RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES6004, doi:10.2205/2008ES000306, 2008

Methods of Petromagnetic Studies

2008ES000306-fig01
Figure 1
[4]  Petromagnetic studies included measurements of sample magnetization and its dependence upon temperature, that is thermomagnetic anaysis (TMA) (Figure 1). TMA was performed with the aid of Curie express balance [Burov et al., 1986], which allowed measuring the intensity of induction magnetization at different temperatures with heating rate of 100o/min. Because of high sensitivity of the equipment, very small samples of less than 0.2 g were used. TMA was performed in the constant magnetic field of 200 mT. The curves of Mi(T) after the first and second runs to 800o were obtained for all samples.

[5]  We estimated the content of goethite, magnetite and titanomagnetite combined (labeled magnetite+titanomagnetite or MT+TM hereafter), and metallic iron. (Some thermomagnetic parameters can be used to gain information about magnetite and titanomagnetite separately; in this particular case, however, we are not interested in this). To achieve this goal, the contribution of each mineral into Mi was determined using the Mi(T) curves and, then, it was divided by the specific saturation magnetization of each mineral. Ms values of 90, 200, and 0.25 Am2 kg-1 for magnetite+titanomagnetite, iron and goethite, respectively, were used.

[6]  To estimate the relative contribution of the total iron content in a rock, we used the intensity of magnetization at 800oC ( M800 ), which is the sum of paramagnetic and diamagnetic magnetizations. For natural minerals, the former is by two or three orders of magnitude higher than the diamagnetic magnetization of quartz and calcite [Rochette et al., 1992]. Hence the paramagnetic component strongly prevails in M800 values for sediments, except for almost purely diamagnetic rocks like limestone or quartz sandstone.

[7]  We used the following indirect features for mineral identification: a) The growth of magnetization above 500oC (Figure 1a, Samples 399 and 402) indicates the presence of pyrite, which is oxidized to magnetite and hematite above 500oC [Novakova and Gendler, 1995]. b) The presence of Curie point around 580-600oC on the Mi(T) curve and the decrease of magnetization and Curie temperature after the first heating points to decomposed titanomagnetite, which becomes partly homogenized during heating (Figure 1a, sample 24). c) The presence of Curie point at 260-300oC on the Mi(T) curve and its lessening and magnetization growth (in contrast to titanomagnetite) after heating to 800oC point to antiferrimagnetic hemoilmenite of intermediate composition, which is the common product of heterophase oxidation of ilmenite; the latter mineral becomes partly homogenized above 800oC and is transformed into a ferrimagnetic state, which leads to the decrease of the Curie temperature and the growth of magnetization. It is possible that oxidation of paramagnetic ilmenite also results in formation of ferrimagnetic hemoilmenite of intermediate composition during TMA; consequently, a new magnetic phase with the Curie point about 250-300oC is created, and the intensity of sample magnetization grows up (Figure 1a, samples 24 and 402).


RJES

Citation: Pechersky, D. M., D. K. Nurgaliev, and V. M. Trubikhin (2008), Native iron in Miocene sediments, Russ. J. Earth Sci., 10, ES6004, doi:10.2205/2008ES000306.

Copyright 2008 by the Russian Journal of Earth Sciences

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