Volatile enhanced dispersal of high velocity impact melts and the origin of tektites

In hypervelocity meteorite impacts, shock energies produce temperatures well above the melting point of a wide area of the impacted target rocks. This produces impact melt during excavation and expansion of the transient crater cavity. The vast majority of this melt is retained in the crater-fill stratigraphy where it may form coherent melt units and/or be variably mixed with non-molten target rocks. A small portion (1-3%) of this melt is ejected from the crater at very high velocities - potentially faster than the impactor itself forming impact glasses and, in rare cases, tektites. Why only some impacts form large volumes of high velocity impact glass and even fewer form tektites remains poorly understood. Many of the expected theoretical controls on the production and dispersal of high-velocity impact melt (target rock type, impact size, impact angle) do not seem to apply; comparison of the volume and nature of ejected melt around complex and simple craters on Earth reveals no systematic relationship to any of these parameters. The geologic evidence suggests that there is another controlling mechanism that promotes production of high velocity impact melt and tektite formation in some impacts. The Darwin impact event shows clearly that the presence of water rich surface layers in the target stratigraphy enhances by orders of magnitude the production of high velocity ejected melt; as hinted at by some numerical models. For tektites from all four strewn fields, the presence of water rich surface layers at the impact site can be inferred and it seems this is the missing feature of the target stratigraphy required to explain tektite origin. (C) 2011 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.