Geochemical processes between steel projectiles and silica-rich targets in hypervelocity impact experiments

Ebert Matthias , Hecht L., Deutsch Alexander , Kenkmann Th. , Wirth R., Berndt J.

Abstract

The possibility of fractionation processes between projectile and target matter is critical with regard to the classification ofthe impactor type from geochemical analysis of impactites from natural craters. Here we present results of five hypervelocityMEMIN impact experiments (Poelchau et al., 2013) using the Cr–V–Co–Mo–W-rich steel D290-1 as projectile and two differentsilica-rich lithologies (Seeberger sandstone and Taunus quartzite) as target materials. Our study is focused on geochemicaltarget–projectile interaction occurring in highly shocked and projectile-rich ejecta fragments. In all of the investigated impactexperiments, whether sandstone or quartzite targets, the ejecta fragments show (i) shock-metamorphic features e.g., planardeformationfeatures (PDF) and the formation of silica glasses, (ii) partially melting of projectile and target, and (iii) significantmechanical and chemical mixing of the target rock with projectile material. The silica-rich target melts are strongly enriched inthe “projectile tracer elements” Cr, V, and Fe, but have just minor enrichments of Co, W, and Mo. Inter-element ratios of thesetracer elements within the contaminated target melts differ strongly from the original ratios in the steel. The fractionationresults from differences in the reactivity of the respective elements with oxygen during interaction of the metal melt with silicatemelt. Our results indicate that the principles of projectile–target interaction and associated fractionation do not depend onimpact energies (at least for the selected experimental conditions) and water-saturation of the target. Partitioning of projectiletracer elements into the silicate target melt is much more enhanced in experiments with a non-porous quartzite target comparedwith the porous sandstone target. This is mainly the result of higher impact pressures, consequently higher temperatures andlonger reaction times at high temperatures in the experiments with quartzite as target material.