SEARCH FOR IMPACTOR REMNANTS PRESERVED IN MICRO-SCALE DEPOSITION LAYERS IN AUSTRALASIAN MICROTEKTITES (Invited Presentation)

High-energy terrestrial meteorite impacts produce large volumes of impact melt and vaporized material; tektites are a specific type of distal impact glass. 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). To this end we characterized microtektites belonging to the Australasian strewn field, acquired from the Ocean Drilling Program. Particularly, we selected microtektites containing multiple μm-scale, compositionally distinct bands for this study, identified by SEM analysis. These bands are similar in size and appearance to condensation features observed at the interfaces of fused, agglomerated, aerodynamically shaped fallout glasses from nuclear explosions, which have been shown to preserve nuclear device source material. This suggests that the analyzed microtektites were also formed by an agglomeration process, and that these bands may preserve impactor material that condensed on their surfaces prior to collision and quenching.

Compositional analysis by EPMA and NanoSIMS revealed that the microtektites are felsic, with high MgO and FeO concentrations (>1.5% and >4% by weight, respectively), consistent with previous studies. Compositional analysis of the bands, however, revealed enrichments in the abundances of Fe, Mg, Ca, Cr, Ni, and Co, and depletion in Si, Al, Na, and K, relative to the bulk composition. No correlation was found between the major oxides in the microtektites away from the bands, suggesting mixing of multiple molten mineral phases. However, simple linear regression analysis of the band compositions showed that the variations of Fe, Mg, and Ca are positively correlated with each other (r² = 0.9), and inversely correlated with K (r² = 0.9) and Si (r² = 0.7). These correlations suggest a common, basaltic source of this vapor component, such as an achondritic impactor.