Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid

Abstract

Abstract-We propose that mesosiderites formed when a 200-400 km diameter asteroid with a molten core was disrupted by a 50-1 50 km diameter projectile. To test whether impacts can excavate core iron and mix it with crustal material, we used a low-resolution, smoothed-particle hydrodynamics computer simulation. For 50-300 km diameter differentiated targets, we found that significant proportions of scrambled core material (and hence potential mesosiderite metal material) could be generated. For near-catastrophic impacts that reduce the target to 80% of its original diameter and about half of its original mass, the proportion of scrambled core material would be about 5 vol%, equivalent to -1 0 vol% of mesosiderite-like material. The paucity of olivine in mesosiderites and the lack of metal-poor or troilite-rich meteorites from the mesosiderite body probably reflect biased sampling. Mesosiderites may be olivine-poor because mantle material was preferentially excluded from the metal-rich regions of the reaccreted body. Molten metal globules probably crystallized around small, cool fragments of crust hindering migration of metal to the core. If mantle fragments were much hotter and larger than crustal fragments, little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally. The rapid cooling rates of mesosiderites above 850 "C can be attributed to local thermal equilibration between hot and cold ejecta. Very slow cooling below 400 "C probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris. We infer that our model is more plausible than an earlier model that invoked an impact at -1 km/s to mix projectile metal with target silicates. If large impacts cannot effectively strip mantles from asteroidal cores, as we infer, we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material. This may help to explain why relatively few olivine-rich (A-type) and metal-rich asteroids (M-type) are known. Some S-type asteroids may be scrambled differentiated bodies.

Plausible parent bodies for enstatite chondrites and mesosiderites: Implications for Lutetia's fly-by

“…A number of contrasting scenarios explaining the formation of the mesosiderites have been published (Mittlefehldt et al, 1998, and references therein; Scott et al, 2001). They all start with the differentiation of a large asteroid (several hundred km in diameter), 4.56 Gyr ago.…”

26 Al IN THE EARLY SOLAR SYSTEM: NOT SO UNUSUAL AFTER ALL

“…For example, NLTT 43806 is aluminum-rich as would be expected if the accreted planetesimal largely was composed of a crust (Zuckerman et al 2011) while PG 0843+516 is iron rich which can be explained by the accretion of a core (Gaensicke et al 2012). Xu et al (2013) found that the abundance pattern of the object accreted onto GD 362 resembles that of a mesosiderite-a rare kind of meteorite that is best understood as a blend of core and crustal material (Scott et al 2001).…”

Geochemistry and origin of metal, olivine clasts, and matrix in the Dong Ujimqin Qi mesosiderite

“…Hassanzadeh et al (1990) suggested that mesosiderites formed by accretion of a molten core accompanied by mantle materials, to a large degree consisting of olivine, onto the basaltic surface of a neighboring asteroid. Scott et al (2001) proposed formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid. Although olivine is a major concern in mesosiderite formation models, there is still no conclusive evidence regarding the origin of olivine in mesosiderites.…”

“…The first class of models assume formation of mesosiderites by impact and mixing of a molten Fe-Ni core from a small differentiated body with the silicates of the basaltic surface of a larger one (Powell 1971;Floran 1978;Wasson and Rubin 1985;Rubin and Mittlefehldt 1993). In the second class of models, mesosiderites were formed by internal processes whereby basaltic crust of an asteroid is mixed with its own core (Hewins 1983;Haack et al 1996;Scott et al 2001). The rare occurrence of olivine clasts is a major concern of both models.…”