Mesosiderite, group A3/4
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Fell May 1879
43° 25' N., 94° 50' W.

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 among 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 through several competing 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 high temperature equilibration, followed by very slow cooling (~0.5°C/m.y.) as the brecciated material was deeply covered by a massive debris blanket. The relatively young Ar–Ar ages of mesosiderites of 3.7–4.1 b.y. reflect this period of very slow cooling. Weakly shocked olivine was sequestered into the core at the time of the catastrophic impact, as molten metal was mixed with cold crustal fragments during re-accretion.

Recent dating of zircons in Estherville by Haba et al. (2014) places the formation age of this mesosiderite (i.e., metal-silicate mixing and crustal remelting) at 4.520 (±027) b.y. Cooling rate studies conducted by Sugiura and Kimura (2015) on a number of mesosiderite samples indicate that Estherville, Vaca Muerta, NWA 2924, and Dong Ujimqin Qi experienced rapid cooling from peak temperatures down to intermediate temperatures, while others including NWA 1242, NWA 1878, Crab Orchard, ALH 77219, and A-882023 cooled much more slowly over the same temperature range.

The conventional theory for mesosiderite formation puts the time of accretion, melting, and crystallization of the large parent body at ~4.56 b.y. ago. A period of impact-melting and metamorphism ensued until 3.9 b.y. ago, by which time the brecciated nature of the mesosiderite parent body had been 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. Estherville has a Sm–Gd-based CRE age of 70 (±7) m.y. (Albrecht et al., 2000). Analyses of cosmogenic noble gases (e.g., 3He, 21Ne, 36Ar, 81Kr) in zircons and silicates from Estherville were conducted by Haba and Nagao (2021) in which similar CRE ages were obtained (see diagram below).

CRE Ages for Zircon, Silicate, and Metal in Estherville
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Diagram credit: Haba and Nagao, MAPS, vol. 56, #5, p. 10 (2021)
'Cosmogenic noble gas nuclides in zircons from the Estherville mesosiderite'

Wang and Hsu (2019) used Pb–Pb chronometry to date 53 merrillite crystals associated with FeNi-metal in the Youxi mesosiderite. Based on the low REE abundances in the Youxi merrillite compared to that in eucrites, they contend that it was formed by oxidation of P in metal during the metal–silicate mixing event rather than during magmatic activity. They derived an age of 3.950 (±0.080) b.y. which they consider represents the timing of merrillite development during the mesosiderite-forming event. An equally plausible timing for the metal–silicate mixing event was ascertained by Haba et al. (2019) using high-precision U–Pb dating of zircons in several mesosiderites. Based on these results they contend that the metal–silicate mixing event occurred 4.52539 (±0.00085) b.y. ago. They propose a scenario in which a hit-and-run collision disrupted the northern hemisphere of Vesta leading to ejecta debris reaccreting to the opposite, southern hemisphere (see schematic diagram below). The deeply buried mesosiderite meteorites were ejected into Earth-crossing orbits by later impacts.

Schematic Illustration of Mesosiderite Formation
Crust (yellow); Mantle (blue); Core (red); Collisional debris (green)
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Diagram credit: Haba et al., Nature Geoscience, vol. 12, #2, p. 512 (2019)
'Mesosiderite formation on asteroid 4 Vesta by a hit-and-run collision'

A more outdated theory for mesosiderite formation 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 presented by D.W. Mittlefehldt (2021), including petrological (e.g., modal, textural, and redox data), compositional (e.g., incompatible lithophile trace elements), and observational (Dawn at Vesta), 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 A3/4 (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.