LAYERED MELTS AND MELT-COVERED ROCKS FROM SMALL IMPACT CRATERS – EVIDENCE FOR MELTING BY AN AIRBURST

Melts at impact craters are usually attributed to shock melting of the target material. However, projectiles that explode in the atmosphere can produce an “airburst” consisting of a high temperature jet of expanding gas and vaporized projectile that impinges on the surface (Boslough, M.B.E., this meeting). The descending “fireball” makes contact with the Earth’s surface, where it expands radially and can also deliver an impactor component to the surface. The fireball can maintain temperatures well above the melting temperature of silicate minerals, and its radial velocity can exceed the sound speed in air. We are studying samples of melts from small craters using a portable XRF to rapidly (3 min. per analysis) and nondestructively analyze the melts for siderophile elements. Small craters with impact melt fragments include: Wabar, Henbury, and Lonar. The impact melt fragments from these craters sometimes exhibit the presence of thin layers of melt. These melts are sometimes stacked into layered samples (Lonar) up to 10 cm thick, and sometimes form coatings around unmelted material or layered melt bodies (Henbury). Surprisingly, our analysis of over 100 samples of impact melts from the 1.8 km diameter Lonar Crater show no evidence for an impactor component, while shocked rocks of all levels are common. Evidence for a chondritic impactor has only been found in tiny spherules in the uppermost ejecta at Lonar by Misra et al., (MaPS, 2009). Samples from the 0.12 km diameter Wabar crater consist of white material coated by dark impact melt. We found the presence of enriched Ni in all the melt coatings of the 9 samples studied. The 18 samples from the Henbury crater field (0.2 – 0.4 km, for example) are the most interesting, with layered melt bombs and a unique melt covered rock. The melts exhibit large correlated Ni and Fe abundances consistent with an impactor component. The existence of a small 4 cm wide tabular rock from Henbury, covered in siderophile-element-bearing melt about 2 mm thick, provide the most convincing data supporting an airburst origin for the melts. The airburst process also explains the lack of intermediately-shocked rocks at the two smaller impact sites, whereas glasses/melts are abundant.