and Results of the Study
[6] The isothermal remanent magnetization (IRM) of the rocks up
to the values of 500 mT was measured to study the carriers of remanent magnetization. Hysteresis and IRM
curves were measured using a coercitivity spectrometer
[Iassonov et al., 1998].
The high-field induced magnetization curves suggest the high concentrations of paramagnetic (PM) and superparamagnetic
(SPM) materials and applied fields of nearly 150 mT were necessary to attain higher than 95% of
saturation IRM. This is typical of the magnetite or magnetic iron sulfides that usually occur in recent lake sediments
[Snowball, 1994].
The small amounts of a high-coercitivity material might have been formed during the recovery of samples as a result
of the very fast oxidation of iron in the sulfides and in other minerals containing ferrous iron.
[7] The following parameters were measured using the IRM curves: normal remanent saturation
magnetization ( Jrs ), saturation magnetization, excluding the effect of the paramagnetic component ( Js ),
the magnetization of the paramagnetic component in the field of 0.5 T ( Jp ), the coercivity force after removing
the effect of the paramagnetic component ( Bc ), the remanent coercitivity force ( Bcr ), the position of the
dJr/dBa derivative peak value (normal magnetization along the
a axis of the Preizah-Neel diagram ( Bda ),
the position of the
dJr/dBb derivative peak value (normal remagnetization along the
b axis of the Preizah-Neel
diagram ( Bdb ), and a superparamagnetic susceptibility in the field of up to 8 mT ( Ksp ).
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Figure 1
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[8] The variations of these magnetic values (Figure 1) show drastic changes in the properties of the
magnetic mineral grains contained in the sediments. The section areas marked by gray bands in Figure 1 are
distinguished by the high magnetization of the sediments (in terms of
Js and
Ksp values) and by the high
magnetic rigidity of the magnetic material ( Bcr, Bda ) contained in the sediments. This figure also shows
some ratios between these parameters. For instance, the
Bcr/Bc and
Jrs/Js values provide information
for the domain structure of the ferrimagnetic materials contained in the sediments
[Day et al., 1977].
The relationships among some of the magnetic parameters of the Khubsugul Lake sediments are shown more
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Figure 2
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clearly in Figure 2. One can see that in terms of their magnetic properties the sediments can be subdivided into
two groups (Figure 2). This is most clearly seen in terms of the relationship between the
Bdb and
Bda parameters (Figure 2d). Table 1 lists the average values of some magnetic parameters for the samples
of both groups (Figure 2).
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Figure 3
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[9] The
Bdb and
Bda values are similar for the samples of group I, and differ for the samples of group
II. The use of a Preisach diagram
[Dunlop and Özdemir, 2001]
allows one to estimate the extent of magnetic interaction in ferrimagnetic grain ensembles. In our case this suggests the
presence of a high magnetostatic interaction between the magnetic grains of the group II samples and of the almost complete
absence of interaction between the magnetic grains in the group I sediments. It can be supposed that the
magnetic material in the group I sediments is represented by single-domain (SD) grains, and that of the
group II sediments, by multidomain (MD) grains. This is also proved by their
Bcr/Bc and
Jrs/Js values (Table 1). The theoretical values reported in the literature
[Dunlop and Özdemir, 2001]
for the assemblages of SD grains are ~1.5 for
Bcr/Bc
and ~0.5 for
Jrs/Js [Dunlop and Özdemir, 2001], yet our samples of group I
showed these values to be ~2.15
0.45 and ~0.37
0.1, respectively, see Table 1. This seems to be associated
with the fact that these samples contained a mixture of SD and MD grains and also superparamagnetic grains
[Dunlop, 2002],
which is proved by the high
Ksp values (Figure 1). The samples of group II contain
mostly MD grains, which is proved by the values of all parameters presented in Table 1. It can, thus, be
supposed that a great difference of the magnetic hysteresis properties of the group I and II samples is
caused by the different sizes of the magnetic grains. The coercitivity spectra of the most typical samples of
groups I and II are presented in Figure 3. The peaks of these spectra plotted for the above-mentioned samples
of groups I and II amount to 84 mT and 48 mT, respectively (Figure 3).
[10] The magnetizations induced in a field of 200 mT (IM) and measured as a function of temperature
provided further evidence for the magnetic minerals. These curves were obtained during the continuous
heating of small samples (~30 cu. mm in size) at a rate of 100o C min
-1 using home-built
sensitive instruments
[Burov et al., 1986].
The high heating rate is supposed to prohibit oxidation of the smallest particles
of magnetic minerals and to allow the recognizing for instance the Curie temperatures
of the very small particles of magnetic iron sulfides, which would have been obstructed at lower heating rates.
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Figure 4
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[11] The typical curves of the two successive heatings of two samples are plotted in Figure 4. The mineral
phases with the Curie temperatures of 560-580o C can represent magnetite (a and b in Figure 4). The faint
features in the first heating cycle around 250-300o C (Figure 4b) may suggest the presence of iron sulfide
(most probably greigite) which disappeared in the second run having been destroyed apparently
[Burov et al., 1986].
The magnetic minerals may have originated in different ways: derived from the catchment soil
erosion and aerial regional (or even global) allogenic input, the biogenic formation of magnetite and/or greigite
[Bazylinski and Moskowitz, 1997]
and the dissolution of iron oxides and the growth of secondary
iron sulfides such as pyrite or greigite
[Tarduno, 1995],
or the formation of new authigenic magnetic mineral phases including magnetite
[Karlin et al., 1987].
Variations of the magnetic properties of the core
samples provide information concerning the potential effects of these processes on the formation of a
magnetic fraction in these lacustrine sediments (Figure 1).

Citation: Nourgaliev, D. K., P. G. Iassonov, L. R. Kosareva, A. Yu. Kazanskii, and A. P. Fedotov (2005), The origin of magnetic minerals in the Lake Khubsugul sediments (Mongolia), Russ. J. Earth Sci., 7, ES3004, doi:10.2205/2005ES000173.
Copyright 2005 by the Russian Journal of Earth Sciences
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