RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES6006, doi:10.2205/2008ES000304, 2008
[2] Cosmic dust particles, in particular of metallic iron and nickel, are often found by satellite measurements in dust clouds, atmosphere, ice cores from Antarctica and Greenland, and oceanic pelagic sediments. On average, about 40,000 tons of cosmic dust is thought to fall annually on the Earth. This amount did not vary more than two-fold either for the last 30 Ky judging by a study of an Antarctic ice core or for the last 80 My as indicated by osmium and iridium isotopes in pelagic sediments in the Pacific. Two jumps up to 500,000 tons/year at ~25 Ma and ~65 Ma from a single column 596 DSDP [Peucker-Ehrenbrink, 1996] are exceptions. Numerous discoveries of metallic iron, usually as spheres and sometimes as flakes are well known. The spatial and temporal distribution and amount of the particles of metallic iron and nickel, however, are poorly known because only the "direct'' methods are used for detecting such particles. An opportunity of obtaining vast and rapid information on metallic iron distribution has been missed.
[3] Thermomagnetic analysis (TMA) is widely used as a part of paleo- and petromagnetic (rock-magnetic) studies of various geologic objects, sediments and sedimentary rocks of different geologic age. As the primary goal of such studies was to characterize the carriers of the natural remanent magnetization, the maximum temperature of TMA was not higher than the Curie point of hematite, i.e. maximum about 700o C. Therefore, any information on composition and concentration of metallic iron particles was completely excluded from the consideration!
[4] In order to trace metallic iron we systematically used the TMA up to 800o in petromagnetic studies of sediments at the K/T boundary [Grachev et al., 2005; Pechersky et al., 2006a]. This paper is the first attempt to review TMA data on the distribution of metallic iron in several sections [Adamia et al., 1993; Grachev et al., 2005; Molostovsky et al., 2006; Pechersky, 2008; Pechersky et al., 2006a, 2006b]; (D. M. Pechersky et al., in press, 2008a,b).
[5] Let us review these results from two points of view: 1) What the spatial distribution of metallic iron is in coeval sediments close to the K/T boundary; and 2) How metallic iron is distributed through time. In both cases, the main attention will be paid to the lithological characteristics and to the relationship with other magnetic minerals like Fe-hydroxides, magnetite, titanomagnetite, which formation and accumulation are of terrestrial provenance, in contrast to metallic iron.
[6] The concentration of magnetite, titanomagnetite, iron and goethite was evaluated by determining the contribution of each mineral Mi in the Mi(T) plots with subsequent normalizing by specific saturation magnetization of each mineral. The following values of Ms were used: ~90 Am2 kg-1 for magnetite and titanomagnetite, ~200 Am2 kg-1 for iron, and 0.25 Am2 kg-1 for goethite. Note that the specific saturation magnetization of goethite varies from 0.02 to 0.5 Am2 kg-1, and the average value was used here. Paramagnetic magnetization of the studied samples was determined with the aid of magnetic measurements.
where Mp is paramagnetic magnetization at room temperature in the field of 500 mT; Md is diamagnetic magnetization at room temperature in the field of 500 mT (diamagnetic magnetization is practically independent of temperature); M20 is "total'' paramagnetic + diamagnetic magnetization derived from the isothermal magnetization curve of a sample at room temperature in the field that is higher than saturation field of magnetic minerals in this sample; M800 is as before but measured at 800o in the same field; 0.274 is the ratio of temperatures 295K/1075K according to the Curie-Weiss law (see the above cited papers for more detail).
Citation: 2008), Metallic iron in sediments at the Mesozoic-Cenozoic (K/T) boundary, Russ. J. Earth Sci., 10, ES6006, doi:10.2205/2008ES000304.
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