Interaction of aluminum projectiles with quartz sand in impact experiments: Formation of khatyrkite (CuAl₂) and reduction of SiO₂ to Si

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• 作者：Christopher Hamann^a ; ^b ; ^{christopher.hamann@mfn-berlin.de" class="auth_mail" title="E-mail the corresponding author} ; Dieter Stö ; ffler^a ; ^c ; Wolf Uwe Reimold^a ; ^c
• 关键词：Regolith ; Impact experiments ; Khatyrkite ; Projectile&ndash ; target interaction ; Asteroids ; Khatyrka
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 November 2016
• 年：2016
• 卷：192
• 期：Complete
• 页码：295-317
• 全文大小：5007 K

文摘

We analyzed the interaction of spherical, 6.36-mm-diameter, Cu-bearing aluminum projectiles with quartz sand targets in hypervelocity impact experiments performed at NASA Ames Vertical Gun Range. Impact velocities and inferred peak shock pressures varied between 5.9 and 6.5 km/s and ∼41 and 48 GPa, respectively. Shocked particles (“impact melt particles”) coated with thin crusts of molten projectile material were recovered from the floors of the ca. 33-cm-diameter craters and the respective ejecta blankets. Through petrographic and chemical (optical microscopy, FE-EMPA, SEM-EDX, and XRF) analysis we show that these particles have a layered structure manifested in distinct layers of decreasing shock metamorphism. These can be characterized by the following physical and chemical reactions and alteration products: (i) complete melting and subsequent recrystallization of the projectile, forming a distinct crystallization texture in the fused metal crust; (ii) projectile–target mixing, involving a redox reaction between Cu-bearing Al alloy und SiO₂, leading to formation of khatyrkite (CuAl₂), Al₂O₃ melt, euhedral silicon crystals, and spherical droplets of silicon; (iii) melting of quartz to lechatelierite and formation of planar deformation features in relic quartz grains; and (iv) shock lithification of quartz grains with fracturing of grains, grain-boundary melting, planar deformation features, and complete loss of porosity. To our knowledge, this is the first report of khatyrkite formed experimentally in hypervelocity impact experiments. These results have implications for the understanding of a similar redox reaction between Al–Cu metal and siliceous impact melt recently postulated for the Khatyrka CV3 carbonaceous chondrite. Moreover, these results bear on the processes that lead to layers of regolith on the surfaces of planetary bodies without atmospheres, such as asteroids in the main belt (e.g., 4 Vesta), and on the Moon. Specifically, impacts of mm-sized projectiles at velocities between 4 and 6 km/s into regolith-covered, asteroidal surfaces in the main belt should yield similar impact melt particles that feature a continuum of shock effects, i.e., partially to completely molten projectile remnants adhering to impact-melted regolith agglomerates, as well as projectile-contaminated impact melts and local shock melting along grain boundaries.