RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6001, doi:10.2205/2005ES000189, 2005
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Figure 27 |
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Figure 28 |
[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.
[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.
[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
E1 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.
[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.
Citation: 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.
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