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Disequilibrium features in experimentally shocked mixtures of olivine plus silica glass powders
... 1970). Shock-induced melts were also demonstrated to preserve high-silica glasses from mixing with other glass compositions. The low-pressure silica glass, named lechatelierite, often mixes incompletely with the other melt before cooling, leading to flow structure (schlieren) when the melt is subjected to shear deformation (e.g., See et al. 1998). Schaal (1982) demonstrated experimentally that shockmelting mixtures of silica glass and olivine powder does not induce full mixing of both melt products. Kinetic factors probably played a role, particularly during quenching of silicate melts, and perhaps even vapors, that would cause the formation of intermediate, deep metastable eutectic solid comp ...
We report the results of high-resolution, analytical and scanning transmission electron microscopy (STEM), including intensive element mapping, of severely thermally modified dust from comet 81P/Wild 2 caught in the silica aerogel capture cells of the Stardust mission. Thermal interactions during capture caused widespread melting of cometary silicates, Fe-Ni-S phases, and the aerogel. The characteristic assemblage of thermally modified material consists of a vesicular, silica-rich glass matrix with abundant Fe-Ni-S droplets, the latter of which exhibit a distinct core-mantle structure with a metallic Fe,Ni core and a iron-sulfide rim. Within the glassy matrix, the elemental distribution is highly heterogeneous. Localized amorphous "dust-rich" patches contain Mg, Al, and Ca in higher abundances and suggest incomplete mixing of silicate progenitors with molten aerogel. In some cases, the element distribution within these patches seems to depict the outlines of ghost mineral assemblages, allowing the reconstruction of the original mineralogy. A few crystalline silicates survived with alteration limited to the grain rims. The Fe- and CI-normalized bulk composition derived from several sections show CI-chondrite relative abundances for Mg, Al, S, Ca, Cr, Mn, Fe, and Ni. The data indicate a 5 to 15% admixture of fine-grained chondritic comet dust with the silica glass matrix. These strongly thermally modified samples could have originated from a finegrained primitive material, loosely bound Wild 2 dust aggregates, which were heated and melted more efficiently than the relatively coarse-grained material of the crystalline particles found elsewhere in many of the same Stardust aerogel tracks (Zolensky et al. 2006).
Transmission electron microscopy was used for characterizing the defect microstructure induced by shock experiments in a single crystal of diopside. The shock-induced defects found in the crystal can be divided in four distinct types:1)
A high density and pervasive distribution of dislocations in glide configuration (glide systems (100)[0
2)
Mechanical twin lamellae, mostly parallel to (100), the (001) twin lamellae are less abundant. li]3)
4)
Heterogeneously distributed tiny molten zones (3 to 20 m size) which, after cooling, appear as a glass with a chemical composition very close to the one of the original diopside.
The present TEM study reveals that the defect micro-structure in shocked diopside consists of a large variety of shock-induced defects. Especially, the amorphous PDFs which were never observed in statically deformed diopside seem to be an important characteristic micro-structural defects in shocked silicate minerals. Although the presence of amorphous PDFs is not yet confirmed for naturally shocked clinopyroxene, we strongly suggest that these features can serve as a diagnostic tool for recognizing impact phenomena on all planetary bodies of our solar system.
Observations of molten mid-ocean ridge basalt (MORB)-molybdenum (Mo) interactions produced by shock experiments provide insight into impact and differentiation processes involving metal-silicate partitioning. Analysis of fragments recovered from experiments (achieving MORB liquid shock pressures from 0.8 to 6 GPa) revealed significant changes in the composition of the MORB and Mo due to reaction of the silicate and metal liquids on a short time scale ( < 13 s). The FeO concentration of the shocked liquid de creases systematically with increasing pressure. In fact, the most highly shocked liquid (6 GPa) contains only 0.1 wt% FeO compared to an initial concentration of 9 wt% in the MORB. We infer from the presence of micrometer-sized Fe-, Si- and Mo-rich metallic spheres in the shocked glass that the Fe and Si oxides in the MORB were reduced in an estimated oxygen fugacity of 10^(−17) bar and subsequently alloyed with the Mo.
The in-situ reduction of FeO in the shocked molten basalt implies that shock-induced reduction of impact melt should be considered a viable mechanism for the formation of metallic phases. Similar metallic phases may form during impact accretion of planets and in impacted material found on the lunar surface and near terrestrial impact craters. In particular, the minute, isolated Fe particles found in lunar soils may have formed by such a process. Furthermore, the metallic spheres within the shocked glass have a globular texture similar to the textures of metallic spheroids from lunar samples and the estimated, slow cooling rate of ⩽ 140°C/s for our spheres is consistent with the interpretation that the lunar spheroids formed by slow cooling within a melted target.
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