Peculiarities of the composition and genesis of the Brahin meteorite

A.I. Bakhtin^*, A.A. Eskin^**, R.Kh. Sungatullin^***, G.V. Sonin^****, R.D. Petrova^*****

Kazan Federal University, Kazan, 420008 Russia

E-mail: ^*Anatoly.Bakhtin@kpfu.ru, ^**eskin.aleksey@gmail.com, ^***rafael.sungatullin@kpfu.ru, ^****g_sonin@mail.ru, ^*****Rimma.Petrova@kpfu.ru

Received April 6, 2017

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Abstract

A polished plate of the Brahin meteorite from the collection of the Geological Museum of the Institute of Geology and Petroleum Technologies of Kazan Federal University has been studied with the use of microprobe analysis. The analyses have been recalculated into the end members, based on which the conclusions have been made about the mineralogical composition of the meteorite. We have found 15 minerals and aluminosilicate glass of a complex composition. The structure of the meteorite is cellular porphyry. The cellular part is composed mostly of plessite, the porphyritic part is made of large olivine crystals. The features of mineral precipitates and their spatial distribution have allowed to conclude that the substance of the Brahin meteorite developed as a result of a fluid-metasomatic transformation of the initially chondrite-like substance under the influence of deep restorative fluids of H₂, CO, CO₂, H₂S etc.

Keywords: meteorites, Brahin meteorite, pallasites, mineral composition, pallasite genesis

Acknowledgments. The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University.

Figure Captions

Fig. 1. Polished plate of the Brahin meteorite.

Fig. 2. Metasomatic inclusions of graphite (black, analysis 1 in Table 1) in olivine (gray).

Fig. 3. Metasomatic inclusions of graphite (dark gray, analysis 2) in olivine (gray, analysis 4). Graphite contains an inclusion (analysis 3) composed of a cryptocrystalline mixture of cohenite (73%) and kamacite (27%).

Fig. 4. The diagram of the equilibrium composition of the system Fe–C–O in dependence of the temperature and CO content (according to B.F. Goncharov and I.S. Solomakhin [4]). Reactions: I – Fe₃O₄ + CO = 3FeO + CO₂; II – FeO + CO = Fe + CO₂; III – 2CO = C + CO₂.

Fig. 5. Segregation of wustite (white, analysis 5) in the micrograin aggregate of wustite-silicate glass (gray, analysis 6). Dark gray is olivine replaced by wustite and silicate glass.

Fig. 6. The position of the lines of the buffer equilibria in the coordinates of fO₂ – T,˚C (from the work [6]). Buffer lines: 1 – SiO₂–Fe₂SiO₄–FeFe₂O₄ (QFM); 2 – FeO– FeFe₂O₄ (WM); 3 – Fe–FeO (IW); 4 – С–СО.

Fig. 7. Zonal position of minerals: olivine (Mg, Fe)₂SiO₄ (dark gray, analysis 7) – chromite (Fe, Mg)Cr₂O₄ (gray, analysis 9) – mackinawite FeS_1–х (light gray, analysis 8) – (cohenite + kamacite) (white, analysis 10) indicates a change of the weakly oxidizing medium (olivine) into the strongly deoxidizing medium (kamacite).

Fig. 8. Tridymite (dark gray, on the right in the upper corner, analysis 11) surrounded by the cryptomerous aggregate of wustite-silicate glass (gray, analysis 6).

Fig. 9. Feldspar (gray, analysis 12) surrounded by the cryptomerous aggregate of wustite-silicate glass.

Fig. 10.The olivine crystal.

Fig. 11. The graph showing changes in the ferruginosity of the olivine crystal f(Оl) shown in Fig. 10 in the profile from the center (points 1, 2,…) to the edge of the crystal (points …9, 10).

Fig. 12. A vein composed of a cryptocrystalline mixture of cohenite (64%) and kamacite (36%) (analysis 14) in olivine.

Fig. 13. Mackinawite veins (analysis 15) in the olivine cracks (analysis 16).

Fig. 14. An inclusion made of the cryptomerous mixture of mackinawite (64%) and heazlewoodite (36%) (analysis 17) in olivine (analysis 18).

Fig. 15. An inclusion of haxonite (black, analysis 19) in the wustite veins (gray, analysis 20) in olivine (dark gray).

Fig. 16. The graphs showing changes in the silicate ferruginosity (f) and the content of metallic iron (Fe_met) in meteorites of different classes marked with numbers in Table 4.

References

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For citation: Bakhtin A.I., Eskin A.A., Sungatullin R.Kh., Sonin G.V., Petrova R.D. Peculiarities of the composition and genesis of the Brahin meteorite. Uchenye Zapiski Kazanskogo Universiteta. Seriya Estestvennye Nauki, 2018, vol. 160, no. 2, pp. 324–338. (In Russian)

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