RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6001, doi:10.2205/2005ES000189, 2005

Petromagnetic Measurements

[92]  The specific magnetic susceptibility (c), specific saturation magnetization (Ms), and the specific remanent saturation magnetization (Mrs)
vary within broad limits (Table 15) depending on the lithological characteristics of the rocks, first of all, their concentration of carbonate material. A positive correlation between Ms and Mrs testifies to the determining role of the concentration of magnetic minerals. The correlations of Ms and Mrs with the magnetic susceptibility c are less clear.

Thermomagnetic data.
[93]  The analysis of the M(T) curves allowed us to identify six magnetic phases (Figure 27):

2005ES000189-fig28
Figure 27
2005ES000189-fig28
Figure 28
[94]  (1)  Tc = 90-150oC, the fraction of this phase Ms is equal to 10-20%, and it is contained in all of the examined samples and corresponds to ferromagnetic iron hydroxides, such as goethite. Assuming that this is goethite with Ms = 0.02 Am2 kg-1, the concentration of this goethite in the sequence was estimated as ranging from 0.5% in its lower part to 2-2.5% in the upper part of section (Figure 28a).

[95]  (2)  Tc = 180-300o C, this phase is present in all of the samples. The contribution of this phase to Ms is equal to 5-40%. After heating to 800oC, the percentage of this phase in many samples notably increases (to 30-90%), and the Curie point usually decreases. Judging from the increase in Ms at decreasing Tc, this phase is hemoilmenite, which partly homogenizes during heating, and, consequently, its Ms(T) curve becomes concave. The check heating of some samples to 1000oC indicates that the Ms(T) curve ceases to be concave, and the Ms notably increases compared with that at heating to 800o C. This corresponds to the diagram of state of hemoilmenite of intermediate composition, which is homogeneous at temperatures above 900oC [Nagata, 1965]. The concentration of hemoilmenite is determined from the results of the second heating (it is read from the Tc vs. hemoilmenite composition diagram from [Nagata, 1965]). This concentration varies from less than 0.0001% to 0.02% (Figure 28b).

[96]  (3)  Tc = 350-360o C was encountered only in sample from layer J during the thermomagnetic analysis of the Mr(T) curves (which testifies that the material contains metallic Ni), but not the M(T) curves. These results suggest that Ni occurs in the form of very small grains, whose concentration is likely below 0.0001% and which are unevenly distributed in layer J.

[97]  (4)  Tc = 550-610o C was identified in all of the samples, and the fraction of this phase Ms ranges from 20% to 60% (Figure 5). This is most probably predominantly titanomagnetite, which is heterogeneously oxidized to magnetite. The latter is, in places, also homogeneously oxidized ( Tc>580o C). The presence of titanomagnetite was confirmed by microprobe analyses: the composition of the grains is identical to those of titanomagnetite from basalts (they contain ~20-25% TiO 2 ) (Table 16). When heated, titanomagnetite grains partly homogenize. The approximate concentration of titanomagnetite and magnetite in the samples varies from < 0.0001% to 0.001% (Figure 28c).

[98]  (5)  Tc = 640-660o C was found only in samples from layer J. The contribution of this phase to Ms equals 10-15%. As samples from layer J contain Ni, it can be suggested that this phase is a Ni-Fe alloy. A simple calculation based on the Tc and Ms values indicates that this can be a phase of the composition Fe3Ni.

[99]  (6)  Tc = 740-770o C was identified in eleven samples, the fraction of this phase in Ms is 10-30%. Upon heating to 800o C, this phase partly or completely disappears. This is metallic Fe with minor admixtures, which oxidizes when heated to 800o C. Its concentration is less than 0.0006%. Practically no metallic Fe was identified in layer J, but it contains a Fe-Ni alloy with Tc = 650-660o C, whose concentration does not exceed 0.0002%. The distribution of Fe in the vertical section is quite homogeneous (Figure 28d).

[100]  After heating to 800o C, half of the samples appear to contain hematite, which is produced by magnetite oxidation and the dehydration of iron hydroxides.

Coercivity of magnetic minerals KC.
[101]  The assemblage of magnetic phases is similar in all of the samples and slightly differs in the carbonate rocks from the bottom part of the sequence and clayey rocks from its upper part. The KC values are the most similar in the Maastrichtian rocks, which are, in turn, closely similar to the Danian layers K, S, and T.

[102]  Beginning with unit L, the KC gradually increases to the limiting field of measurements of 800 mT (Table 15) and farther up gradually changes: from sample from unit M to those from S and T, the curve first shows a plateau and then a clearly pronounced maximum at 130-160 mT and a minimum at ~400 mT.

[103]  The coercivity KC of layer J is principally different in its low-coercive field in having a maximum or a plateau at 25-40 mT. In the rest of it, the KC of layer J is similar to those of samples from units N and O. The changes in the coercivity in layer J is also seen from the behavior of the Hcr (Table 15).

[104]  Judging from the values of Hcr and Mrs/Ms (Table 15), the rocks are dominated by single-domain and pseudosingle-domain grains. The low coercivity of magnetic grains in layer J is not reflected in Mrs/Ms or Hcr/Hc (thus highlighting the absence of relations with the sizes of magnetic grains) but rather results from the presence of Ni and its alloys with Fe.

[105]  There are no correlations of such characteristics of the coercivity spectra as the position and heights of the major extrema with the Fe concentrations, a fact that can be explained only by a low contents of this phase. The weakly pronounced correlation for goethite is presumably explained by the occurrence of a complicated assemblage of iron hydroxides in sedimentary rocks. The contents of magnetite and hemoilmenite are related through a weak positive correlation. This implies that the coercivity spectra of the rocks are determined mostly by the presence of magnetite and hemoilmenite grains. In addition to these minerals, layer J (its low-coercivity part) contains grains of metallic Ni and its alloy with Fe.

Anisotropy.
[106]  We measured the anisotropy of magnetic susceptibility ( A<undef> k ) and remanent saturation magnetization ( Ars ). The former is a characteristic of all minerals, magnetic, paramagnetic, and diamagnetic, while the latter is typical only of magnetic minerals. In general, both types of anisotropy behave similarly (with rare exceptions, see Table 15). The values of Ak of the main group of samples range from 1 to 1.1 and increase to > 1.1 only in four samples, whereas the values of Ars of most samples lie within range of 1.12-1.36 and decrease to Akle1.1 only in four samples. The likely reason for this is that the paramagnetic and diamagnetic constituents of sediments are generally isotropic, although calcite and clay minerals are anisotropic ( Ak is equal to 1.13 for calcite and 1.2-1.35 for clays [Rochette et al., 1992]), and the distribution of their symmetry axes is close to the matrix one.

[107]  In terms of Ars, virtually all of the sediments (with rare exceptions: layers K and T ) are anisotropic, and this anisotropy is only insignificantly dependent on the rock composition. From units A to R, Ars varies within a narrow range and notably increases in the upper part of the sequence, in units U - W. Within individual units, Ars varies insignificantly. The exception is clay layer J, whose anisotropy shows the greatest scatter: from 1.02 to 1.32.

[108]  Most of the sediments are characterized by a foliation magnetic fabric, E>1, and only occasional layers have either Esim1 or very weak linearity ( E<1 (Table 15). The aforementioned features can be explained by the presence of elongated grains of magnetic minerals, the compaction of the sediments, some structuring of the sediments under the effect of flows and currents, etc. The occurrence of anisotropy and foliation testifies that the magnetic minerals (which are the major causes of the magnetization of the rocks) are of terrigenous nature. Conceivably, isotropic samples from units K and T contain authigenic magnetic minerals.

Variations in magnetic characteristics in the vertical section.
[109]  The vertical section of the rocks displays two levels of c, Mrs , and Ms: Maastrichtian calcareous marls and Danian clays, which include layers K, S, and T (Table 15). The occurrence of these levels is confirmed by the distribution of magnetite, hemoilmenite, and goethite in the vertical section and the absence of any systematic trend in the distribution of metallic iron (Figure 28). A notable differences between the vertical distributions of c, on the one hand, and Mrs and Ms, on the other, seem to be controlled by the significant contribution of paramagnetic material and, most significantly, tiny superparamagnetic grains, to the susceptibility.

[110]  Layer J does not differ from other layers in the content of magnetic minerals (Figure 28) but has obviously lower coercivity (Table 15). The latter fact is likely explained by the presence of Ni and Fe-Ni alloy. The distributions of magnetite, hemoilmenite, and "goethite" in the vertical section are generally similar. These minerals seem to have been accumulated simultaneously, under lithological control, without any relation to the Cretaceous-Paleogene boundary.


RJES

Citation: Grachev, A. F., O. A. Korchagin, H. A. Kollmann, D. M. Pechersky, and V. A. Tsel'movich (2005), A new look at the nature of the transitional layer at the K/T boundary near Gams, Eastern Alps, Austria, and the problem of the mass extinction of the biota, Russ. J. Earth Sci., 7, ES6001, doi:10.2205/2005ES000189.

Copyright 2005 by the Russian Journal of Earth Sciences

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