Iron projectile fractionation processes in siliceous glass from small impact craters

Detection of extra-terrestrial geochemical components in melt generated during meteorite impact provides diagnostic evidence that can be used to confirm a hypervelocity impact event, and in some cases, classify the projectile. However, projectile contamination is often present at sub-percent levels, and can be difficult to detect. In contrast, meteoritic abundances in glass from small impact craters (<1 km diameter) formed by iron meteorites can be anomalously high, which has been attributed to glass originating from the projectile-target interface. Emulsion textures, immiscible liquids, metal spherules, and non-meteoritic siderophile element ratios have been cited as evidence that the projectile component is typically fractionated in impact glass. Here we present compositional data for impact glass from the Henbury crater field in Australia, where the largest crater is 145 m in diameter and the subgreywacke target rock and IIIAB iron projectile are geochemically distinct. Mixing models (Fe-Si, Ni-Co, Cr-Ir) and high platinum group element abundances indicate average projectile contributions ranging from 3 to 13 % in Henbury glass, comparable to ranges reported in glass from the Kamil (Egypt) and Wabar (Saudi Arabia) impact craters. However meteoritic siderophile element ratios (Fe:Ni, Fe:Co, Ni:Co) in Henbury glass appear nearly unfractionated, whereas Wabar and Kamil glasses have more fractionated ratios. Observed variations are attributed to fractionation of meteoritic Ni by formation of immiscible Ni-rich spherules during oxidation of meteoritic iron, and subsequent separation of Ni-rich spherules from glass during ejection. The Henbury glass sample analyzed is interpreted as an example of an interface melt that quenched prior to extensive oxidation and phase separation, and thus may represent one of the least fractionated samples of melt from the projectile-target interface described thus far, whereas Wabar and Kamil glasses record more evidence of fractionation processes. These results further highlight the influence of metal spherule formation on the composition of ejected glass from small impact structures formed by iron meteorites and provide new insights that explain textural features observed in natural impact glasses.