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
[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
2005ES000173-fig02
Figure 2
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
[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 pm 0.45 and ~0.37 pm 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
[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).


RJES

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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