Folco, Luigi; Bland, Philip A.; D'Orazio, Massimo; Franchi, Ian A.; Kelley, Simon P. and Rocchi, Sergio
Extensive impact melting on the H-chondrite parent asteroid during the cataclysmic bombardment of the early solar system: Evidence from the achondritic meteorite Dar al Gani 896.
Geochimica et Cosmochimica Acta, 68(10) pp. 2379–2397.
DaG 896 is an olivine-rich microporphyritic rock of komatiitic composition. Both the olivine composition (Fa(17.5+/-2.1), [Mn/Mg] = 0.0061) and the bulk oxygen isotopic composition (delta(17)O = +2.55, delta(18)O = + 3.50) indicate that DaG 896 is a sample of the H-chondrite parent body. The bulk chemistry shows an H-chondritic distribution of lithophile elements, whereas chalcophile and siderophile elements are strongly depleted, indicating formation through whole-rock melting (or nearly so) of H-chondrite material, nearly complete loss of the metal plus sulfide component, and crystallization without significant igneous fractionation. Superheated, severely shocked chondritic relics (similar to10 vol%), typically in the form of corroded lithic fragments <100 mum in size intimately distributed within the igneous lithology, indicate that melting was triggered by a highly energetic impact, which possibly induced shock pressures of similar to80-100 GPa. The relatively young 3.704 +/- 0.035 Ga Ar-40- Ar-39 crystallization age is consistent with the impact melting origin, as magmatism in the asteroid belt was active only in the first hundred million years of solar system history.
1ased on textural data and thermodynamic crystallization modelling, we infer that DaG 896 crystallized from a liquidus temperature of similar to1630degreesC under relatively slow cooling rates (similar to10degreesC h(-1)) to -1300degreesC, before quenching. The two-stage cooling history indicates that a reasonable formation environment might be a dike intruding cooler basement below a crater floor. Metal-silicate fractionation may have been accomplished, at least at the centimeter-scale of the studied meteorite sample, through differential acceleration of immiscible liquids of different density during the intense flow regimes associated with the excavation and modification stages of the cratering mechanism. Alternatively, DaG 896 may represent a surface sample of a differentiated melt body at the floor of an impact crater, as gravitational settling appears to be an effective process at the surface of a chondritic parent asteroid: for metal particles 1 to 10 mm in size, typically observed in partially differentiated impact melt rocks, Stokes' Law indicates a settling velocity > 1 in h(-1) during the first few hours of crystallization on asteroidal bodies of >25 kin radius.
The similar to3.7 Ga age of DaG 896 nearly overlaps with the slightly older resetting ages of H-chondrites (all impact melts) available from the literature, indicating that the H-chondrite parent asteroid underwent extensive impact melting at the enduring of the cataclysmic bombardment of the early solar system. Such an age overlap may also indicate early disruption of the initial H-chondrite parent asteroid.
The close similarity between the bulk composition and degassing age of DaG 896 and silicate inclusions in HE iron meteorites is further evidence in support of a common origin by impact melting and metal-silicate segregation on the H-chondrite parent asteroid. Our new high-precision oxygen isotopic measurements of H-chondrites (Delta(17)O = 0.77 +/- 0.04) Should be extended to IIEs to verify this possible petrogenetic link.
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