Mineralogy of Antarctic aubrites, Yamato-793592 and Allan Hills-78113: Comparison with non-Antarctic aubrites and E-chondrites

Two Antarctic aubrites, Yamato (Y)-793592 and Allan Hills (ALH)-78113,were mineralogically studied, for comparison with minerals in non-Antarctic aubrites and enstatite chondrites. The Antarctic aubrites are breccias consisting of coarse-grained enstatite fragments and fine-grained matrix. ALH-78113 has 200-300μm dark clasts that are fine-grained aggregates of silicate and opaque minerals. FeO-rich pyroxene (up to Fs_) occurs in the dark clasts. One dark clast has K-feldspar. These dark clasts seem to be exotic inclusions with distinct mineralogy. Daubreelite in the two Antarctic and non-Antarctic aubrites is lower in Zn than those in EH3-5 chondrites. This reflects the depletion of volatile elements in aubrites. Hydrated Na-Cr-sulfides were also found. Djerfisherite is a common accessory mineral in aubrites. It is characterized by low contents of Cu and Na, and high content of Ni, in comparison to djerfisherite in EH3-5 chondrites. Y-793592 has many roedderite grains. The occurrences of roedderite, Na-Cr-sulfides and djerfisherite in aubrites suggest that Al_2O_3 relative to alkali elements may have been fractionated during nebular or magmatic process

Phosphate-bearing microspherules in chondrules of unequilibrated ordinary chondrites

Microspherules, up to ca. 200μm in size, composed mainly of metal cores and various amounts of phosphates usually at the rim are described in chondrules and igneous-textured clasts from ordinary chondrites of petrologic types 3 and 4 and one type 5 chondrite. Phosphates found in microspherules in petrologic types 3 and 4 are mostly whitlockite and those in type 5 include apatite as well. Their chemical composition is almost identical to those in equilibrated ordinary chondrites. One of those spherules in Chainpur shows quite complex texture with native copper and lawrencite-like phase. Though many points on the formation history of phosphate-bearing bodies are left as future problems, their xenolithic origin was indicated by the features of some of those spherules. They were probably formed from molten droplets of metal and schreibersite (in the same event as the chondrule formation?) followed by oxidation reaction in advance of their being captured by coarser silicate chondrule melt

Discovery of metallic residues of the Rochechouart meteorite in basement rocks

The Rochechouart structure of Triassic or early Jurassic age is deeply eroded to the level of the crater floor. At Champonger, the crater floor could be recognized by the association of shatter cones in shocked basement rocks overlain by low temperature suevite. Metal veinlets and particles were found to occur in Champonger basement gneisses within shock produced inter- and intragranular microfractures in various minerals. The metals very probably are condensates of the vaporized impacting projectile and were probably implanted as metal vapor. The condensed metallic vapor was tightly sealed within these fractures. The metals consist of an alloy containing on the average 73 wt. % Fe, 17 wt. % Cr and 8 wt. % Ni. No Mn was found, small amounts of Co may be present. Due to the high Cr content we suggest that the Rochechouart meteorite was a stony and not an iron as previously proposed by Janssens et al. (1977).La structure de Rochechouart qui date du Trias ou du début du Jurassique, est très érodée, jusqu'au niveau du fond du cratère. A Champonger, celui-ci a pu être repéré par l'association de « shattercones » dans les roches choquées du socle, recouvertes de suevite. Des veinules et des particules métalliques ont été découvertes dans les gneiss du socle, dans des microfractures inter-et intragranulaires induites par le choc dans divers minéraux. Les métaux sont très probablement des condensats du projectile impactant vaporisés et ont dû être implantés sous forme de vapeur. Les vapeurs métalliques condensées ont été scellées dans ces fractures. Il s'agit d'un alliage contenant en moyenne 73 % Fe, 17 % Cr et 8 % Ni (en poids), il n'a pas été décelé de Mn, on trouve de petites quantités de Co. D'après la forte teneur en Cr, on peut supposer que la météorite de Rochechouart n'était pas métallique, ainsi que l'ont proposé Janssens et al. (1977) mais une météorite pierreuse.Horn Wolfgang, El Goresy Ahmed. Discovery of metallic residues of the Rochechouart meteorite in basement rocks. In: Bulletin de Minéralogie, volume 104, 4, 1981. 12e assemblée générale de l'I.M.A. - Orléans – Juillet 1980. Deuxième partie : inclusions magmatiques / silicates / gemmes / « open session »

Shock Events in the Solar System: The Message from Minerals in Terrestrial Planets and Asteroids

Impacts are central to the origin and evolution of planets of the Solar System. The shapes of craters, which can reach up to 1,000 km in diameter on the Moon, provide critical information on the large-scale dynamics of the impact and related shock. Minerals formed at high pressure and temperature found in shocked terrestrial rocks and meteorites give additional and complementary insights on the shock process at a smaller scale, typically from a few micrometers to a few millimeters. Local flaws in rocks, such as voids and mineral interfaces, are the preferential sites for the formation of high-pressure melts and minerals. Calculations based on the physics of shocks and the thermodynamics and kinetics of mineral transformations provide orders of magnitude for the duration, transient pressure, and prevailing temperature conditions of shock events. Case studies on shocked terrestrial and extraterrestrial materials illustrate the links between these parameters and impact duration. Many of the high-pressure mineral phases of olivine, pyroxenes, feldspars, silica, phosphates, titanium oxide, and carbon have been discovered in these heavily shocked rocks and provide unique opportunities to study the high-pressure minerals that exist in the deep Earth

A shock-produced (Mg, Fe)SiO3 glass in the Suizhou meteorite

Ovoid grains consisting of glass of stoichiometric (Mg, Fe)SiO3 composition that is intimately associated with majorite were identified in the shock veins of the Suizhou meteorite. The glass is surrounded by a thick rim of polycrystalline majorite and is identical in composition to the parental low-Ca pyroxene and majorite. These ovoid grains are surrounded by a fine-grained matrix composed of majorite-pyrope garnet, ringwoodite, magnesiowstite, metal, and troilite. This study strongly suggests that some precursor pyroxene grains inside the shock veins were transformed to perovskite within the pyroxene due to a relatively low temperature, while at the rim region pyroxene grains transformed to majorite due to a higher temperature. After pressure release, perovskite vitrified at post-shock temperature. The existence of vitrified perovskite indicates that the peak pressure in the shock veins exceeds 23 GPa. The post-shock temperature in the meteorite could have been above 477 degrees C. This study indicates that the occurrence of high-pressure minerals in the shock veins could not be used as a ubiquitous criterion for evaluating the shock stage of meteorites.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202