At the interface of silica glass and compressed silica aerogel in Stardust track 10: Comet Wild 2 is not a goldmine

Abstract

Abstract-In Stardust tracks C2044,0,38, C2044,0,39, and C2044,0,42 (Brennan et al. 2007 and Stardust track 10 (this work) gold is present in excess of its cosmochemical abundance. Ultra-thin sections of allocation FC6,0,10,0,26 (track 10) show a somewhat wavy, compressed silica aerogel/silica glass interface which challenges exact location identification, i.e., silica glass, compressed silica aerogel, or areas of overlap. In addition to domains of pure silica ranging from SiO 2 to SiO 3 glass, there is MgO-rich silica glass with a deep metastable composition, MgO = 14 AE 6 wt%, due to assimilation of Wild 2 Mg-silicate matter in silica melt. This magnesiosilica composition formed when temperatures during hypervelocity capture reached >2000°C followed by ultrafast quenching of the magnesiosilica melt when it came into contact with compressed aerogel at~155°C. The compressed silica aerogel in track 10 has a continuous Au background as result of the melting point depression of gold particles <5 nm that showed liquid-like behavior. Larger gold particles are scattered found throughout the silica aerogel matrix and in aggregates up to~50 nm in size. No gold is found in MgO-rich silica glass. Gold in track 10 is present at the silica aerogel/silica glass interface. In the other tracks gold was likely near-surface contamination possibly from an autoclave used in processing of these particular aerogel tiles. So far gold contamination is documented in these four different tracks. Whether they are the only tiles with gold present in excess of its cosmochemical abundance or whether more tiles will show excess gold abundances is unknown.

GEMS, hydrated chondritic IDPs, CI‐matrix material: Sources of water in 81P/comet Wild 2

“…), which is no surprise. A single outlier composition, MgO = 27.5 wt%, is close to the “enstatitic” eutectic composition in the MgO‐SiO 2 phase diagram (Rietmeijer ).…”

“…This DME (deep metastable eutectic) magnesiosilica composition indicates that this melt quenched rapidly , which is no surprise. A single outlier composition, MgO = 27.5 wt%, is close to the "enstatitic" eutectic composition in the MgO-SiO 2 phase diagram (Rietmeijer 2016a).…”

“…) aside from rare, round glassy objects superficially resembling GEMS in the bulbous part of Type B and Type C tracks (Joswiak et al. ; Leroux and Jacob ; Rietmeijer , ). Isotopically anomalous presolar GEMS are in fact but a fraction (1–6%) of GEMS in anhydrous chondritic IDPs, that is, the vast majority of GEMS have a solar system origin (Keller and Messenger ).…”

“…The GEMS-like objects in Stardust are probably the results of mixing [Mg-Fe-Si-O] and [Fe-Ni-S] melts (Rietmeijer 2015) but this process cannot explain the presence of GEMS in the matrix of CP IDPs. The STARDUST mission to Jupiter-Family (J-F) comet 81P/Wild 2 (hereafter Wild 2) ) did not provide compelling evidence for GEMS (Zolensky et al 2006) aside from rare, round glassy objects superficially resembling GEMS in the bulbous part of Type B and Type C tracks (Joswiak et al 2012;Leroux and Jacob 2013;Rietmeijer 2015Rietmeijer , 2016a. Isotopically anomalous presolar GEMS are in fact but a fraction (1-6%) of GEMS in anhydrous chondritic IDPs, that is, the vast majority of GEMS have a solar system origin (Keller and Messenger 2011).…”

Material of Cometary Nuclei and Asteroids Can Be Studied in the Earth’s Orbit

“…Until now, it was possible to study the composition of those cometary nuclei and asteroids whose meteorite materials fell to the Earth's surface (Churyumov et al 2016). Every day, 100-1000 tons of meteorites falls here (Churyumov et al 2014;2015;Vid'Machenko 1995;Vidmachenko 2009;Vidmachenko et al 1979;Vidmachenko & Morozhenko 2014;Vidmachenko & Steklov 2013).…”