RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES6004, doi:10.2205/2008ES000306, 2008 | |||||||||||||||||||||
In memory of our friend and colleague Boris Vladimirovich Burov | |||||||||||||||||||||
Native iron in Miocene sedimentsD. M. PecherskySchmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia D. K. Nurgaliev Geological Faculty, Kazan State University, Kazan, Tatarstan, Russia V. M. Trubikhin Geological Institute, Russian Academy of Sciences, Moscow, Russia Contents
Abstract[1] With the aid of thermomagnetic analysis up to 800o C we studied the detailed pattern of how native iron is distributed in Miocene sediments of two sections, Khalats (Turkmenia) and Kvirinaki (Georgia), which are more than 1500 km apart. Out main result is that enrichment by native iron particles was synchronous in both sections. This phenomenon is of extraterrestrial origin and does not depend upon local conditions of deposition. The length of iron-enriched interval (from 12.6 to 12.2 Ma) over a vast area cannot result from a single impact event. Introduction[2] According to numerous data, the particles of metallic (native) iron and nickel are common on the Earth surface, but their temporal distribution is very poorly studied. Unfortunately, thermomagnetic analysis (TMA) of sediments had not been used for this purpose. Recently, TMA up to 800oC was employed during petromagnetic (rock-magnetic) studies of sediments at the K/T boundary [Grachev et al., 2005; Molostovsky et al., 2006; Pechersky et al., 2006a, 2006b]; (D. M. Pechersky et al., in press, 2008a,b). As a result, it has been found that metallic iron in low concentration, usually below 0.001%, is widespread in sediments [Pechersky, 2008a, 2008b]. No enrichment by metallic iron was found close to the K/T boundary. A possible influence of oxidation of iron particles and their re-deposition has led to an idea that it is worth studying how iron particles are distributed in younger sediments, of Miocene and Pliocene ages; however, we abstained from investigating present-day sediments, which may be contaminated by anthropogenic input. The Khalats (Turkmenia) and Kvirinaki (Georgia) sections were selected for such studies as both have already been sampled. These sections satisfy main provisions for such studies, which run as follows: they are about 1500 km apart, local factors, including possible oxidation of iron particles and their re-deposition, can be ruled out; the sediments accumulated in very different physical-geographic environment; the sediments contain abundant fossils for reliable biostratigraphic dating; magnetostratigraphical information is available for both sections (Tables 1 and 2). The Khalats section comprises continental sediments, mostly cobble conglomerates with lenses of sand, sandstone and sandy siltstone. Fine-grained varieties were sampled for paleo- and petromagnetic studies. According to biostratigraphic data, this section represents a nearly continuous succession of Middle Miocene and early Late Miocene age, from the upper part of the Sakaraul-Kotsakhurian stage to the late Sarmatian stage (Table 1). Correlation of paleomagnetic data with the magnetochronostratigraphic scale of Berggren et al. [1995] showed that the Khalats section includes magnetic chrons from C5Cn to C4Ar, which corresponds to the age interval from 16.5 to 9.5 Ma (Table 1). In contrast to the Khalats sections, the Kvirinaki section is composed of marine terrigenous sediments with noticeable carbonate content. According to biostratigraphic data, this section encompasses a large part of the Middle and Late Miocene, from the Tarkhanian stage to the Middle Sarmatian stage, with a large hiatus between the Chokrakian and Karaganian stages (Table 2). Correlation of paleomagnetic data with the magnetochronostratigraphic scale of Berggren et al. [1995] showed that the Kvirinaki section includes magnetic chrons from C5Br to C5n, which corresponds to the age interval from 15.9 to 10.2 Ma with the gap from 14.6 to 13.0 My (Table 2). Thus the sections cover similar intervals, which is very important for establishing the synchronism, or the lack thereof, of iron-enriched levels. [3] This paper presents the results of magnetolithologic and magnetomineralogical study of the Khalats and Kvirinaki sections, the distribution of metallic iron particles being out primary goal. Methods of Petromagnetic Studies
[5] We estimated the content of goethite, magnetite and titanomagnetite combined (labeled magnetite+titanomagnetite or MT+TM hereafter), and metallic iron. (Some thermomagnetic parameters can be used to gain information about magnetite and titanomagnetite separately; in this particular case, however, we are not interested in this). To achieve this goal, the contribution of each mineral into Mi was determined using the Mi(T) curves and, then, it was divided by the specific saturation magnetization of each mineral. Ms values of 90, 200, and 0.25 Am2 kg-1 for magnetite+titanomagnetite, iron and goethite, respectively, were used. [6] To estimate the relative contribution of the total iron content in a rock, we used the intensity of magnetization at 800oC ( M800 ), which is the sum of paramagnetic and diamagnetic magnetizations. For natural minerals, the former is by two or three orders of magnitude higher than the diamagnetic magnetization of quartz and calcite [Rochette et al., 1992]. Hence the paramagnetic component strongly prevails in M800 values for sediments, except for almost purely diamagnetic rocks like limestone or quartz sandstone. [7] We used the following indirect features for mineral identification: a) The growth of magnetization above 500oC (Figure 1a, Samples 399 and 402) indicates the presence of pyrite, which is oxidized to magnetite and hematite above 500oC [Novakova and Gendler, 1995]. b) The presence of Curie point around 580-600oC on the Mi(T) curve and the decrease of magnetization and Curie temperature after the first heating points to decomposed titanomagnetite, which becomes partly homogenized during heating (Figure 1a, sample 24). c) The presence of Curie point at 260-300oC on the Mi(T) curve and its lessening and magnetization growth (in contrast to titanomagnetite) after heating to 800oC point to antiferrimagnetic hemoilmenite of intermediate composition, which is the common product of heterophase oxidation of ilmenite; the latter mineral becomes partly homogenized above 800oC and is transformed into a ferrimagnetic state, which leads to the decrease of the Curie temperature and the growth of magnetization. It is possible that oxidation of paramagnetic ilmenite also results in formation of ferrimagnetic hemoilmenite of intermediate composition during TMA; consequently, a new magnetic phase with the Curie point about 250-300oC is created, and the intensity of sample magnetization grows up (Figure 1a, samples 24 and 402). The Results of Petromagnetic Studies[8] TMA shows that the following magnetic minerals are present in the sediments from the Khalats and Kvirinaki sections:
[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).
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).
[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. 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.
[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. Conclusions[34] With the aid of thermomagnetic analysis up to 800oC we revealed the detailed pattern of how metallic iron particles are distributed in space and time, which is impossible to detect with "direct methods''. Our main discovery is the synchronous enrichment by iron particles in Miocene sediments of the Khalats and Kvirinaki sections that are more than 1500 km apart. This phenomenon that occurred 12.6-12.2 Ma, is most likely to be global and space-connected. It does not depend upon local deposition, sediment composition, redox conditions, etc. Long duration of this phenomenon and its global scale cannot have resulted from a single impact event. ReferencesBerggren, W. A., D. V. Kent, C. C. Swisher III, and M. Aubry (1995), A revised Cenozoic geochronology and chronostratigraphy, Geochronology Time Scale and Global stratigraphic Correlation, Society for Sedimentary Geology (SEPM), Special Publication, (54), 129. Burov, B. V., D. C. Nourgaliev, and P. G. Yasonov (1986), Paleomagnetic Analysis (in Russian), 167 pp., Kazan State University, Kazan. Grachev, A. F., O. A. Korchagin, H. Kollmann, D. Pechersky, and V. Tselmovich (2005), A new look at the nature of the transitional layer at the K/T boundary near Gams, Eastern Alps, Austria, and the problem of the mass extinction of the biota, Russ. J. Earth Sci., 7, ES6001, doi:10.2205/2005ES000189. [CrossRef] Molostovsky, E. A., V. A. Fomin, and D. M. Pechersky (2006), Sedimentogenesis in Maastrichtian-Danian basins of the Russian plate and adjacent areas in the context of plume geodynamics, Russ. J. Earth Sci., 8, ES6001, doi:10.2205/2006ES000206. [CrossRef] Novakova, A. A., and T. Gendler (1995), Metastable structural-magnetic transformations in sulfides in course of oxidation, J. Radioanal. Nucl. Chem., 190, (2), 363. Pechersky, D. M. (2008a), Enrichment of sediments by Fe-hydroxides at the Mesozoic-Cenozoic boundary: A compilation rock-magnetic data, Izv. Phys. Solid Earth, (3), 65. Pechersky, D. M. (2008b), Metallic iron in sediments at K/T boundary, Russ. J. Earth Sci., 9, 1. Pechersky, D. M., A. F. Grachev, D. Nourgaliev, V. Tselmovich, and Z. Sharonova (2006a), Magnetolithologic and magnetomineralogical characteristics of deposits at the Mesozoic/Cenozoic boundary: Gams section (Austria), Russ. J. Earth Sci., 8, (3), ES3001, doi:10.2205/2006ES000204. [CrossRef] Pechersky, D. M., D. K. Nurgaliev, and Z. Sharonova (2006b), Magnetolithologic and magnetomineralogical characteristics of deposits at the Mesozoic/Cenozoic boundary: Koshak section (Mangyshlak), Izv. Phys. Solid Earth, (10), 99. Rochette, P., M. Jackson, and C. Aubourg (1992), Rock magnetism and interpretation of anisotropy of magnetic susceptibility, Reviews of Geophysics, 30, 209. Received 19 July 2008; accepted 22 August 2008; published 14 September 2008. Keywords: cosmic metallic iron, sediments, themomagnetic analysis, Curie point. Index Terms: 1029 Geochemistry: Composition of aerosols and dust particles; 1512 Geomagnetism and Paleomagnetism: Environmental magnetism; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques; 2129 Interplanetary Physics: Interplanetary dust. ![]() Citation: 2008), Native iron in Miocene sediments, Russ. J. Earth Sci., 10, ES6004, doi:10.2205/2008ES000306. (Copyright 2008 by the Russian Journal of Earth SciencesPowered by TeXWeb (Win32, v.2.0). |