Impact Melting of Ordinary Chondrite Regoliths and the Production of Fine- Grained

Introduction: The detailed study of individual lunar soil grains provides evidence that the major optical properties of the lunar surface are primarily related to the production of fine-grained (< 20 nm, super-paramagnetic) Fe-particles in agglutinitic impact melts and to iron-rich vapor deposits on the surfaces of individual grains [1]. These Fe-rich materials are derived from oxidized species due to high post-shock temperatures in the presence of solar-wind derived H 2 [2]; part of the Fe-rich grain surfaces may also be due to sputtering processes [1]. Identical processes were recently suggested for the optical maturation of S-type asteroid surfaces [3, 4], the parent objects of ordinary chondrites (OCs). OCs, however, do not contain impact-produced soil melts [5], and should thus also be devoid of impact-triggered vapor condensates. The seeming disparity between [3, 4] and [5] can only be understood if all OCs resemble relatively immature impact debris, akin to numerous lunar highland breccias [5, 6, 7]. It is possible to assess this scenario by evaluating experimentally whether impact velocities of 5-6 km/s, typical for the present day asteroid belt, suffice to produce both impact melts and fine-grained metallic iron. Experimental Conditions: We used 125-250 µm powders of the L6 chondrite ALH85017 [8]. These powders were aliquots from fines that were produced by collisionally disrupting a single, large (461g) chunk of this meteorite during nine impacts [9] and by subjecting the resulting rubble to an additional 50 impacts, akin to [10]. As a consequence, the present shock-recovery experiments employ target materials of exceptional fidelity (i.e., a real chondrite that was impact pulverized). The target powders were packed into tungsten-alloy containers to allow for the potential investigation of freshly produced, fine-grained iron and impacted by stainless-steel and tungsten flyer plates; the packing density varied between 38 and 45% porosity. Peak pressures ranged from 14.5 to 67 GPa and were attained after multiple reverberations of the shock wave at the interface of the silicate powder and metal container. Pressures in the 50 to 70 GPa range should be fairly typical for asteroid impacts at ~ 5-6 km/s [e.g., 7, 10], yet we note that these pressures refer to those at the projectile/target interface only and that most crater ejecta on OC parent-bodies will have experienced much lower stresses. Results: As illustrated in Fig. 1a, all initial porosity disappeared by 14.5 GPa and a dense network of interlocking grains, some disaggregated and mechanically deformed, resulted. …