RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES3001, doi:10.2205/2006ES000204, 2006

Methods of Petromagnetic Studies

[10]  Several 1- to 2-cm cubes were sawn from each sample were used for standard isothermal petromagnetic studies, and pieces less than 1 cm in size were used for thermomagnetic analysis (TMA). The cubes being not strictly defined in size, the measured values were reduced to the weight of samples, i.e. specific susceptibility and specific magnetization were determined. Petromagnetic studies included measurements of the specific magnetic susceptibility c, and the hysteresis and anisotropy characteristics Ac and Ars; results of the measurements are presented in Table 1. The susceptibility was measured with the KLY-2 bridge, the remanence was measured with the JR-4 spin magnetometer, and the magnetization curves in a constant field of up to 0.5 T and hysteresis characteristics of samples were examined with the help of a coercivity spectrometer in an automatic regime [Burov et al., 1986; Yasonov et al., 1998]. The magnetization curves enabled the determination of the following characteristics: the specific saturation remanence ( Mrs ), the specific saturation magnetization without the paramagnetic+diamagnetic component ( Ms ), the coercivity without the effect of the paramagnetic+diamagnetic component ( HC ), and the remanent coercivity ( HCr ). The ratios of the hysteresis parameters HCr/HC and Mrs/Ms provide constraints on the domain state, i.e. the sizes of magnetic grains [Day et al., 1977]. However, this should be done with due regard for the effect of superparamagnetic grains of the rock [Dunlop, 2002a, 2002b]. The magnetization curves of superparamagnetic particles are obtained from measurements with a coercivity spectrometer. After reaching the maximum field of magnetization, the remanence behavior is measured at the stage of the dropping field and, in the absence of superparamagnetic and magnetoviscous particles, the remanence should remain constant. In practice, rock studies reveal a decrease in Mr caused by the presence of superparamagnetic particles. In relatively small-scale fields (to 100 mT) this curve is virtually undistorted and can be used for estimating properties of superparamagnetic particles.

[11]  The magnetization of the paramagnetic+diamagnetic component was estimated from curves of magnetization in constant fields exceeding the saturation fields of magnetic components in rocks (the values of this component are given in parentheses in the Mp column of Table 1). If the saturation field of magnetic components is unattainable, the resulting value of the paramagnetic magnetization can be overestimated [Richter and van der Pluijm, 1994].

[12]  Using hysteresis parameters, the ensemble of magnetic minerals present in samples can be subdivided according to their coercivity. This can be demonstrated most clearly using coercivity spectra of the remanent magnetization [Egli, 2003; Robertson and France, 1994; Sholpo, 1977].

2006ES000204-fig02
Figure 2
[13]  Thermomagnetic analysis of rock samples was performed using an express Curie balance [Burov et al., 1986] measuring the temperature dependence of the induced magnetization at a heating rate of 100oC min-1. Such a high heating rate was possible due to a high sensitivity of the apparatus allowing one to use a very small sample no more than 10 mm3 in volume. The temperature gradient within such a small sample does not exceed 10oC. The thermomagnetic analysis was carried out in a constant magnetic field of 200 mT or (more rarely) 500 mT; however, some samples have a higher saturation field, so that a certain induced magnetization was actually measured and its value in such magnetic species as magnetite, hemoilmenite, and metallic nickel and iron is the saturation magnetization Ms, whereas a high saturation magnetic field may characterize some grains of hemoilmenite and ferromagnetic hydroxides of iron. The curves M(T) of the first and second heatings to 800oC were obtained for all samples. To estimate the sample concentrations of magnetite, iron, hemoilmenite, and "goethite", the contribution of a given magnetic mineral to the Ms value was determined from the curves M(T), and this value was divided by the specific saturation magnetization of the mineral. The following values of Ms were accepted: ~90 A m2 kg-1 for magnetite, ~200 A m2 kg-1 for iron, and ~0.02 A m2 kg-1 for goethite; for hemoilmenite Ms varied from ~4 to ~40 A m2 kg-1 with TC varying from 300oC to 200oC [Nagata, 1961]. The hemoilmenite concentration was determined from M(T) of the second heating when homogenization of hemoilmenite takes place and its appreciable part becomes ferrimagnetic and less coercive; accordingly, we assume in this case that we deal with the saturation magnetization. Undoubtedly, not all grains of hemoilmenite and ilmenite became homogenized, as is evident from the form of the curve M(T) close to hyperbolic; accordingly, the resulting estimate of the hemoilmenite concentration is the lower limit for its values in the samples studied. The above considerations concerning the partial homogenization of hemoilmenite when heated to 800oC is confirmed by results of check heatings of some samples to 1000oC, resulting in complete or nearly complete homogenization of hemoilmenite, as is evident from the disappearance of the concavity in the curve M(T) and a well-defined Curie point (Figure 2). These results correspond to the state diagram of hemoilmenite of an intermediate composition, where its homogeneous state region lies above 900oC [Nagata, 1961]. According to petrographic and chemical data, the Gams sediments contain noticeable amounts of iron hydroxides [Grachev et al., 2005]. The value Ms = 0.02 A m2 kg-1 accepted here is minimal [Bagin et al., 1988], but even in this case we obtain a lower limit of iron hydroxide concentrations because they include paramagnetic varieties and, moreover, the saturation field of holocrystalline goethite is higher than the TMA magnetic field.

[14]  The values of Ms near 800oC, where the contribution of magnetic minerals vanishes, can be used as lithologic control; namely, they provide constraints on the relative contributions of the paramagnetic (paramagnetic hydroxides of iron, clays, etc.) and the diamagnetic (carbonates and quartz) components in the deposits. Negative and positive values of Ms at 800oC determine, respectively, the diamagnetic ( Md ) and the paramagnetic ( Mp ) magnetization components (Table 1). We used the 800oC values of M because accurate estimation of paramagnetic and especially diamagnetic magnetizations, as well as their separation at room temperature, is very difficult.

[15]  Thermomagnetic curves were obtained in some samples from the remanent magnetization Mr(T) by measurements with a spin magnetometer made on the basis of the ION-1 magnetometer equipped with a furnace [Burov et al., 1986]. The sample measured has a volume of about 1 cm3, and the heating rate amounts to 25oC min-1.

[16]  To gain additional information on the properties of magnetic minerals, we also used thermomagnetic curves obtained during successive heatings of samples to various temperatures. For example, this allowed us to trace mineralogical alterations in samples during their heatings and distinguish them from Curie points.

[17]  Results of the thermomagnetic studies are presented in Table 2 and several figures.

[18]  Along with the petromagnetic studies, microprobe analysis with the use of the Camebax microanalyser was performed for the magnetic fractions extracted from sediments of the layers K, L, M, O, P, and W. Material of a fraction was mounted as a washer 26 mm in diameter prepared with the use of a strong permanent magnet onto a conductive tape with an adhesive film on its both sides. Microprobe measurements were carried out at an accelerating voltage of 20 kV and a beam current of 10 nA. Under these conditions, the effective diameter of the probe amounted to about 2-3  m m, which was regularly verified using fine phases. We measured the concentrations of TiO2, FeO, MgO, MnO, Cr2O3, Al2O3, SiO2, CaO, Ni, and Cu (Table 3).

[19]  On the whole, the results of the microprobe analysis and TMA complement each other. For example, (a) the microprobe and TMA results yield divergent constraints on the titanomagnetite composition, indicating decomposition of titanomagnetite grains. (b) The TMA data from sample J4 are evidence for the presence of nickel, whereas the latter is not discovered in the magnetic fraction of this lamina; on the contrary, nickel is discovered in the magnetic fractions of samples J5 and L6 but it is not fixed from the TMA data. This is evidence for a local and very irregular distribution of nickel particles.


RJES

Citation: Pechersky, D. M., A. F. Grachev, D. K. Nourgaliev, V. A. Tsel'movich, and Z. V. Sharonova (2006), Magnetolithologic and Magnetomineralogical Characteristics of Deposits at the Mesozoic/Cenozoic Boundary: Gams Section (Austria), Russ. J. Earth Sci., 8, ES3001, doi:10.2205/2006ES000204.

Copyright 2006 by the Russian Journal of Earth Sciences

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