Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid

We propose that mesosiderites formed when a 200-400 km diameter asteroid wi th a molten core was disrupted by a 50-150 km diameter projectile. To test whether impacts can excavate core iron and mix it with crustal material, we used a low-resolution, smoothed-particle hydrodynamics computer simulation . For 50-300 km diameter differentiated targets, we found that significant proportions of scrambled core material (and hence potential mesosiderite me tal material) could be generated. For near-catastrophic impacts that reduce the target to 80% of its original diameter and about half of its original mass, the proportion of scrambled core material would be about 5 vol%, equi valent to similar to 10 vol% of mesosiderite-like material. The paucity of olivine in mesosiderites and the lack of metal-poor or troilite-rich meteor ites from the mesosiderite body probably reflect biased sampling. Mesosider ites may be olivine-poor because mantle material was preferentially exclude d from the metal-rich regions of the reaccreted body. Molten metal globules probably crystallized around small, cool fragments of crust hindering migr ation of metal to the core. If mantle fragments were much hotter and larger than crustal fragments, little metal would have crystallized around the ma ntle fragments allowing olivine and molten metal to separate gravitationall y. The rapid cooling rates of mesosiderites above 850 degreesC can be attri buted to local thermal equilibration between hot and cold ejecta. Very slow cooling below 400 degreesC probably reflects the large size of the body an d the excellent thermal insulation provided by the reaccreted debris. We in fer that our model is more plausible than an earlier model that invoked an impact at similar to1 km/s to mix projectile metal with target silicates. I f large impacts cannot effectively strip mantles from asteroidal cores, as we infer, we should expect few large eroded asteroids to have surfaces comp osed purely of mantle or core material. This may help to explain why relati vely few olivine-rich (A-type) and metal-rich asteroids (M-type) are known. Some S-type asteroids may be scrambled differentiated bodies.