ESTHERVILLE

At 5:00 P.M. on May 10, in Estherville, Iowa, several large masses and hundreds of small iron nodules fell after a fireball was seen and sonic booms heard. Over 700 pounds of material was recovered, including one mass of ~437 pounds and one of 151 pounds. The largest mass was divided between the London, Paris, and Vienna Museums while the location of the smaller mass is unknown. Hundreds of the atmospherically ablated iron nodules are preserved at Yale's Peabody Museum.

This polymict breccia includes iron inclusions together with large areas of silicates including olivine, pyroxene, and plagioclase. Mineralogical studies have determined that matrix olivines and olivine clasts are most likely xenoliths from separate parent bodies, which were assimilated together onto the mesosiderite planetesimal during impact late events (Hassanzadeh et al., 1990). Lithic clasts of eucritic and diogenitic material are present.

The formation of mesosiderites on their parent body has been explained by several theories. A recent model based on smoothed-particle hydrodynamics calls for the disruption and re-accretion of a 200–400 km differentiated asteroid with a molten core. The impactor is calculated to have been a 50–150 km body with an impact speed of 5 km/s. This event initially caused rapid cooling (~0.1°C/y.) from thermal equilibration, followed by very slow cooling (~0.5°C/m.y.) as the brecciated material was deeply covered by a massive debris blanket. The Ar–Ar ages of mesosiderites of 3.7–4.1 b.y. reflect this very slow cooling. Weakly shocked olivine was sequestered into the core at the time of the catastrophic impact, as molten metal was mixed with crustal fragments during re-accretion.

A more conventional theory calls for the accretion, melting, and crystallization of the large parent body ~4.56 b.y. ago. By ~4.47 b.y. ago, crustal remelting had occurred and metal–silicate mixing was taking place. This was followed by a period of impact melting and metamorphism until 3.9 b.y. ago, by which time the brecciated nature of the mesosiderite parent body was established. It was at this time, 3.9 b.y. ago, that a major thermal event occurred, raising temperatures to as high as 500°C. A likely cause for this event is the collisional disruption and gravitational reassembly of the asteroid. The surface breccias were buried under a deep regolith where slow cooling and annealing proceeded. Subsequent impacts excavated this deeply buried material and some of it was ejected into space, establishing a range of cosmic-ray exposure ages for mesosiderites of ~10–340 m.y.

A more outdated theory has the basaltic crust of a molten parent body founder and sink through the mantle to the metallic core where mixing occurred. Subsequent collisions exposed this stony-iron layer and delivered fragments to Earth. It is notable that the O-isotopic values of the mesosiderites are almost identical to those of the HED suite of meteorites, implying that a genetic link exists between these disparate groups (Greenwood et al., 2006). Conversely, multiple line of evidence indicate that separate parent bodies were probably involved.

In the classification scheme of Floran, 1978 and Hewins, 1984, Estherville was assigned as a transitional member to group 3A and 4A (see the Bondoc page for further information about the grouping scheme). Calculations based on cosmogenic radionuclides show that Estherville had a pre-atmospheric diameter of at least 62 cm. Its cosmic-ray exposure age of ~70 m.y. is similar to that of Crab Orchard and Chinguetti, suggesting a common ejection event for these three mesosiderites. The Estherville specimen shown above is an 18.6 g complete slice, which exhibits a stony matrix containing iron inclusions and numerous olivine crystals.

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