An Almahata Sitta EL3 fragment: Implications for the Unique Thermal History of Enstatite Chondrites

Abstract Almahata Sitta is a polymict breccia, consisting of many kinds of clasts. Here we present our mineralogical and petrological results on an EL3 fragment, MS-177 from Almahata Sitta. This fragment shows a typical type 3 chondritic texture, consisting of well-defined chondrules often with olivine, isolated silicate minerals, and opaque nodules. Although these components are typical of EL3 chondrites, the mineral abundances and compositions are different from the other EL3s. Diopside is highly abundant. On the other hand, perryite and daubreelite were not found. The major pyroxene is orthoenstatite, and the silica phase is quartz. Fe-Ni metal has relatively high P contents. Troilite is enriched in Cr and Mn. Keilite and buseckite are present in MS-177. From the mineralogy and texture, MS-177 experienced a high-temperature event under subsolidus conditions. Shock-induced heating for a short duration might explain this high-temperature event. This is supported by shock-induced darkened feature of MS-177. We suggest that other E3 chondrites also experienced heating events under such subsolidus conditions on their parent bodies. On the other hand, the high abundance of diopside cannot be explained by a secondary thermal event and may have been a primary feature of MS-177, formed before accretion to the parent body.