Thermal and shock history of Almahata Sitta meteorite inferred from structure refinement of pyroxene and Mössbauer spectroscopy of Fe‐Ni metal

Abstract

AbstractThe crystal structures of orthopyroxene (En86.3Fs8.6Wo5.1, space group Pbca) and pigeonite (En81.7Fs8.8Wo9.5, space group P21/c) from the Almahata Sitta ureilite (fragment#051) have been refined to R1 indices of 3.10% and 2.53%, respectively, using single‐crystal X‐ray diffraction data. The unit formulas were calculated from electron microprobe analysis, and the occupancies at the M1 and M2 sites were refined for both pyroxenes from the single‐crystal diffraction data. The results indicate a rather disordered intracrystalline Fe2+‐Mg cation distribution over the M1 and M2 sites, with a closure temperature of 726(±55)°C for orthopyroxene and 704(±110)°C for pigeonite, suggesting fast cooling of these pyroxenes. The Mössbauer spectrum of the Fe‐Ni metal particles of Almahata Sitta ureilite (fragment#051) is dominated by two overlapping magnetic sextets that are assigned to Fe atoms in Si‐bearing kamacite, and arise from two different nearest‐neighbor configurations of Fe* (=Fe+Ni) and Si atoms in the bcc structure of kamacite; (8F*, 0Si) and (7Fe*, 1Si). In addition, the spectrum shows weak absorption peaks that are attributed to the presence of small amounts of cohenite [(Fe,Ni)3C], schreibersite [(Fe,Ni)3P], and an Fe‐oxide/hydroxide phase. The fast cooling of pyroxene to the closure temperature (after equilibration at ~1200°C) and the incorporation of Si in kamacite can be interpreted as due to a shock event that took place on the meteorite parent body, consistent with the proposed formation history of ureilites parent body where a fast cooling has occurred at a later stage of its formation.