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).
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Figure 29
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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
Tc
570o 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
Tc
500o 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).
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Figure 30
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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
Tc
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.
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Figure 31
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[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).

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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