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Neumann lines in a mesosiderite |
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| Meteorite - NWA 6266, mesosiderite. |
| Figure 1. Scale bar 300 µm. |
| Meteorite: NWA 6266, mesosiderite. |
| Neumann lines in kamacite structures. |
| Neumann Lines in a Mesosiderite
Meteorite: NWA 6266 (mesosiderite) Figure 1. Neumann lines (shock-deformation twins) in kamacite within NWA 6266. Scale bar: 300 µm. Mesosiderites are a rare class of stony-iron meteorites, one of the two principal groups of stony-irons (the other being pallasites). They consist of roughly equal proportions of metallic Fe–Ni and silicate material and remain relatively uncommon compared with ordinary chondrites or iron meteorites. The metallic portions are composed primarily of kamacite and taenite. The silicate fractions are dominated by orthopyroxene, plagioclase, and lesser olivine, and are predominantly crustal in character—basaltic, gabbroic, and orthopyroxenitic lithologies similar to those of the Howardite–Eucrite–Diogenite (HED) clan—rather than deep mantle material. Neumann Lines Neumann lines (also known as Neumann bands) are fine, parallel or intersecting straight features visible in polished and etched sections of kamacite. They represent mechanical twins formed by shock-induced plastic deformation under high strain rates, typically at pressures of ~10–20 GPa or greater. In NWA 6266 and other mesosiderites, these features record impact events that occurred after the initial assembly of metal and silicate components on the parent body. As secondary structures, Neumann lines provide key evidence of the collisional history of mesosiderite parent asteroids. Petrogenetic Context The long-standing interpretation that mesosiderites originated at a core–mantle boundary has been largely abandoned. That model applies primarily to pallasites, which contain abundant olivine intimately mixed with metal. Mesosiderites differ in three critical respects: their silicates are predominantly crustal, they lack the abundant olivine expected from a deep mantle source, and they display clear brecciation and complex impact processing. Current mainstream models favor impact-driven mixing of molten or partially molten metal—most likely derived from a disrupted core—with crustal silicates. Two leading scenarios are: • Low-velocity accretion of large metallic core fragments onto the basaltic surface of another differentiated asteroid (the model developed by Wasson & Rubin, 1985). • Catastrophic disruption and reassembly of a differentiated body that still retained a molten core. Alan Rubin has been a principal advocate of the impact-mixing interpretation. In the 1985 Nature paper with John T. Wasson, and in subsequent work with David W. Mittlefehldt (1992, 1993), he argued that mesosiderites formed through collisional processes that delivered core-derived metal into crustal regoliths, followed by further impact gardening, localized remelting of silicate clasts, and metamorphism. These studies explicitly frame mesosiderites as products of dynamic mixing rather than undisturbed samples of a core–mantle interface. The presence of Neumann lines in the kamacite of NWA 6266 records later shock metamorphism that deformed the metal without fully melting it. Despite their brecciated character, mesosiderites can exhibit surprising mechanical resilience; the ductile metal matrix helps absorb and dissipate impact energy, reducing the likelihood of complete fragmentation. As noted by Dr. Randy Korotev (Washington University in St. Louis), the kamacite–taenite metal component plays a critical role in this energy absorption. The combination of metal–silicate mixing, brecciation, and secondary shock features such as Neumann lines makes mesosiderites among the most complex meteorite types, preserving a record of violent collisional processes in the early Solar System. |
| References • Meteoritical Bulletin Database (entry for NWA 6266). • Mittlefehldt, D. W., et al. (various works on mesosiderites). • Buchwald, V. F. (1975). Handbook of Iron Meteorites (for general context on Neumann lines). • Korotev, R. L. (personal communication or related statements on mesosiderite durability). |
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