Pure metallic iron and hercynite in Eucrite imply a thermal condition after impaction

Introduction: The Howardites-Eucrite-Diogenite (HED) clan of meteorites are generally considered to be derived from the Vesta asteroid [1-2]. Previous study demonstrated the pressure of the impact event on vesta is below 15GPa, as well as few high-pressure minerals have been found in HED meteorites [3-4]. Thermal metamorphic event is a significant stage of Vesta evolution, and the temperature is ranging from ~700 to 1000 °C [5]. Iron metal veins (most with minor Ni) had been reported in HED meteorites, which is attributed to the fluid deposition during the fluid-driven secondary alteration stage on Vesta [6]. Spinel is a common minor mineral in HED meteorites, dominantly of Cr-spinel [7]. Our work focuses on the observation of pure metallic iron and hercynite particles in the shocked melt pocket of Northwest Africa 11592 (NWA 11592). According to the results of HR-TEM observation and quantitative EDS mapping, the pure metallic iron and hercynite were determined in the meteorite, indicating a thermal condition of Vesta. Sample and analytical techniques: NWA 11592 was found in Algeria, 2016, and was classified as basaltic Eucrite. The polished thin section of NWA 11592 was observed and analyzed by the FEI Scios dual-beam scanning microscope. The mineral composition was analyzed by JEOL 8230 electron microprobe (EPMA). Two FIB cross sections were prepared by the FEI Scios focus ion beam and analyzed by FEI Talos field-emission scanning transmission electron microscope. Results: Petrology of host rock and shocked melt pocket. NWA 11592 is a basaltic Eucrite that mainly consists of pyroxene and plagioclase with an ophitic texture, as well as minor amounts of olivine, troilite, chromite and ilmenite. Augite exsolution lamella was universally observed within the pyroxene. The average compositions of host pyroxene (Fs57.8-63.1Wo2.05-6.75) and augite (Fs26.6-27.4Wo44.4-45.3) suggest an average equilibrium temperature about 850°C [8]. Most Plagioclase crystals (An88.1Or0.7) exhibit atypical crystallization characteristics (not completely transformed into maskelynite), and its Raman spectrum shows diffuse broad peaks. All these mineral grains exhibit numerous thin cracks, which are attribute to the impact events on Vesta. There are two thin shock-induced melt veins (~100μm wide) and one shocked melt pocket (~300μm wide) are observed in NWA 11592 (Fig 1a.). Both of veins exhibit amorphous glass with the composition of mixture of pyroxene and Plagioclase in host rock. Our work focused on the melt pocket with typical quench texture characterized by the dark needle-like material within the fine-grained matrix, as well as abundant metallic iron particles (~200nm) attach to the needlelike material (Fig 1c.).