[meteorite-list] Shirokovsky

From: Jeff Kuyken <jeff_at_meteoritecentral.com>
Date: Wed Jul 7 16:32:01 2004
Message-ID: <001901c4633b$d8fc2500$12348690_at_mandin4f89ypwu>

G'day Frederic,

The official position of the Nomenclature Committee is stated in their Shirokovsky statement below.

http://meteoriticalsociety.org/bulletin/shirokovsky.pdf

Cheers,

Jeff Kuyken
I.M.C.A. #3085
www.meteorites.com.au

"Shirokovsky pallasite", special statement.

Specimens of an object known as the "Shirokovsky pallasite," recently acquired

by a variety of public and private collections, are probably not meteorites. The

petrology and geochemistry of this object strongly suggest that it has a terrestrial

origin. Below is an account of the 1956 bolide that may have produced meteorites

(still undiscovered), followed by a description of the probable pseudometeorite.

At 03:30 UT on 1956 Feb 1, a fireball shining brighter than the sun and leaving

a smoke trail was observed by numerous eyewitnesses in an area of about 500 km2

across Russia. The fireball disappeared in 5-6 seconds but the trail was visible for

an hour. Windows in nearby villages were broken by the shock wave. A meteorite

reportedly fell on the frozen Shirokovsky reservoir (58?48'N, 57?57'E), situated on

the Kosva River near Shirokovsky village and the cities of Ugle-Uralsk and Kizel,

Producing a 42-cm diameter hole in 80-cm thick ice. 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. In early 2002,

anonymous searchers found many fragments, totaling ~150 kg, of iron-rich

material at the site.

Mineralogy and petrography (M. A. Nazarov, Vernadsky Inst., L. A. Taylor,

University of Tennessee): The material is breccia-textured, with angular silicate

clasts in a metal-rich matrix. Clasts consist mainly of olivine fragments up to 1 cm

(Fa3-39, avg = Fa12, CaO up to 1 wt%, NiO up to 0.3 wt%, Fe/Mn = 8-34 at., avg =

21) and rare diopside grains (Fs3-19Wo43-50). Olivine contains small inclusions of

diopside, roedderite (?), Mg-rich wustite and magnesioferrite. There is no reaction

zone between olivine fragments and the metallic matrix, however diopside has

reacted with the matrix to produce unknown Ca,Fe,Mg-rich phases, larnite and

merwinite. The matrix is metal-wustite eutectic containing minor Ca-rich fayalite.

The metal contains 20-47 wt% Ni (avg = 26.5) and 0.8-2.2 wt% Co (avg = 1.3).

Phosphorus and Cu were not detected. Sulfides, phosphides, phosphates, chromite,

and Al-bearing phases were not found.

Superficially, the mineralogy resembles that of pallasites, and the Fa number

and Fe/Mn ratio of the olivine are comparable to those of pallasite olivines.

However the high Ni of the olivine is distinct from that in all metal-rich meteorites.

The metal-wustite eutectic has never been documented in meteorites. Accessory

minerals are atypical of meteorites, and accessory phases typical of stony-iron

meteorites are absent. Olivine chemistry (Fa, Fe/Mn, CaO, NiO) resembles that in

some terrestrial carbonatites. Olivine was not equilibrated with the matrix melt,

which crystallized quickly under highly oxidizing conditions.

Mineralogy and petrography (T. Bunch and J. Wittke, Northern Arizona Univ.):

contains subequal proportions of fragmental olivine and metal-rich matrix. Olivine

cores average Fa11.4, rims Fa17, and have lower Fe/Mn than pallasites. Olivine has

inclusions of ferroan magnesioferrite (5-80 ?m), magnesiowustite (Mg0.7Fe0.3), and

an alkali-rich Ca phosphate (<30 ?m). Metal composition (wt%), Ni = 23.6, Co =

1.8, Mn = 0.12, Ti = 0.11, Cr = 0.25. Metal contains vermicular inclusions of

magnesian magnetite (4-8 vol%), magnesian kirschteinite (4-8 vol%), and rare

chromite, plagioclase (An89), and olivine (Fa41).

Instrumental neutron activation analysis (M.A. Nazarov and G.M. Kolesov,

Vernadsky Inst.): Composition of metal-wustite eutectic, Ni =14 wt%, Co = 1.03

wt%, Au = 540 ppb, Ag = 2700 ppb, Pt = 339 ppb, Os = 3.8 ppb, Ir = 4.3 ppb, and

Ru = 40 ppb. The Ni and Co contents are similar to those in some pallasites. The

Au, Ir and Os concentrations are all significantly lower than those in pallasites.

Pt/Ir is 40? the solar ratio, similar to that of Cu-Ni ore deposits.

Oxygen isotopes (D. Rumble III, Carnegie Inst. Wash.): Olivine of the

Shirokovsky specimen is on the terrestrial oxygen isotope fractionation line, ?17O

= +2.44?, ?18O = +4.63?, ?17? = +0.002.

Noble gases (Yu. A. Shukolyukov and L. Schultz, Max-Plank Inst.): olivine

has 20Ne/22Ne = 7.4 and 21Ne/22Ne = 0.038, close to the ratios of planetary gas. No

cosmic component was found in He, Ne, or Ar isotopic compositions. This

suggests that the specimen has never been in space, it came from the interior of a

large meteoroid, or it had a very short cosmic exposure history. The small size of

the hole in the reservoir ice and the lack of other meteorite fragments indicate that

the pre-atmospheric mass was on the order of 1 m in diameter, too small to lack

cosmic noble gases. Very short cosmic exposure histories are not typically

observed in meteorites. Some radiogenic 40Ar is present; if the K content of the

olivine is similar to that of Omolon olivine (8 ppm), then the K-Ar age of the

Shirokovsky sample would be about 270 Ma.

Nuclear tracks (L.L. Kashkarov, Vernadsky Inst.): Cosmic-ray tracks, common

in pallasite olivine grains, were not found in olivine from the Shirokovsky

specimen. This is consistent with the results of the noble gas studies. Fission

tracks are possibly present.

Thermoluminescence studies (A.I. Ivliev, Vernadsky Inst.) Induced TL spectra

of olivine are similar to those of olivine of terrestrial peridotites in having a strong

peak at 125oC and a smaller one at 275oC. Pallasite olivine from Marjalahti is

completely different, with a strong peak at 240oC and a weaker one at 160oC.



----- Original Message -----
  From: meteoriteshow
  To: Meteorite List
  Sent: Tuesday, July 06, 2004 6:05 PM
  Subject: [meteorite-list] Shirokovsky


  Dear All,

  As I mentioned it about a week ago, Christophe and I attended both Ensisheim and Ste Marie aux Mines shows last month. This year, there was nobody proposing any slice of "Shirokovsky" in Ensisheim, but Mikhail Medvedev from St Petersburg was in Ste Marie, and sold again some slices of this "meteorite". A good friend of mines bought a slice for 200.00 Euros, and was quite ennoyed when he was told later that this is not a meteorite.
  I know that there was lots of controversial talks about Shirokovsky, but I am not 100% sure of the final position (if any) about it.
  Could any of you provide some information, I would appreciate.
  Thanks and kind regards,

  Frederic Beroud
  www.meteoriteshow.com
  IMCA #2491


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