RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES3001, doi:10.2205/2006ES000204, 2006
[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].
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Figure 2 |
[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.
Citation: 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.
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