Electron microscopy of pyroxene in the Almahata Sitta ureilite

Abstract

Abstract– We performed scanning electron microscope–electron backscatter diffraction (SEM‐EBSD) and focused ion beam–transmission electron microscopy (FIB‐TEM) investigations of pyroxene in the Almahata Sitta ureilite (sample #7). The pyroxenes (mg# = ∼0.92) are present as minute individual grains (10–20 μm in size) showing a polycrystalline texture, and they are both low‐Ca and high‐Ca pyroxenes. Although their Ca contents are as low as Wo2–3, the EBSD analysis shows that low‐Ca pyroxenes are clinopyroxene (P21/c). The obtained pyroxene equilibration temperature (1240–1280 °C) is consistent with the previous studies on many ureilites. In low‐Ca pigeonite, (001) augite exsolution lamellae (10–15 nm wide) develop in the pigeonite host (20–45 nm wide), and a similar exsolution texture was observed in augite. The exsolution wavelength of pyroxene (typically 30–60 nm) gives the cooling rate of 0.2–5 °C h−1. Such a rapid cooling probably records quenching from high temperature (1240–1280 °C) down to 1000 °C (estimated from the exsolution pair) due to the breakup of the ureilite parent body (UPB) while it was still hot. The pyroxene microstructure of Almahata Sitta is within the range of known ureilites and is most similar to that of Allan Hills (ALH) A81101 with the affinity of polycrystalline texture. The coarser exsolution texture of ALHA81101 pyroxene suggests slower cooling history than Almahata Sitta. However, direct comparison is difficult because of different pyroxene compositions. Because ALHA77257 has a similar pyroxene composition to Almahata Sitta and does not show visible pyroxene exsolution, it should have cooled faster than Almahata Sitta. Rapid, albeit variable cooling rates observed in different ureilite samples may suggest that they originated from UPB fragments of different size.