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

1,2Christopher Hamann, 1,3Dieter Stöffler, 1,3Wolf Uwe Reimold
Geochimica et Cosmochmica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.07.018]
1Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin, Germany
2Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74–100, 12249 Berlin, Germany
3Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany
Copyright Elsevier

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 5.9–6.5 km/s and ∼41–48 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 analyses (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 4–6 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.

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