Discussion and Conclusions

[34]  Our data obtained on spinel from the K/T boundary layer in the Gams sequence, Eastern Alps, indicate that the mineral is contained in these rocks in two distinct populations. One of them is dominated by Cr-spinel that was provided for the sediments by eroded high-pressure metamorphic rocks of eclogite composition, which are widespread in the Eastern Alps. This spinel is not only typical of the whole K/T boundary layer but was also found in the overlying Maestrichtian clays. The other population of spinel principally differs from the first one in having high Ni concentrations and is contained in all parts of the boundary layer.

[35]  A principally important problem is the genesis of the Ni spinel, which was found, along with sediments at the Cretaceous-Paleogene boundary, also in cosmic dust, micrometeorites, cosmic spherules, and the fusion crusts of meteorites [Toppani and Libourel, 2003], i.e., is not directly related to any impact event.

[36]  The following facts should be taken into consideration under the discussion of Ni spinel origin:

[37]  1. Ni spinel from the Gams section differs by the high content of zinc (up to 5.6% in a rust crust), that is not characteristic for other known sections at the K/T boundary.

Figure 10
[38]  2. Alongside with well expressed crystals octahedral forms, two grains of drop-like mode of Ni spinel has been found (Figure 10).

[39]  3. It is considered, that Ni spinel is confined to 1-3 mm rust colored layer which has deposited in less than 100 years [Robin et al., 1992; etc.], while in the Gams section it meets in all parts of the transition layer.

[40]  4. Recently, in globules from the Baransky hydrothermal system (Iturup Island) have been found out such mineral phases as trevorite (Ni, Fe) Fe2O4, magnesioferrite MgFe2O4, chromite FeCr2O4, cuprospinel CuFe2O4 together with native iron [Rychagov et al., 1997]. This finding opens the another way in the study of Ni spinel formation as a result of the mantle fluid activity during the volcanic eruptions. In this connection it is necessary to note the discovery of Ni spinel in spherules from the Paleocene flood basalts in Western Greenland [Robin et al., 1996]. High Cu contents in these spherules exclude their formation from meteoritic material. It is important that the enclosed basalts have the abnormal Ir concentrations (up to 6.8 ng g-1 ) [Robin et al., 1996].

[41]  Thus, the question of origin Ni spinels from the transitional layer at the K/T boundary remains opened and demands the further researches.


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