RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES6004, doi:10.2205/2008ES000306, 2008

Discussion

[24]  It is important to illustrate the different lithological conditions of deposition, which may, or may not, affect the accumulation of metallic iron particles.

[25]  1) First of all, the steadily uniform concentrations of magnetic minerals in the Kvirinaki sediments point to unwavering regime of their accumulation, in contrast to more variable accumulation of magnetic minerals in the Khalats deposits (Figures 2 and 3).

[26]  2) Pyrite is present in the upper part of the Kvirinaki section and is not found in the Khalats section altogether. Consequently, reducing conditions that govern the pyrite formation and preservation, prevailed over most the Kvirinaki sequence, with oxidizing conditions predominating at the base and the very top of the section and thus accounting for maghemitization of magnetite and titanomagnetite. In contrast, the nearly entire Khalats section is characterized by low-temperature oxidation of magnetite and titanomagnetite and maghemite formation. Hematite, which is not found in all the Kvirinaki samples, is present in many Khalats samples.

[27]  3) The magnetite+titanomagnetite content in the Kvirinaki sediments is generally higher than in the lower part of the Khalats section but is lower than in the upper part of the same section.

[28]  4) The base of the Kvirinaki section is perceptibly enriched by goethite (2-4%), while its concentration is less than 1-1.5% in the Khalats section. The upper parts of both sections contain about 0.5% of this mineral. [29]  5) The lower part of the Kvirinaki section is relatively rich in paramagnetic iron compounds ( M800 = 0.03-0.04 Am 2 kg -1 ), whereas the upper part and the Khalats section contain lesser amounts of paramagnetic iron compounds, the values of M800 being ~0.02 Am2 kg-1 and 0.005-0.02 Am2 kg-1, respectively.

2008ES000306-fig09
Figure 9
2008ES000306-fig10
Figure 10
[30]  Therefore, the accumulation of iron particles superimposed on different backgrounds that are created by local physical-geographic peculiarities of deposition of coeval sediments. Note the only interval of relative enrichment (up to 0.004%) by metallic iron particles is recognized in both sections (Figures 2a and 3a). Judging by Curie temperatures of 750-760oC (Figure 6), the composition of iron particles is uniform and similar in both sections. According to magnetostratigraphic data, these iron-enriched intervals with ages of 12.6-12.15 My (Figures 9 and 10) are nearly synchronous. The positions of iron concentration maximums, however, are at the base of the interval (12.55 My) in the Khalats section and in the upper part of the interval in the Kvirinaki section (12.25 My). On the other hand, we cannot guarantee such a high precision of correlation. Secondly, even if the correlation is perfect, it is difficult to believe into the absolutely equal amounts of deposited iron particles over the distance of 1500 km. The latter was further tested in the following way: two additional samples were selected ~1 cm away from samples #38 and #41 with iron content of 0.004% and 0.0015%, respectively, from the Khalats collection; similarly, two more samples were taken close to the samples #395 (0.0013%) and #399 (0.004%) from the Kvirinaki collection. These additional samples, 38a (0.003%), 41a (0.002%), 395a (0.0019%) and 399a (0.0021%), reveal the elevated concentration of metallic iron too, the observed values being similar for the sister-samples. This indicates that the elevated concentration of metallic iron in the 12.6-12.15 My interval is real indeed, but the values themselves may vary from 0.002 to 0.004% even for the same stratigraphic level. So it is not surprising that these values vary over 1500 km.

[31]  It is interesting that three narrow maximums of iron content of 0.001-0.002% in the upper parts of both sections are also synchronous within the error limits, the ages of these maximums being 10.2, 10.95, and 11.5 My for the Khalats section and 10.26 Ma, 10.85 Ma, and 11.7 My for the Kvirinaki section (Figure 10). These maximums are found in the sediments that accumulated under very different conditions, i.e., oxidizing in the Khalats section and reducing in the Kvirinaki section.

[32]  The above correlation of iron-enriched intervals in sediments from remote sections indicates a global event of cosmic iron precipitation on the Earth at these times. Judging by the duration of 0.4 My of the main iron-enriched interval, it cannot have resulted from a single, whatever huge, impact event but had to be a series of global synchronous events or an a prolonged process.

[33]  The long hiatus that is present in the Kvirinaki section is entirely overlapped in the Khalats one (Figure 9). As testified by these data, no perceptible events of iron particles precipitation occurred during the 14.6-13 Ma interval. Before that, a detectable amount of iron particles is found in the Kvirinaki area in the 15.6-14.9 Ma interval; their concentration reaches 0.001%, with the a gap between 15.3 and 15.1 Ma, where no metallic iron is found. This no-iron interval (15.2-15.0 Ma) coincides with elevated iron concentration of 0.0016% in the Khalats area (Figure 9). This misfits that are also found for the peaks in the 12-10 Ma interval, can be attributed to inaccuracies in dating, to imperfect correlation between the sections and geomagnetic polarity time scale, and, finally, to the impression of the scale itself.


RJES

Citation: Pechersky, D. M., D. K. Nurgaliev, and V. M. Trubikhin (2008), Native iron in Miocene sediments, Russ. J. Earth Sci., 10, ES6004, doi:10.2205/2008ES000306.

Copyright 2008 by the Russian Journal of Earth Sciences

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