MELTING AND CHEMICAL PROJECTILE-TARGET INTERACTION IN HYPERVELOCITY AND LASER EXPERIMENTS

The detection and identification of meteoritic components in impact-derived rocks are of great value for confirming an impact origin and for reconstructing the origin of extraterrestrial material that repeatedly stroke the Earth during its entire life time [1]. The processes of mixing projectile matter into target and impactite materials, however, are poorly understood. We present results from hypervelocity experiments that have been performed at the two-stage acceleration facilities of the Fraunhofer Ernst-Mach-Institute (Freiburg). In addition laser-induced melting experiments were done with a laser welding facility of the Technical University of Berlin. Our results are based on experiments using a Campo del Cielo iron meteorite as projectile and (quartz-rich) Seeberger Sandstone as target material.

Both types of experiments show partials melting of target and projectile material. The melts are more or less heterogeneous depending on the relative proportions of minerals phases that were melted. During mixing of projectile and target melts the Fe of the projectile is preferentially partitioned into the silicate target melt over Ni and Co. The hypervelocity experminents produced shocked quartz with pdf’s (in the sandstone target), that are also enriched in Fe and Ni. Inter-element fractionation between projectile and target occurs in different impact stages. (A) After shock compression with formation of PDFs in Qtz, diaplectic glass or lechatelierite, and during early unloading, <1 % of projectile matter is added to the glass phases without detectable fractionation. (B) Later, when waste heat triggers melting of the sandstone, molten projectile is mixed with the sandstone melt and significant element fractionation occurs. Fe is selectively enriched in the silicate melt; Ni and Co are enriched over Fe in co-existing projectile spherules. This is in accordance with observations in nature [e.g. 2].

[1] Koeberl C. (1998) Geol. Soc. Spec. Pub. 140, 133-153. [2] Mittlefehldt D. W. et al. (1992) Meteoritics 27, 316-370.