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:
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Figure 2
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Figure 3
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[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).
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Figure 4
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[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
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Figure 5
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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).
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Figure 6
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[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).
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Figure 7
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[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
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Figure 8
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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.

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.
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