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

The Results of Petromagnetic Studies

[8]  TMA shows that the following magnetic minerals are present in the sediments from the Khalats and Kvirinaki sections:

2008ES000306-fig02
Figure 2
2008ES000306-fig03
Figure 3
[9]  1) A magnetic phase with T C = 90-150oC is present in most samples but disappears after the first heating; its share in Mi is 5-20%. This phase is likely to be weakly ferromagnetic Fe-hydroxides like goethite (Figures 2 and 3).

[10]  2) A magnetic phase with T C = 180-300oC that is present in many samples accounts for 0-40% of Mi. After heating to 800oC, the contribution of this phase often increases, while the Curie temperature decreases, which is typical for hemoilmenite of intermediate composition. During heating, this mineral becomes partly homogenized and/or ilmenite is transformed into hemoilmenite (Figure 1a, sample 402).

[11]  3) A magnetic phase with T C = 200-370oC is found in most samples but disappears after heating to 300-400o (Figure 1a, sample 38); hence, as a rule, this is not a Curie temperature but the result of transformation of maghemite into hematite. Judging by considerable decrease of magnetization intensity after heating ( Mt/Mo is often less than 0.5, Figures 2 and 3), a significant part of magnetite and titanomagnetite in the sediments of both sections is maghemitized.

[12]  4) A magnetic phase with T C = 510-640oC is present in all studied samples from both sections (Figures 2 and 3); its contribution into Mi ranges from less than 5% to 90%. As a rule, this phase is preserved during heating, but its concentration usually decreases, while the Curie temperature either remains unchanged (magnetite) or shifts to lower values (titanomagnetite). After heating to 800oC, titanomagnetite grains become partly homogenized; the latter feature allows us to state that titanomagnetite is present in many samples. Below, the combined contribution of magnetite and titanomagnetite (MT+TM) is used for analysis.

[13]  5) A magnetic phase with T C = 670-680oC is present in lower amount in some Khalats samples but is absent altogether in the Kvirinaki section. After heating to 800oC, this phase appears in all pyrite-bearing samples as a result of pyrite oxidation at high temperatures. Very likely, it is hematite.

[14]  6) A magnetic phase with T = 720-780oC (Figure 1b) is the main goal of this study. This is metallic iron with minor impurities. It is present in many samples, and its contribution into Mi ranges from 0 to 60% (Figures 2 and 3). This phase partly or completely oxidizes after heating to 800oC.

[15]  7) Above 500oC, magnetization of many Kvirinaki samples grows considerably, with a peak at 540oC (Figure 1a, samples 399 and 402). This indicates the presence of pyrite, which oxidizes into magnetite and hematite above 500oC [Novakova and Gendler, 1995].

[16]  Let us now analyze the distribution of main magnetization carriers in the sediments of the Khalats and Kvirinaki sections.

Khalats.
[17]  The concentrations of Fe-hydroxides like goethite, magnetite+titanomagnetite and the total iron content as estimated by M800 value vary considerably along the section, partly repeating each other (Figure 2). The concentrations of magnetite+titanomagnetite and goethite correlate with Mt/Mo ratio (Figure 2b,c,e and Figure 4d,e); this correlation is negative, which points to a large contribution of maghemite and hence considerable low-temperature oxidation of magnetic minerals. The low-temperature oxidation zone is more distinct in the lower part of the section (700-820 m interval), where the concentrations of magnetite+titanomagnetite and goethite are higher (Figure 2b, c), and hematite is more common.

[18]  The distribution of metallic iron looks dissimilar (Figure 2a): its concentration does not exceed 0.001%, and it is absent (more precisely, it is not detected by TMA). Two intervals with higher metallic iron content differ from this background: one with iron content up to 0.004% encompasses the 625-557 m part of the section (samples 38, 39, 41, and 42, Table 1, Figure 2a) and the second less clearly defined interval at 930-909 m with iron content up to 0.002% (samples 23, 24, and 25, Table 1, Figure 2a). According to correlation of paleomagnetic data from the Khalats section with geomagnetic polarity time scale, the upper iron-enriched interval covers the uppermost part of the C5Ar chron and about half of the C5An chron, i.e., about 12.6-12.2 Ma (Table 1). The lower iron-enriched interval is within a reverse subchron in the middle part of the C5Bn chron and hence is 15.2-15.0 Ma in age (Table 1).

2008ES000306-fig04
Figure 4
[19]  The similar variation in concentrations of goethite, magnetite+titanomagnetite, and M800 through the section is supported by significant correlations between these components (Figure 4). This is likely to indicate a similar terrestrial process of their accumulation. In contrast, the correlation between the above components and metallic iron is much weaker or absent altogether. This is mainly due to the large number of samples where metallic iron is not detected; a weak positive
2008ES000306-fig05
Figure 5
correlation, which is clearer for Fe-goethite (Figure 5a) and Fe-MT+TM (Figure 5b), is found in the remaining samples. Earlier in [Pechersky, 2008b], a significant positive correlation between cosmic metallic iron and goethite, magnetite, and titanomagnetite of definitely terrestrial origin, which has been found in many sedimentary sections from Europe and Asia, was accounted for by re-deposition of iron particles. This correlation is better visible for the logarithms of concentrations, and the logarithmic scale is used in the figures of this paper. As the zero value is absent in this data presentation, the value of 0.00001% (the lowest measurable concentration) was arbitrarily ascribed to the samples, for which no metallic iron was detected by TMA. It is worth stressing that a weak correlation in the Khalats sediments (Figure 5) indicates that metallic iron originated from both direct extraterrestrial input (cosmic dust and/or meteorites) and subsequent re-deposition. It is important that, as a rule, the highest values of metallic iron concentration do not fit the general trends (Figure 5).

2008ES000306-fig06
Figure 6
[20]  The Curie temperatures of iron particles are confined to a narrow range from 730oC to 770oC with a peak at 750oC, particularly, in the iron-enriched interval (Figure 6a). Hence the particles are composed of nearly pure iron with minor impurities.

Kvirinaki.
[21]  With respect to the Khalats section, magnetic minerals are more uniformly distributed in the Kvirinaki section. Note also that the distributions of all magnetic components, metallic iron in particular, are different below and above the hiatus between 320 and 322 m (samples 390 and 391, Table 2, Figure 3). In the lower 100-meter part of the section below this large hiatus, the behavior of goethite, magnetite+titanomagnetite, and M800 is similar (Figure 3b,c,d). Similarly with the Khalats section, a zone of increased low-temperature oxidation, with the highest concentrations of goethite and magnetite+titanomagnetite and, correspondingly, the lowest Mt/Mo ratio values, is found at the base (350-400 m) of the Kvirinaki section (Figure 3b,d,e). The main difference between these two sections is the presence of pyrite over a large interval of the Kvirinaki sections. This is well manifested in TMA data by the growth of magnetization intensity and the Mt/Mo ratio above 500oC and is accounted for by transformation of pyrite into magnetite (Figures 1a, 3e). According to these data, pyrite is present in the 236 meter-thick interval out of the total thickness of 400 m. Pyrite is present at the interval, where the concentrations of other magnetic and paramagnetic minerals are very steady, while that of pyrite varies by an order of magnitude (Figure 3). We conclude that pyrite formation and preservation point to reducing environment of this part of the Kvirinaki section. The distribution of metallic iron particles clearly differs from those of other magnetic and paramagnetic components, but in general similarity is present too (Figure 3). In particular, somewhat elevated concentration of metallic iron, up to 0.001%, appears to be detected in the lower part of the section below the hiatus, that is where the concentrations of magnetic and paramagnetic minerals is elevated as well. In contrast, no iron is detected by TMA in the upper part of the section, where the concentrations of magnetic and paramagnetic minerals are relatively lower (Figure 3), and just three jumps of iron content up to 0.0015-0.002% are found (Figure 3a). An "anomalous'' eighteen-meter thick interval that is relatively enriched, up to 0.004%, by metallic iron particles stands out from the general background (Figure 3a). According to magnetostratigraphic data, this interval covers the uppermost part of the chron C5Ar and about half of the C5An chron, i.e., from 12.6 to 12.2 Ma (Table 2, Figure 3a).

2008ES000306-fig07
Figure 7
[22]  Similarly to the Khalats data, a positive correlation exists between M800, and the goethite and MT+TM concentrations (Figure 7), whereas no correlation is found for pyrite (Figure 7e). Note that the goethite content is perceptibly lower in the pyrite zone than outside of it (Figure 7e), which is likely due to oxidizing and reducing environment in these zones. It means that the origin and accumulation of magnetic minerals (most probably terrigenous) clearly differ from those of pyrite. The correlation between the above components and metallic
2008ES000306-fig08
Figure 8
iron is much weaker (Figure 8): it is virtually absent between iron and M800 (Figure 8c), or iron and pyrite (Figure 8d). Besides, metallic iron is absent altogether from many samples (Figure 8). Thus it is more likely that a considerable part of iron particles in sediments came directly from the cosmic dust and/or meteorites. On the other hand, the general patterns of separate minerals concentrations in this section that have been described above are similar, which is likely to indicate some re-deposition of iron particles. This is not related, however, for the narrow iron-enriched interval, which is certainly anomalous with respect to terrestrial minerals, like goethite, magnetite+titanomagnetite, pyrite, and paramagnetic iron compounds ( M800 ) (Figures 3 and 8). It is worth stressing that the iron-enriched interval straddles the maghemite and pyrite zones (Figure 3) and it thus independent of the redox conditions in sediments.

[23]  The Curie temperatures of iron particles mainly range from 730oC to 770oC, with T C = 760oC in the iron-enriched interval (Figure 6b). Hence the particles are composed of nearly pure iron with minor impurities.


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

Powered by TeXWeb (Win32, v.2.0).