Formation of mesosiderites by low-velocity impacts as a natural consequence of planet formation

Abstract

Mesosiderites are an enigmatic class of stony-iron meteorites composed of roughly equal amounts of pyroxenitic and basaltic silicates and metallic iron–nickel olivine is rare. We suggest here that, during the period of planet formation, mesosiderites originated by the low-velocity collisions of large metallic core fragments with the surface of a differentiated asteroid-size body. Relative velocities of the asteroids native to this region were low (≤1 km s⁻¹) because it was distant from massive protoplanets. However, a few differentiated asteroids were destroyed by high-velocity collisions with interlopers perturbed by protoplanets into highly eccentric orbits. These collisions reduced mantles and crusts to small silicate fragments but left cores in the form of large, durable metal fragments. Mesosiderite-like pyroxenite–basalt–metal mixtures were formed when large core fragments accreted at low velocities to the regolith of an intact asteroid; the metal contents of other regolithic regions remained low. Most of the olivine in mesosiderite and howardite breccias we attribute to the mantles of the disrupted parent bodies. The low (12–20 mg g⁻¹) olivine contents indicate that the amount of debris accreted from disrupted asteroids was small relative to the volumes of the regoliths.