MYSTERY OF SHIROKOVSKY FALL
Fall, February 1, 1956, 3:30 UT
58° 48' N, 57° 57' E
Russia, Ural, Perm district
The fall. In the early morning of 1956, a fireball shining brighter than the Sun and leaving a smoke trail was observed by numerous eyewitnesses over an area of about 500 km2. The fireball disappeared in 5-6 seconds, but the trail was visible for an hour. A powerful explosion caused a weak earthquake. Windows in nearby villages were broken by the impact wave. The meteorite fell on the frozen Shirokovsky reservoir situated on the Kosva River near the village of Shirokovsky and the cities of Ugle-Uralsk and Kizel. The meteorite made a 42 cm diameter hole in the 80 cm thick ice sheet and sank to the bottom at a depth of 23 m. Magnetic particles enriched in Ni were extracted from the ice surrounding the hole. Several attempts by divers to recover the meteorite on the bottom were not successful. It was not until early 2002, after 3 years of searching, that searchers found some iron pieces at the fall site. Many fragments with a total mass of about 150 kg were collected. Two slices were transferred to the Vernadsky Institute for study and research.
The slice of Shirokovsky rock (http://www.pallasite.ru/)
Mineralogy and Petrography. M.A. Nazarov, Vernadsky Inst.;L.A. Taylor, University of Tennessee): This rock is a melt-matrix breccia, in outward appearance similar to a stony-iron meteorite. The clast population consists mainly of angular olivine fragments (up to 1 cm), but rare diopside clasts are also present. Olivine contains small inclusions of diopside, roedderite (?), Mg-rich wustite, and magnesioferrite. There was obvious reaction zone between the olivine fragments and the iron-metal matrix. In contrast, diopside fragments have reacted with the melt-matrix to produce Ca,Fe,Mg phases, some not readily identifiable. Larnite and merwinite were found among these reaction products. Olivine is Fa3-39 ; (av=Fa12, and contains appreciable CaO (up to 1 wt%), and NiO (up to 0.3 wt%). Fe/Mn (at.) of olivine is 8-34 (av = 21). Diopside is Wo43-50 Fs3-19. The melt-matrix is metal-wustite eutectic mixture. Minor Ca-rich fayalite is also present in the eutectic texture. The metal contains 20-47 wt% Ni and 0.8-2.2 wt% Co. P and Cu were not detected (i.e., <0.04%). No sulfides, phosphides, phosphates, chromites, or Al-bearing phases were found in the samples. Obviously, olivine of the Shirokovsky rock was not equilibrated with the matrix melt. The clast-laidenn melt was cooled and crystallized rapidly, under oxidizing conditions.
In mineralogy, the studied samples resemble pallasites, and the Fa number and the Fe/Mn ratio of the olivine are compatible with those of pallasite olivines. However, the high Ni content of the olivine is distinctly different from that of most meteorites. Granted, there are Ni-rich olivines in CK chondrites, but such rocks contain metal little to no native Fe metal. In addition, the metal-wustite eutectic has never been documented in meteorites. Furthermore, the Shirokovsky samples do not contain typical accessory phases of stony-iron meteorites, and the accessory phases are completely atypical for all meteorites. In chemistry (Fa, Fe/Mn, Ca, Ni), the Shirikovsky olivines are similar to those from terrestrial carbonatites.
Trace elements (M.A. Nazarov, G.M. Kolesov, Vernadsky Inst.). The metal-wustite eutectic mixture was analyzed with INAA resulting in the following analyses: 14 wt% Ni, 1.03 wt.% Co, 540 ppb Au, 2700 ppb Ag, 339 ppb Pt, 3.8 ppb Os, 4.3 ppb Ir, and 40 ppb Ru. The Ni and Co contents are similar to those in pallasites. The Ir and Os contents are slightly lower than that in pallasites. However, the Pt/Ir ratio is much higher than cosmic and similar to that of terrestrial Cu-Ni ore deposits.
Oxygen isotopes (D.Rumble III, Carnegie Inst.). Olivine of the Shirokovsky samples has a terrestrial oxygen isotopic composition: d
17O = +2.44‰, d
18O = +4.63‰ D17O = 0.002
Noble gases(Yu.A.Shukolyukov, L.Shultz, Max-Plank Inst.). 20Ne/22Ne=7.4 and 21Ne/22Ne=0.038of Shirokovsky olivine are close to those of the planetary gases. No cosmic component occurs in the He, Ne, and Ar isotopic compositions. This suggests that either the Shirokovsky rock has never been in space, or it had been, it had an extremely short cosmic exposure history. Note also that the hole in the ice was only 42 cm in diameter, and therefore, a body of such size must have some cosmic noble gases. Some radiogenic 40Ar is present. If the K content of the olivine is similar to that of the Omolon olivine (8 ppm), then the K-Ar age of the Shirokovsky sample should be about 270 Ma.
Nuclear tracks (L.L.Kashkarov, Vernadsky Inst.). Cosmic-ray tracks were not found in the Shirokovsky olivine grains. This corroborates the results of noble gas study. However, some fission tracks are possibly present.
TL studies (A.I.Ivliev, Vernadsky Inst.) The TL spectra of the Shirokovsky olivine are completely different from pallasite olivines and similar to the olivines of terrestrial peridotites.