SEARCH FOR REMNANT IMPACTOR SIGNATURES IN MICRO-SCALE DEPOSITION LAYERS IN AUSTRALASIAN MICROTEKTITES

Large-scale terrestrial meteorite impacts produce large volumes of impact melt and vaporized material. The vaporized impactor condenses and mixes with the molten ejecta, forming mm- to cm-scale distal impact glasses known as tektites. This material is found over large geographic regions, such as the Australasian strewn field. The composition of most Australasian tektites is representative of melted upper continental crust, however, the nature and origin of the impactor remains unconstrained, mostly due to the low abundance of any meteoritic admixture (with exception of the Ivory Coast tektites). Near-surface nuclear explosions produce fallout, which share certain physicochemical similarities to tektites. Recent studies on fallout have shown that they preserve layers of condensed nuclear device material on their surfaces. This is particularly obvious in agglomerates, which preserve distinct condensation features at the agglomerate interfaces. We postulate that tektites may similarly preserve condensed impactor material. We investigated microtektites from the Australasian strewn field, using spatially-resolved analytical techniques including EPMA and NanoSIMS. We identified two microtektites containing multiple μm-scale, high-Z bands similar to the condensation features observed at the interfaces of agglomerated fallout glasses. The microtektites are felsic (SiO₂ ~63 wt.%) in composition, with relatively high MgO (>1.5 wt%) and FeO (>4 wt%) concentrations. The high-Z bands revealed relative enrichments in the abundances of Fe, Mg, Ca, Cr, Ni, and Co, and depletion in Si, Al, Na, and K. We interpret the relative enrichment of Co and Ni in the bands as indicative of condensed impactor contribution. The Co/Ni and Cr/Ni variations across the samples are consistent with recent studies suggesting a basaltic source for this vapor component, such as an achondritic impactor.