Moderate velocity oblique impact sliding: Production of shocked meteorite textures and palaeomagnetically important metallic spherules in planetary regoliths
We detail the production of metallic spherules in laboratory oblique shock impact experiments, and their applicability (1) to textures in a partly shock-melted chondritic meteorite and (2) to the occurrence of palaeomagnetically important fine iron or iron alloy particles in the lunar regolith. Samples recovered from 29–44 GPa, 800 ns, experiments revealed melting and textures reminiscent of metallic spherules in the Yanzhuang H-chondrite, including “dumbbell” forms and other more complex morphologies. Our experiments demonstrate that metallic spherules can be produced via oblique impact sliding at lower velocities (1.85 km s^(−1)) than are generally assumed in previous work associated with bulk-shock melting, and that oblique impact sliding is a viable mechanism for producing spherules in shock-induced veins in moderately shocked meteorites. Significantly, our experiments also produced fine metallic (iron alloy) spherules within the theoretical narrow size range (a few tens of nanometers for slightly ellipsoidal particles) for stable single-domain (SSD) particles, which are the most important palaeomagnetically, since they can record lunar and planetary magnetic fields over geological time periods. The experiments also produced spherules consistent with superparamagnetic (SP) and multidomain (MD) particle sizes. The fine SSD and SP particles on the lunar surface are currently thought to have been formed predominantly by space weathering processes. Our experiments suggest that oblique shock impact sliding may be a further means of producing the SSD and SP iron or iron alloy particles observed in the lunar regolith, and which are likely to occur in the regoliths of Mercury and other planetary bodies
