Geochemical interaction between projectile and target in hypervelocity cratering experiments

Geochemical projectile-target interaction was studied by means of 6 hypervelocity impact experiments and 4 laser-induced melting experiments, using sandstone and quartzite targets and projectiles (steel and iron meteorite) rich in siderophile (Ni, Co, Mo, W) and some lithophile tracer elements (Cr, V). The high chemical contrast between target and projectile facilitated tracing of very small projectile material within the target. During the cratering process ejecta fragments were formed showing shock- metamorphic features such as planar deformation features (PDF) in quartz, the onset to complete transformation of quartz to lechatelierite, partial melting of the sandstone (quartzite), and partially molten projectile, mixed mechanically and chemically with target melts. These highly shocked projectile-rich fragments are mainly ejected in a steep angle (~70° - ~80°) compared to the impact angle, which is in accordance with previous impact experiments. The laser experiments provided an additional control on the melting behavior and chemical mixing of projectile and target materials. This was especially important to constrain the heterogeneity of projectile and target melts prior to their mixing. During mixing of projectile and target melts, whether impact or laser experiments, Fe, Cr and V of the projectiles are preferentially partitioned into target melts compared to Ni, Co, Mo and W, yielding inter-element ratios in the sandstone or quartzite melt which completely differ from the element ratios of the projectiles. Due to the loss of the more lithophile elements (e.g., Cr, V) the projectile droplets are enriched in the siderophile elements (e.g., Co, Ni, Mo, W). This inter-element fractionation results from differences in the reactivity of the respective elements with oxygen during incorporation of metal melt into silicate melt. Data from Meteor Crater (Arizona, USA) and the Wabar craters (Saudi Arabia) show trends similar to those observed in the mesoscale laboratory craters of the MEMIN project. Melting of projectile requires temperatures higher than expected from the calculation of maximum shock pressures. Three mechanisms for enhanced thermal input are suggested: 1) friction and deformation (i.e., plastic work) of the projectile, 2) more effective transfer of kinetic energy to porous material (in case of the sandstone) including the local increase of shock pressure due to pore collapse, and 3) heat transfer from shock compressed air during projectile flight (projectile pre-heating). The results indicate that the principles of projectile-target interaction and associated fractionation do not depend on impact energies (at least for the selected experimental conditions) and water saturation of the target. Partitioning of the projectile tracer elements is intensified in experiments with non-porous quartzite compared to the porous sandstone target. This is mainly the result of higher shock pressure and the related higher shock and post-shock temperatures including a longer time-span with physical conditions sufficient to partition of elements between projectile and target melt. The intensified partitioning processes in impact experiments with quartzite as target material led to the formation of an Fe enrichment zone surrounding projectile droplets. During quenching of the ejecta the Fe enrichment zone experiences a phase separation into an Fe-rich liquid (Lfe) and a Si-rich liquid (Lsi). This liquid immiscibility occurs between melts with strong chemical differences and forms conspicuous emulsions textures. The incorporation of steel and Fe meteorite matter into sandstone melt of the laser experiments is also accompanied by phase separation of iron rich and silica rich melts. In addition, this feature was recently described in the impact glass of the Wabar crater.