RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES3001, doi:10.2205/2006ES000204, 2006
Results of Petromagnetic Measurements
Specific magnetic susceptibility (χ), specific
saturation magnetization (Ms), and specific saturation remanent
magnetization (Mrs) (Table 1)
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Figure 3
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[20] The values
of these characteristics vary within wide limits, generally
reflecting the main lithologic properties of the rocks such as the contributions
of diamagnetic material (calcite and quartz), paramagnetic material (Fe-bearing
clays and Fe hydroxides), and magnetic minerals of terrigenous origin;
accordingly, the magnetization is lowest in Maestrichtian marls and Danian
interbeds K, S, and T enriched in diamagnetic calcite and quartz, whereas the
sandy-clayey sediments of the upper part of section (layers U, V, and W) are
most magnetic. Both groups of sediments are largely affected by paramagnetic
material whose magnetization is about 10-20 times higher than the saturation
magnetization (
Ms ) of magnetic minerals (Table 1), and the amount of
paramagnetic material in the sandy-clayey deposits is about three times larger
compared to marls and limestones (Table 1). The positive correlation between
Ms
and
Mrs (Figure 3) implies a decisive role of both
concentrations of magnetic minerals. The correlation of
Ms and
Mrs
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Figure 4
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with the magnetic susceptibility is less distinct (Figure 4). Apparently,
the susceptibility is appreciably affected by the contributions of paramagnetic,
diamagnetic (divergences in the weakly magnetic region), and superparamagnetic
(divergences in the strongly magnetic region) materials; these effects are largely
eliminated from
Ms and are absent in
Mrs. The "divergences" in
Ms and
Mrs in sample W can be accounted for by the presence of
numerous fine magnetic grains making a small contribution to
Ms.
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Figure 5
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Figure 6
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Figure 7
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TMA data
[21] (Table 2). The analysis of the curves
M(T)
(Figure 5) and their derivatives (Figure 6) has identified seven magnetic phases.
[22] (1)
TC = 90-150oC, the phase accounts for 10-20%
Ms. It is present
in all samples studied (Table 2) and is destroyed upon heating (Figure 5). Most likely,
it consists of ferromagnetic iron hydroxides of the goethite type. Assuming that
this is goethite with
Ms
= 0.02 A m2 kg-1, we obtain that its concentration
varies in the section from 0.2-0.5% in marls of the Maestrichtian and in the
lens K and interbeds S and T of the Danian to 2-3% in the sandy-clayey sediments
(Figure 7a).
The bulk concentration of iron (Fe2O3 ) in the deposits varies,
respectively, from 2% to 6-8%
[Grachev et al., 2005].
Therefore, the amount of paramagnetic iron varies from ~1.5 to 3-4%. This
is the iron of paramagnetic iron hydroxides and/or iron-bearing clayey minerals.
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Figure 8
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[23] (2)
TC = 180-300oC, the phase is present in all samples except the boundary
layer J (after heating, this phase also arises in J samples). It accounts for
5-40%
Ms (Table 2). After heating to 800oC, its fraction in many samples
increases by 30-90%, and the Curie point generally decreases (Figure 5 and Table 2).
Successive heatings of samples (e.g. sample K, Figure 8) reveal that this
rise takes place only after heating to 800oC. An increase in
Ms associated
with a drop in
TC implies that this is hemoilmenite partially homogenized
during heating; as a result, the curve
Ms(T) is typically concave. Check
heatings of some samples to 1000oC showed that the concavity of the curve
Ms(T) disappears, and the value
Ms is noticeably larger compared to
the results of heating to 800oC (Figure 2). This corresponds to the state diagram
of hemoilmenite of an intermediate composition for which the region of the
homogeneous state lies above 900oC
[Nagata, 1961].
Results of the second heating were used to determine the hemoilmenite concentration
(from the
TC
versus hemoilmenite composition diagram
[Nagata, 1961]).
This concentration varies from less than 0.0001% to 0.02% (Figure 7b). The magnetic
fractions of all samples studied with the microprobe contain large numbers of ilmenite
grains (clasts); the latter are often well-preserved relatively large plates more than
50
m
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Figure 9
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m in size (e.g. see Figures 9a, b). Their concentrations in the sediments amount
to a few tenths percent. The composition is close to pure ilmenite (Table 3). They
often contain intergrowths and lamellae of rutile. No hemoilmenite grains were
observed whose composition corresponds to a Curie point of 200-300oC. The
majority of the hemoilmenite grains are very fine (smaller than the probe size),
as is evident, for example, from their high coercivity (see below), and the
concentration of hemoilmenite with
TC = 180-300oC is one to two orders lower
than the concentration of ilmenite. Very thin lamellae of hemoilmenite (tenths
and hundredths of micron) are poorly observable in the ilmenite grains, and
their composition could not be measured by the microprobe 2-3
m m in size.
Moreover, ilmenite is well drawn away by a magnet, apparently, due to
hemoilmenite inclusions.
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Figure 10
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[24] It is possible that Mg-Al-ferrospinels with similar Curie points (200-300oC)
could form during laboratory heatings. In this case, rocks must contain
silicates containing iron, magnesium, and aluminum and decomposing at high
temperatures (e.g. see
[Bagin et al., 1976, 1977;
Gapeev and Tsel'movich., 1988]).
The study rocks (particularly, sandy-clayey sediments) contain a sufficient amount
of components necessary for such a process
[Grachev et al., 2005].
However, no correlation of the amount of this magnetic phase with Fe, Mg, and Al
concentrations is observed.
For example, in samples from the layer J, containing
the highest concentrations of the above elements (7-8% Fe2O3, 17-19% Al2O3,
and 2.6-3% MgO), the magnetic phase with
TC = 200-300oC virtually does not form
during successive heatings: the curves
M(T) nearly coincide up to 850oC
(Figure 8), whereas the concentration of this magnetic phase obviously rises
after heating above 800oC in a sample from the lens K (Figure 8), in which the
concentrations of the above elements are much lower (3.5-4.7%Fe2O3,
5.7-8.9%Al2O3, and 0.9-1.3%MgO). Another argument is provided by the
thermomagnetic study of the magnetic fraction extracted by a permanent magnet
from samples of the layers L and W and their "nonmagnetic" residues. As seen
from Figure 10a, a Curie point of about 250oC is fixed precisely in the magnetic
fraction; the relative amount of the latter is small (less than 20%), and the
curves
M(T) of the second and third heatings lie appreciably lower than the
first heating curve, which may be caused by destruction of nearly half of
magnetite possibly due to its heating-related oxidation. One might expect that
Mg-Al-ferrospinels would form most intensely from the nonmagnetic fraction, but
this is not observed: its heatings caused no alterations (Figure 10b).
[25] (3)
Tb = 340-370oC, the phase is observed in all samples of the layer
J and in samples of the layers R and T. As seen from the data of successive heatings,
this phase is generally destroyed after heating to 300oC
(Table 2, Figure 8); i.e. in the vast majority of cases, this is not a Curie point but a result of
destruction of a magnetic mineral. Most likely, this is the usual process of the
transformation of maghemite into hematite.
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Figure 11
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[26] The thermomagnetic and microprobe examination of sample J6 and its magnetic
fraction revealed metallic nickel with a Curie point of about 360oC in two
pieces less than 3 mm in size from the upper (sample J6-6) and the middle (J6-4)
parts of the layer J (for more details, see
[Grachev et al., 2005])
and in samples J2 and J3. The curves
M(T) from the remaining samples,
including samples J6-1, 2, 3, and 5 and even a small piece taken near sample J6-6,
yield no evidence for metallic nickel (Table 2), but the latter is detected from
the curves
Mr(T) of several samples (Figure 11), although there are samples
(e.g. J4-1) that do not contain nickel from
Mr(T) as well (Figure 11). These
results imply that, first, nickel exists as very fine grains whose average
concentration in the layer J is apparently less than 0.001 (~0.02, ~0.01,
and ~0.1% in pieces from samples J3-2, J6-4, and J6-6, respectively); therefore,
they have no signature in the value of
Ms but contribute to
Mrs.
Second, the detection of metallic nickel only in individual minute samples is evidence
for its local and very irregular distribution in the layer J. Apart from the layer
J, an intergrowth of pure nickel and copper was discovered in sample L6
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Figure 12
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(Figure 12). Thermomagnetic analysis have not discovered nickel in the layer L,
supporting its very irregular distribution. The presence of individual Ni grains
in the layer L is possibly due to the erosion of the upper part of the layer J
and the redeposition of Ni particles that settled mainly during the deposition
of the upper part of the layer J.
[27] (4)
TC = 550-610oC, the phase is present in all of the studied
samples of the section and accounts for 20% to 60% of
Ms
(Table 2 and Figure 5). After heating, this phase is generally preserved, but its amount usually decreases
and
TC in several samples shifts to the left. Only in two cases, in samples K and
T, the amount of magnetite increases after heating (Table 2 and Figure 5). Often
this is titanomagnetite successively oxidized to magnetite; in turn, the latter
is often single-phase oxidized ( TC>580o C). After a fast laboratory heating
to 800oC, titanomagnetite grains are partially homogenized. This feature implies
the presence of titanomagnetite in samples from the Maestrichtian layers B, C,
E, G, and H and from the Danian layers J, R, V, and W. In the other layers,
titanomagnetite is absent, but magnetite is present; this is valid for the
layers L, M, N, and U, distinguished by the highest concentrations of magnetic
minerals. The absence of titanomagnetite is confirmed by the microprobe data:
solely magnetite that does not contain titanium is discovered in the layers K,
L, M, O, and P (Table 3). The magnetic fraction from the layer W includes very
fine grains dominated by ilmenite (Table 3), and titanomagnetite is fixed only
from TMA data. The presence of titanomagnetite in the layer J is confirmed by
microprobe data: the composition of grains is close to titanomagnetites typical
of basalts (TiO
2
20-25%)
[Grachev et al., 2005].
Approximate estimates of the magnetite and titanomagnetite concentrations in
the samples vary from
< 0.0001% to 0.001% (Figure 7c). Moreover, the presence
or the absence of titanomagnetite and its concentration correlates in no way with
the concentrations of magnetite, i.e. they have different sources. Magnetite clasts
of the inspected magnetic fractions very often contain well-preserved single crystals
(octahedral, Figures 9c, d), which is evidence for a near source area or in situ
crystallization of magnetite. Such crystals of pure magnetite are evidently of nonmagmatic
origin.
[28] (5)
TC = 640-660oC, the phase is present only in samples from the layer J
(Table 2) and accounts for 10-15%
Ms. After heating to 800oC, this phase
is destroyed, implying that this is not hematite. Taking into account the presence
of nickel in samples from the layer J, we may suppose that this is a Fe-Ni
alloy, and a simple calculation of
TC and
Ms for iron and nickel shows that
this can be Fe3Ni, which is confirmed by the microprobe data
[Grachev et al., 2005].
[29] (6)
TC = 660-670oC, the phase is only present in a sample from the lens K and
is preserved after heating to 800oC. It is evidently hematite. After heating to
800oC, hematite forms in half of the samples studied and has
TC = 660-680oC
(Table 2).
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Figure 13
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[30] (7)
TC = 740-770oC, the phase is present in 19 samples and its contribution
to
Ms amounts to 10-30% (Table 2, Figures 5 and 6).
After heating to 800oC,
this phase is partially or completely destroyed (Figure 5). Evidently, this is
fine grains of metallic iron with minor admixtures that oxidizes during heating
to 800oC. Individual balls of pure iron were discovered by the microprobe in
samples J2 and M4 (Figure 13). Its concentration is small, less than 0.0006%.
In the layer J, the thermomagnetic analysis did not discover metallic iron, but
a magnetic species with
TC = 640-660oC is present in the layer (see above);
probably, it is a Fe-Ni alloy with a concentration of no more than 0.0002%. The
along-section distribution of metallic iron (including its alloy with nickel) is
rather uniform (Figure 7d).
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Figure 14
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Coercivity of magnetic minerals and coercivity spectra.
[31] As seen from
coercivity spectra (CS) presented in Figure 14, all samples have similar
ensembles of magnetic grains. The CS of the Maestrichtian marls are least
different and very close to the CS of the layers K, S, and T in the Danian
deposits. The spectra exhibit a smooth increase to a maximum at 100-140 mT and
a subsequent drop to a minimum at ~400 mT followed by a rise until a field of
500 mT. The CS extrema in the sandy-clayey deposits virtually disappear beginning
from the layer L: the CS smoothly increase until a limiting field of 800 mT
used in the measurements (Figure 14). In the upward direction along the section,
the CS are gradually transformed into marl-similar CS: in the interval from
samples M to S and T, a plateau first appears and, in the overlying layers,
it is transformed into a distinct maximum at 130-160 mT and a minimum at ~400 mT.
The CS of the uppermost horizons of the section are similar to those of the
Maestrichtian marls (Figure 14).
[32] Against this background, the CS of the layer J is markedly distinguished in its
low coercivity region by a maximum or a plateau at 25-40 mT. In the remaining
part, the CS of the layer J is similar to CS from samples of sandy-clayey
deposits, particularly in the layers N and O (Figure 14).
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Figure 15
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[33] The CS being stretched, its integral characteristics such as
HCr and
Mrs/Ms are smoothed and
its dependences on the compositions of rocks and minerals are averaged (Table 1),
but a decrease of coercivity is seen in the
HCr of the layer J. Judging from
the values of
HCr and
Mrs/Ms (Table 1 and Figure 15),
single-domain (SD) and pseudosingle-domain (PSD) magnetic grains prevail in the rocks,
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Figure 16
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but the vast majority of points in Day plot (Figure 16) lie in the multidomain region,
which is due to the presence of a large number of superparamagnetic grains. In
high fields, their
Ms effect can be eliminated together with the paramagnetic
effect (if the superparamagnetic grains are very fine). However, in a lower
field of the order of
HC, the susceptibility of these grains is high and, for
this reason, remagnetization takes place in a field much smaller than the real
HC value. This leads to overestimation of the ratio
HCr/HC.
The superparamagnetic magnetization curve is not linear, as in the paramagnetic case
(at room temperature), but hyperbolic, typical of ferri- and ferromagnetic species.
Eliminating the paramagnetic magnetization through a linear approximation, we do
not remove the superparamagnetic contribution in
Ms ( Mrs
is not
influenced by the superparamagnetism). This decreases the ratio
Mrs/Ms.
As a result, points in the Day plot are displaced to the right and downward. Overall,
notwithstanding the distortion of concrete values, the
HCr/HC-Mrs/Ms diagram displays a general tendency (Figure 16). As seen from the Day plot, the finest SD
magnetic grains are present in the layers K and T
(Table 1 and Figure 16). The low coercivity of magnetic grains in the layer J has no effect on
Mrs/Ms and
HCr/HC, thereby emphasizing that these ratios
are unrelated to the size of magnetic grains.
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Figure 17
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[34] Figure 17 presents the magnetization curves (up to fields of 100 mT) of
superparamagnetic particles in samples studied. For clearness, the curves are
normalized to the maximum superparamagnetic magnetization. They can be used to
establish which superparamagnetic grains, coarse or fine, prevail in the
spectrum. Very rapid saturation is evidence for the presence of coarse
particles, and prolonged saturation indicates fine grains. On the other hand,
rapid saturation implies the presence of grains with large values of
Ms.
Our
examples show that samples from the layer J are saturated much more rapidly than
the remaining samples, which means that larger and probably more magnetic grains
are present in the samples J. We consider them as nickel grains. The slowest
saturation is observed in samples of sandy-clayey rocks, i.e. they contain the
finest and probably the least superparamagnetic grains (e.g. goethite
and hemoilmenite whose concentrations in the sandy-clayey deposits is
appreciable higher compared to marls (Figures 7 and 17)).
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Figure 18
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[35] Now, we consider the correlation of such CS characteristics as the position and
height of main extrema with the concentration of magnetic minerals
(Figure 18). Such a correlation is absent in the case of metallic iron, which can be
attributed to its small concentration. We relate a weak correlation for
"goethite" to the presence of a complex association of iron hydroxides that
includes, high coercivity grains of needle goethite, its predominantly low
coercivity earthy varieties, and paramagnetic hydroxides of iron. A positive
correlation with magnetite+titanomagnetite and hemoilmenite is observed,
particularly, in sandy-clayey rocks (Figure 18). This suggests that the CS of
the studied rocks is mainly controlled by grains of magnetite, titanomagnetite,
and hemoilmenite. We emphasize that the ilmenite grains that are present in
noticeable amounts in the sandy-clayey sediments are paramagnetic at room
temperature, i.e. they do not contribute to CS. In the layer J, most likely,
metallic nickel and its alloy with iron, make the main contribution to the low
coercivity part of the spectrum.
Anisotropy.
[36] We measured the anisotropy of the magnetic susceptibility
A<undef>c and the saturation remanent magnetization
Ars. The first involves
all minerals, magnetic, superparamagnetic, paramagnetic, and diamagnetic, while the
second is related solely to magnetic minerals. On the whole, both types of anisotropy
behave, with rare exceptions, similarly (Table 1). The
Ac values of the main
group lie within the limits 1-1.1 and only four samples yielded
Ac>1.1;
whereas the main part of
Ars varies from 1.12 to 1.36, and only four samples
yielded
Ac
1.11. Apparently, this is due to the fact that the paramagnetic
and diamagnetic parts of the sediments are, on the whole, isotropic, although
calcite and clayey minerals are anisotropic ( Ac is 1.13 in calcite and
1.2-1.35 in clays, whereas quartz is isotropic
[Rochette et al., 1992])
and the distribution of their symmetry axes in the studied sediments is close to
chaotic. Therefore, the magnetic susceptibility anisotropy is determined in our
case by magnetic minerals. The
Ars values show that, with rare exceptions
(samples K and T), the studied sediments are anisotropic and the anisotropy
depends weakly on the composition of the rocks. In the interval from A to R, the
anisotropy of
Ars varies within close limits but is appreciably enhanced
in the upper horizons U-W. Within each layer,
Ars varies within narrow
limits except for the boundary clay J, where the anisotropic scatter is widest, from
1.02 to 1.32.
[37] In the vast majority, the sediments of the section possess a foliation fabric
( E>1 ) and only some of its beds are characterized by either
E
1 or a very
weak lineation ( E<1, samples from the Maestrichtian layers B, C, G, and H)
(Table 1). All of the aforesaid can be accounted for by the presence of
elongated grains of magnetic minerals, compaction of the sediments, a certain
influence of currents, and so on. The presence of anisotropy and a magnetic
fabric is evidence for a terrigenous origin of magnetic minerals that are the
main carriers of magnetization in the sediments. Authigenic magnetic minerals
are likely present in isotropic samples (K, T, and others).
Apart from the normal magnetic fabric (the minimum susceptibility is
perpendicular to the bed plane), intervals of the inverse fabric (the maximum
susceptibility is perpendicular to the bed plane) are also identified. The
latter are the beds I, L-Q, V, and W, although they contain normal fabric
samples as well (Table 1). Such an inverse magnetic fabric is characteristic of
siderite and other Fe carbonates, with their easy magnetization axis being
perpendicular to the c symmetry axis
[Rochette et al., 1992].
However, appreciable amounts of siderite and the like are not observed in
the sediments studied; moreover, as noted above, the paramagnetic and diamagnetic
parts of
Ac are, rather, isotropic. Therefore, the inverse fabric is more
likely related to magnetic minerals. The normal and inverse magnetic fabrics
determined from the susceptibility and remanent magnetization coincide, which
additionally confirms the conclusion on the noticeable contribution of magnetic
minerals to the magnetic susceptibility (Table 1). The inverse fabric determined
from the magnetic susceptibility is known for needle goethite and elongated (uniaxial)
SD grains of magnetite. In both cases, the susceptibility is minimal along the
longer axis of the grain, i.e. the easy magnetization axis is perpendicular to
the elongation direction of the grain
[Rochette et al., 1992];
the same is true of the remanence of needle goethite
[Bagin et al., 1988].
Inverse fabrics of rocks determined from remanent magnetization have repeatedly
been observed and are often related to a tectonic factor
[Rochette et al., 1992].
In our case, the undeformed state of sediments of the sequence excludes a tectonic
factor as the cause of the inverse magnetic fabric. The amount of magnetic anisotropy
and the characteristics of the magnetic fabric do not correlate with the composition
and concentration of magnetic minerals, but the following general tendency can be
noted: the magnetic fabric is invariably normal in marls in which the
concentration of goethite is much smaller compared to the sandy-clayey sediments
(Tables 1 and 2); therefore, it is likely that the inverse fabric is primarily
related to the presence of needle goethite in sediments.
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Figure 19
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Behavior of magnetic properties along the section.
[38] Two levels of
c,
Mrs, and
Ms are clearly fixed in the section: (1) weakly magnetic
Maestrichtian marls underlying the layer J, the lens K, and the interbeds S and
T in Danian sediments; and (2) more magnetic sandy-clayey sediments of the layers J
and L-W (Table 1, Figure 19). These two levels are generally recognizable in the
along-section distributions of magnetite, hemoilmenite, and goethite
(Figures 7a, 7b, and 7c) but are absent in the distributions of titanomagnetite (Figure 7c) and metallic
iron (Figure 7d). The noticeable distinction in the along-section behavior of
c, on the one hand, and
Mrs and
Ms, on the other hand, is
evidently due to an appreciable contribution of paramagnetic material and fine
superparamagnetic grains to the susceptibility. Within these levels, we can see
fluctuations in the magnetization closely correlating with variations in the
concentration of hemoilmenite, magnetite, and goethite (relative maximums
at
- 14-12, 4, and 22-26 cm), implying a certain cyclicity in the accumulation
of magnetic minerals; the same pattern is observed in minimums of
c,
Mrs,
and
Ms ( - 16, 2, and 18-20 cm) correlating with the anomalous layers K, S,
and T. The cyclicity "wave" is most pronounced in the behavior of
Mrs
(Figure 19c). The layer J does not differ in the overall concentration of magnetic
minerals from other levels (Figure 19), but it is distinguished by the lowest
coercivity (Figure 15). We relate the latter circumstance to the presence of
nickel and an iron-nickel alloy in the layer J. The concentration of magnetic
minerals is lowest in the layers K, S, and T, containing predominantly SD
magnetic grains; i.e. the cyclicity in the accumulation of magnetic minerals is
also expressed in the mean size of their grains (Figure 15). Moreover, samples
from the layers K, S, and T differ from the remaining samples by isotropy
and a substantial increase in the amount of secondary magnetite due to laboratory
heating (Table 2). Therefore, they contain some authigenic (isotropic) magnetic
and paramagnetic minerals (e.g. pyrite) that are oxidized during heating and
produce magnetite. Specific features of the layers K, S, and T emphasize
the cyclicity of the sedimentation process.
[39] The along-section distributions of goethite, hemoilmenite, and magnetite are
generally similar, which implies concurrent accumulation of these minerals under
the lithologic control. The chaotic distribution of metallic iron is unrelated
to both lithologic properties of the sequence and the K/T boundary
(Figure 7d). Titanomagnetite is present at nearly all levels of the Maestrichtian deposits
and in the layer J, whereas it occurs only in the upper part of the section
in the Danian sandy-clayey sediments (samples R, V, and W). Unlike magnetite,
goethite, and hemoilmenite, the concentration of titanomagnetite, wherever it is
present, varies insignificantly at all levels. We may state that the presence of
titanomagnetite is independent of the lithology of the sequence; rather, taking
into account its composition typical of basalts, it characterizes volcanic
eruptive activity and the dispersal of titanomagnetite by air. The magnetite
concentration is controlled by lithology, although with a certain lag: it is
very low (occasionally vanishing) in the Maestrichtian marls up to the layer K
(including the layer J) and, only beginning from the layer L, the magnetite
concentration increases by about an order (Figure 7c). The hemoilmenite grains
exhibit a similar pattern: the hemoilmenite concentration increases
substantially (by more than five times) above the layer J
(Figure 7b). Lithologic control is most pronounced in the goethite accumulation: an abrupt
increase in its concentration is observed precisely in the layer J (Figure 7a).
[40] Lithologic control is also traceable in values of
Ms near 800oC,
where the contribution of magnetic minerals vanishes and, accordingly, one may gain
constraints on the relative paramagnetic ( Mp ) and diamagnetic ( Md )
fractions in the magnetization of the sediments (Table 1). Overall, the values of
Ms at 800oC are positive, i.e. paramagnetic, in the sandy-clayey part
of the section and negative, i.e. diamagnetic, in the limestones. More specifically,
we may speak of relative paramagnetic and diamagnetic fractions, because noticeable
amounts of diamagnetic carbonates and quartz can be present in the sandy-clayey
beds, and the same is true of paramagnetic clayey minerals and iron hydroxides
in the marls. Thus, the Maestrichtian marls contain paramagnetic material, as is
seen from the small values
Mp = (1-2)
10-5 A m2 kg-1
(samples B and G, Table 1) and the small value
Md =
- 2
10-5 A m2 kg-1
in the other Maestrichtian marls. The magnetic susceptibility of paramagnetic minerals is 30
to 300 times higher than the susceptibility of diamagnetic materials
[Rochette et al., 1992].
Accordingly, given such small values of
Md and
Mp, the ratio
between the paramagnetic and diamagnetic materials in the rocks under consideration
should be at least 1/30; i.e. if, for example, about 2% Fe2O3 were present in
marls, the coinciding values of
Md and
Mp would require more than 60%
of diamagnetic calcite and/or quartz. Pure diamagnetic chalk from the Koshak
section has
Md =
- (26-35)
10-5 A m2 kg-1
[Pechersky et al., 2006].
This value gives an idea of the significance of the paramagnetic admixture in
the Gams deposits. The similar values of
Md and
Mp in the Maestrichtian
marls indicate the homogeneity of these rocks. These characteristics vary from +6
to
- 12
10-5 A m2 kg-1
in the lens
K and the Danian interbeds S
and T, whereas we have
Mp = (15-36)
10-5 A m2 kg-1
in
the sandy-clayey sediments. The layer J differs little in this lithologic indicator:
Mp = (26-36)
10-5 A m2 kg-1.
|
Figure 20
|
[41] Now, we compare the behavior of the susceptibility (Figure 19a), saturation
magnetization (Figure 19b), and concentration of magnetic minerals
(Figure 7) with the behavior of
Md and
Mp and the bulk concentration of iron,
the main magnetization carrier in the rocks
(Figure 20). As seen from the comparison
between these figures, the along-section distributions of susceptibility and
paramagnetic magnetizations at room temperature and at 800oC agree best with
each other and with the Fe2O3 concentration. It is clearly seen that the
regimes of Fe accumulation in the Maestrichtian and Danian parts of the
sequence, fairly homogeneous in each of the parts, are different and rhythmic
variations in the sedimentation conditions lead to a decrease in iron in the
layers K, S, and T. This cyclicity is recognizable in the accumulation of both
magnetic minerals and paramagnetic iron. This correlation is weaker in the
behavior of
Ms and the concentration of magnetic minerals. This can be due
to the fact that more than half of iron in the deposits is present in the
paramagnetic form. Thus, the total concentration of the iron oxide in
goethite+magnetite+titanomagnetite+hemoilmenite does not exceed 3%, whereas the
concentration of Fe2O3 in the sandy-clayey sediments varies from 6% to 8%;
accordingly, a half of iron is concentrated in paramagnetic hydroxides of iron
and clayey minerals, which differed in the accumulation regime from magnetite
and hemoilmenite (apparently, of volcanic-terrigenous origin) and, even to a
greater extent, from titanomagnetite (apparently, of volcanic origin).

Citation: Pechersky, D. M., A. F. Grachev, D. K. Nourgaliev, V. A. Tsel'movich, and Z. V. Sharonova (2006), Magnetolithologic and Magnetomineralogical Characteristics of Deposits at the Mesozoic/Cenozoic Boundary: Gams Section (Austria), Russ. J. Earth Sci., 8, ES3001, doi:10.2205/2006ES000204.
Copyright 2006 by the Russian Journal of Earth Sciences
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