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

Characteristics of Transitional Layer J at the K/T Boundary Derived from the Results of Thermomagnetic and Microprobe Analysis

[111]  Thermomagnetic analysis (TMA) was carried out in samples 1-10 mm3 in volume, and minerals for microprobe analysis (MPA) were separated from 100-200 mm3 of the material. TMA was originally conducted on a series of samples taken in different parts of layer J. Then one of the samples, sample J6, was divided into six units (Figure 4), and each of them was subjected to TMA. Simultaneously, magnetic minerals for MPA were separated from each of the units by using a powerful permanent magnet. The sizes of the separated mineral particles ranged from a submicrons to tens of microns. The characteristics of layer J are described in detail below (they are listed in order from the bottom to the top).

2005ES000189-fig29
Figure 29
Sample J-1.
[112]  According to TMA data, this sample contains notable amounts of titanomagnetite with Tc = 540o C (Figure 29a), and this is confirmed by MPA data, which point to the presence of titanomagnetite and ilmenite, sometimes in aggregates. The composition of TM grains (Table 15) does not correspond to the Curie point, a fact testifying to the heterogeneous alteration of TM grains (their oxidation and decomposition). The grains are small and oxidize and disappear when heated (as can be inferred from the drastic drop in the magnetization value at heating to > 700oC) and partly homogenize (the Curie point shifts to the left to 510oC after the first heating of the sample, which also results in a decease in the magnetization). A small bend on the curve at 300o C that disappears after the second heating is likely caused by maghemite, which is transformed into hematite at this temperature. Its fraction relative to the titanomagnetite content is approximately 10%.

Sample J-2.
[113]  Judging by the TMA data, the samples contains (a) maghemite, (b) magnetite with Tcsim570o C, and, perhaps, also (c) metallic iron (Figure 29b). The sample was "underheated", and it was thus difficult to definitely determine whether it contained iron, but an iron spherule was identified under the microprobe (The similar iron spherule was found in unit M.). The sample is very weakly magnetic, and a notable contribution to its magnetization is made by paramagnetic material. We failed to separate titanomagnetite and magnetite grains from the clayey material of this layer. Compared with sample J -1, this sample contains approximately 20 times less magnetic minerals.

Sample J-3.
[114]  The TMA data on this sample are similar to those on sample J -1 and suggest the presence of titanomagnetite, whose Tcsim500o C and shifts to 480o C as a result of the partial homogenization of the mineral, and the magnetization dramatically drops (Figure 29c). In addition to titanomagnetite, the rock contains magnetite with Tc = 590o C. This sample differs from sample J -1 in having much lower (by more than one order of magnitude) concentrations of magnetic minerals, and the horizontal segment of the curve more probably testifies to the absence of metallic iron. One particle of magnetite and one of ilmenite were found in this sample under a microprobe (both of them are no larger than 10  m m), along with numerous minute grains of supposedly magnetite.

Sample J-4.
[115]  Judging from the TMA data, this sample contains (a) maghemite (the curve shows a bend corresponding to the maghemite-hematite transition), (b) metallic Ni ( Tc = 350o C), (c) magnetite (  Tc = 590o C), and (d) iron ( Tc>730o C, determined by an extrapolation). Ni is the predominant phase. The total content of magnetic minerals is roughly 15 times lower than in sample J -1 (Figure 29d).

2005ES000189-fig30
Figure 30
Sample J-5.
[116]  According to TMA data (Figure 29e), the sample contains (a) maghemite (bend on the curve at 170-300o C), (b) magnetite with Tc = 590o C, (c) "hemoilmenite" with Tc = 300o C (concave curves of the second and third heating), and (d) iron (?) with Tcsim 700-750o C (extrapolation). Three small Ni grains with awaruite were found in the magnetic fraction by microprobe analyses (Figures 30).

Sample J-6.
[117]  Ni with Tc= 350oC was found in this sample (as the predominant phase) by MTA and was confirmed by MPA (Figure 29f). This richness of the rock in Ni is of local character and was not detected in any other samples from layer J.

[118]  In general, the results of MPA and TMA contribute each other, and their analysis led us to the following conclusions:

[119]  The lower part of layer J (unit J1 ) is slightly enriched in titanomagnetite and ilmenite, which have compositions typical of these minerals from basalts. It can be concluded that this layer provides record of the deposition of volcanic particles, when the largest particles had been already settled, and the settling process gradually attenuated. No titanomagnetite was found in higher units J4 and J5, but Ni appears in the rocks starting from layer J4. It appears in aggregates with awaruite (Figure 30) and in unit J5 and as metallic spherules of practically pure Ni in unit J6.

[120]  The distribution of Ni particles within layer J is extremely uneven: single particles start to appear in unit J4, and the amount of the particles reaches a maximum in the uppermost part of layer J (numerous Ni beads were found in unit J6 ), with this enrichment in Ni detected only in one sample. The Ni beads range from a submicrons to tens of microns.

2005ES000189-fig31
Figure 31
[121]  Finally, the mineralogical analysis of the heavy fraction from sample J 2 has revealed single platelets of copper. The microprobe study of these particles confirms that they contain not only pure Cu but also particles with up to 30% Au and 1-2% Ni (Figure 31). The morphology of these particles is unusual (Figure 21).


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

Powered by TeXWeb (Win32, v.2.0).