Mesosiderite, group A
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Found 1923, the crater known since 1920
27° 38' S., 117° 17' E.

Approximately 12.2 kg of mesosiderite fragments, comprising both oxidized and unoxidized material, have been found in and around a small impact crater in Western Australia. A high abundance of sub-mm-sized fragmentation particles, calculated to weigh 40 kg, were found by Smith and Hodge (1996) during their studies of soil from the crater site. Dalgaranga is the only crater known to have been created by a mesosiderite, and is the smallest known crater in Australia, measuring 70 feet across and 11 feet deep. Because the eastern side of the crater is higher than the western side, it is believed that the impactor approached from the west. This is a relatively recent impact event estimated by some to be ~270,000 years old. Dalgaranga contains a high abundance of plagioclase consistent with the mesosiderite petrologic class A, but no metamorphic grade (0–4) has been assigned (see the Bondoc page for further information about this classification scheme).

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. Dalgaranga has a Sm–Gd-based CRE age of <10 m.y. (Begemann et al., 1976).

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.

The Dalgaranga specimen shown above is a 20 g end section exhibiting a silicate groundmass with a high abundance of metallic grains, but lacking any of the larger silicate or metallic inclusions observed in most larger cut sections of Dalgaranga. The photo below shows the shallow Dalgaranga Crater with bedrock basement.

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Photo courtesy of Valdi (The Life of Valdi—Blogger)