New evidence of meteoritic origin of the Tunguska cosmic body
Graphical abstract
Figures a, b, c, d. SEM images of the Tunguska diamond–lonsdaleite–graphite micro-samples: a,b—the general form of the samples, c—lamellar structure of the sample, d—inclusion of troilite crystal in carbon matrix, indicated by arrow.
Introduction
The extremely powerful Tunguska blast of an unknown extraterrestrial object occurred at June 30, 1908 near the river of Podkamennaya Tunguska (60°54'07″N 101°54'16″E) in an unpopulated part of Siberia, Russia. The blast was estimated to be equivalent to 3–5 megatons of trinitrotoluene (e.g. Boslough and Crawford, 2008), and it burned and flattened taiga forests over an area >2000 km2. The origin of the Tunguska blast was explained by a huge meteorite impact (e.g. Yavnel, 1957, Florensky, 1963, Longo et al., 1994, Serra et al., 1994, Longo, 2007, Gasperini et al., 2007, Gasperini et al., 2009, Badyukov et al., 2011) or by a comet (e.g. Florensky et al., 1968a, Florensky et al., 1968b, Golenetsky et al., 1977, Ganapathy, 1983, Zbik, 1984, Nazarov et al., 1990, Hou et al., 1998, Kolesnikov et al., 1999, Kolesnikov et al., 2003, Kolesnikov et al., 2005, Rasmussen et al., 1999, Gladysheva, 2007), or by a cosmic body. However, no clear differences between comet and meteorite impacts were established (Dolgov et al., 1973, Nazarov et al., 1983, Hou et al., 2000, Hou et al., 2004, Kolesnikov et al., 2005). A less-supported concept of a deep plume originating from the lower most mantle, which ‘penetrated’ the lithospheric plate beneath Siberia and escaped to the atmosphere causing a gigantic blast in the Tunguska vicinity, was recently proposed (Skublov et al., 2011). Geochemical data from the natural materials in the epicenter of the Tunguska blast sometimes fit well to either a comet blast or meteorite impact (iron or chondritic), in many other cases researchers cannot separate meteoritic characteristics from those that would refer to a comet. The problem of this uncertain interpretation is that no large fragments of meteorite were ever identified in the Tunguska blast area. The interest to Tunguska was renewed a decade ago after new samples were collected in the vicinity of Lake Cheko, which was proposed to be a filled impact crater formed at the time of the Tunguska event (Gasperini et al., 2007). The meteorite impact crater hypothesis of Gasperini et al. (2007) was rejected by Collins et al. (2008). Later Gasperini et al. (2009) provided more evidence that the crater of interest was formed at the time of the Tunguska event.
In this context we continue to study Tunguska samples that are available in the archives of the Institute of Geochemistry and Mineral Physics of the Academy of Sciences (Ukraine), and are using new high-resolution analytical and imagining techniques. These samples were collected and documented in 1978 during scientific expeditions organized and financed by the Academy of Sciences of the former Ukrainian SSR. Using high resolution scanning (SEM) and transmission electron microscopy (TEM), focused ion beam (FIB) technique for TEM sample preparation, nano-secondary ion mass-spectrometry (NanoSIMS) and synchrotron-based X-ray microfluorescence beam technique, we have studied several small samples of carbon-bearing materials from the Tunguska epicenter.
Section snippets
Previous work
Studies carried out in early 1979–1980s showed that peat samples from the Tunguska site include ‘amalgamated’ polycrystalline micro-aggregates of diamond, lonsdaleite, graphite and troilite (Table 1) which contain traces of Co, Ni, Cr, Rb, Cs, Sr, Ba, Sc, Zr, Ir, Th, Hf, La, Ce, Nd, Sm, Eu, Tb, Yb, Rb, Fe, S, Si and Al (e.g. Kvasnitsa et al., 1979, Kvasnitsa et al., 1980, Sobotovich et al., 1980, Sobotovich et al., 1985). While concentrations of La, Ce, Sm, Eu, Tb, Yb and others corresponded to
Samples and analyses
In this paper we present results of our studies of five fragments of several grains of diamond–lonsdaleite–graphite intergrowths found in peat collected from the Northern peat bog near Kulik's izba situated close to the Tunguska blast epicenter (Fig. 1). The sample of the peat was selected from the 3 m2 area. Composition of the peat does not depend on the chemical composition of the underlying rocks, so moss (Sphagnum fuscum)—oligotorf gets minerals from the air as a result of loss of aerosols.
SEM study
The studied fragments are characterized by “chondrule-like” shape with “lamellar-like” platy microstructures (Fig. 2) which obliterate the microporosity observed on the samples surface. The secondary electron images (Fig. 3) show a fragment of three intergrown allotrops of carbon (diamond, lonsdaleite and graphite), which contain particles of troilite (crystals, plates and films). The frequency of occurrence of the four different phases is as follows: diamond>lonsdaleite>graphite>troilite (
Discussion and conclusions
This study shows that the Tunguska samples consist of three allotropes of carbon, diamond>lonsdaleite>graphite with an average carbon isotopic ratio δ13C=−15.6±2‰. We observed that nanoinclusions of troilite, taenite, γ-Fe and schreibersite in the carbonaceous matrix associate with cracks, pores, or extend out from single grains of troilite as tiny veins. We speculate that these minerals were formed by impregnation of melt into solid carbonaceous matrix crystallizing during rapid cooling.
Aknowledgments
VK thanks the German Science Foundation (DFG), Bonn-Bad Godesberg for a travel grant. Part of this research conducted in the Lawrence Livermore National Laboratory was supported by LAB-FEE Research Grant (LD-JM-BJ-IH). The work performed at Beamline X27A, National Synchrotron Light Source (NSLS) of the Brookhaven National Laboratory. It was supported in part by the US Department of Energy—Geosciences (DE-FG02–92ER14244 to The University of Chicago—CARS). Use of the NSLS was supported by the US
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