Possible records of space weathering on Vesta: Case study in a brecciated eucrite Northwest Africa 1109

Abstract

Records of space weathering are important for understanding the formation and evolution of surface regolith on airless celestial bodies. Current understanding of space weathering processes on asteroids including asteroid‐4 Vesta, the source of the howardite–eucrite–diogenite (HED) meteorites, lags behind what is known for the Moon. In this study, we studied agglutinates, a vesicular glass‐coating lithic clast, and a fine‐grained sulfide replacement texture in the polymict breccia Northwest Africa (NWA) 1109 with electron microscopy. In agglutinates, nanophase grains of FeNi and FeS were observed, whereas npFe0 was absent. We suggested that the agglutinates in NWA 1109 formed from fine‐grained surface materials of Vesta during meteorite/micrometeorite bombardment. The fine‐grained sulfide replacement texture (troilite + hedenbergite + silica) should be a result of reaction between S‐rich vapors and pyroxferroite. The unique Fe/Mn values of relict pyroxferroite indicate a different source from normal HED pyroxenes, arguing that the reaction took place on or near the surface of Vesta. The fine‐grained sulfide replacement texture could be a product of nontypical space weathering on airless celestial bodies. We should pay attention to this texture in future returned samples by asteroid exploration missions.

Evidence of metasomatism in the interior of Vesta

“…Meanwhile, considering the difficulty that reaction 2 requires the existence of almost pure SiO 2 melt/vapor (not involving Ca, Al, Cr, and Ti) in the interior of Vesta, reaction 1 which only requires mobility of S 2 and MgO seems more likely to interpret the texture and compositions for the coexisting olivine, Opx-II, troilite, and metal 25,29 . Lost of some elements from the system during sulfidation of silicate minerals has been observed in the literature 29,32,33 .…”

“…If that the sulfidation of olivine had taken place in the interior of Vesta as discussed above, an external source for the sulfur-rich medium is unlikely 32,33 . Instead, the sulfur-rich medium should be derived from the Vestan interior, although its exact nature and source cannot be constrained based on the current observations.…”

“…A few types of metasomatism on Vesta have been reported based on investigations on HED meteorites 25,27,28,[32][33][34][35][36][37][38] . However, most of the metasomatic textures were correlated to surface or subsurface processes on Vesta [32][33][34][35][36][37][38] . Zhang et al 25 have observed a similar replacement texture in a diogenite meteorite NWA 7183 and proposed that the sulfidation of olivine might have taken place in the interior of Vesta.…”

Shock‐induced phase transformation of anorthitic plagioclase in the eucrite meteorite Northwest Africa 2650

“…A thin foil containing grossular and kyanite was prepared with a FEI Scios focused ion beam (FIB) system at the Institute of Geochemistry, Guiyang, China. The thin foil is approximately 100 nm in thickness after being extracted and thinned at various accelerating voltages and beam currents, the cutting and thinning conditions are the same to that in Wang et al (2019). The FIB foil was observed with a FEI Tecnai F20 TEM instrument at Nanjing University at both high-angle annual dark field scanning transmission electron microscopy (HAADF-STEM) and bright field TEM modes.…”

The sulfurization recorded in tridymite in the monomict eucrite Northwest Africa 11591

“…However, the reduced phases of metasomatism have only been reported in a few HED meteorites. For example, a few high-Fe pyroxene fragments or lithic clasts were partially replaced by a fine-grained and troilite-rich mineral assemblage in the polymict brecciated Northwest Africa (NWA) 2339 and NWA 1109 eucrite, which were considered to be formed via in situ reduction of sulfurization by S-rich vapors due to impact heating on the surface of Vesta (Zhang et al 2013;Wang et al 2019). In addition, intergrowths of troilite-orthopyroxene were found in some howardites, brecciated diogenites, and even in an unbrecciated noritic diogenite, which is suggested to have been formed via reactions between diogenitic olivine and the S-rich vapors (Hewins 1981;Zhang et al 2018Zhang et al , 2020.…”